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Ceramic Troubleshoo Troubleshooting ting Body Bloating This problem occurs after a clay body matures to the point that the surface seals due to glass development but before generation of gases from decomposition of organic, carbonate or sulfate materials has completed. The internal pressures bubble the clay (since it has softened to the point of being flexible). This problem is most common in terra cotta bodies that have been over fired. Some bodies are very sensitive to over firing: for example the existence of manganese granular in a body (used to create visual speckling effects) will almost certainly generate small bloats if the body is fired even one cone past its recommended maximum. Clay bodies made from native materials that have not been ground to 200 mesh are more likely to bloat at some stage in their melting process whereas bodies made from refined materials can completely melt without ever going through a bloating bloating stage.
Bloat ng ng in an over ﬁ red red terra co a body. It is OK at cone 4 it is very dense and strong but suddenly suddenly bloa bloat ng ng begins at at cone cone 5. Such bodies must must be be fired fired at at lower lower temperatures temperatures to avoid avoid this this volatility.
Body Cracking and Dunting During Firing At the Medalta Potteries (in Medicine Hat, Alberta, Canada) during the 1920s they made stoneware crocks up to 60 gallons. These monsters weighed more than 200 pounds and had walls between one and two inches thick. These crocks were made from the same clays that are employed in Plainsman H550 functional stoneware today and the glaze was typical of the feldspar recipes we still use. Even if you could fabricate one of these and figure out how on earth they dip-glazed a 250 pound unbisqued vessel, it would certainly crack into many pieces as it split during firing heat up and dunted during cooling. What was the firing secret? Simple. Energy was cheap, huge beehive kilns the size of a house could be fired for less than a dollar a month! Kilns were hard brick and massive and the firing cycle was one week. That's right, seven days. To me the moral of this story is that firing needs to be tuned to the ware. Consider another case that the average potter would find equally mystifying. In industry today it is common for roller kilns to fire stoneware in a 2-3 hour cold-to-cold cycle. My experience tells me that this is impossible. How do they do it? Obviously, they can only do this with ware that is lighter. But still, it is amazing, there must be much more to it. Here are a few factors: First, they use a body that employs low lignite, large particle, lower plasticity clays in the lowest possible proportion so water and gases can be vented out quickly. quickly. Second, they fire smarter. There are firing consultants in industry that do nothing but design custom firing curves for manufacturers. In the firing of any body there are periods when temperature rise can 1
be much faster and others when it must be slower. But how much faster and where? A thermogravimetric analysis (TGA) test weighs a clay sample during firing to determine when it expels the most gases. A differential thermal analysis (DTA) test reveals the periods of firing where the body is exothermic and where it is endothermic. With this and other information one can design a firing curve that provides for the shortest possible firing time. Third, commercial kilns fire very evenly, some expose ware to less than a degree of gradient. Draft is one factor, many kilns have burners that double as blowers and create a kind of 'hurricane' within the kiln that exposes every part of ware to heat. A fourth factor is not directly related to cracking but I will mention it. Modern glazes are formulated to be fast-fire. They are low in boron and remain unmelted until just before the end of the firing. This makes allowance for easy channeling of gases of body decomposition before the glaze melts. How long should a firing be? If ware is cracking and you don't want to get into complicated analysis of your firing curve then it should just be longer, it is a relative thing. However it does not take brain surgeons to fire smarter also. Hold at boiling point as long as possible (over night candling is best) and go up (and especially down) through quartz inversion slower (1050F, 570C). In electric kilns there is no draft, this is a real problem in avoiding gradients; you have no choice but fire slower in the hopes of getting a more even firing.
Example of of dunting, dunting, where a crack crack has has released released the the stresses stresses produced produced by by uneven uneven thermal thermal contraction contraction during cool ‐down in the kiln. This usually usually happens happens by by cooling cooling too quickly quickly through through quartz inversion.
Cracking during heatup. They They start start inward inward on on the concave angles. It It is is important important to to create shapes not prone prone to cracking and and smooth, smooth, compress and and round round abrupt abrupt contours contours and and areas areas prone prone to cracks (to deny deny them them a place to start).
This crack crack likely likely starting starting during bisque. It It started started at at a a sharp angled angled indent indent on on the outside (that (that coincided coincided with with a thin wall wall section) section) and and grew grew around around the the perimeter perimeter (not (not visible). visible). From there it it branched branched to to the base.
This crack began as stresses created during uneven drying (the rim was allowed to get ahead of the base). A thinner section (that happened during throwing) was exploited by the stresses and a crack appeared during heatup, likely likely during during the bisque.
Dealing With Chrome Flashing Close your eyes and imagine a nice pink pastel glaze. Now imagine that you wanted a nice white instead! During the ACerS convention in St. Louis I attended an eye-opening presentation by Stan Sulewski of Pfalzgraff (they are a well known porcelain table ware manufacturer in the US). After hearing what he said I came to better appreciate the synergy between chemistry and physical properties involved with glaze opacity and color. c olor. Potters absorbed in reactive and a nd artware glazes often forget how difficult it can be to make a proper white semi-gloss food-safe glaze. Engineers at the company faced a dilemma: •
The tin opacified glaze flashes pink because chrome used in the darker colored glazed ware volatilizes and reacts with the tin to form chrome tin pink hues in the white. The zircon opacified glaze had excessive metal marking. Refractory and angular zircon particles protrude from the surface when their population is too high (even when particle size is very fine).
The obvious solutions of dedicating a kiln to non-chrome-bearing wear or eliminating chrome containing glazes were not feasible. Thus the objective was clear: Adjust the recipe of the tin glaze to have low metal marking and white color without pink flashing. The first and most obvious approach of simply blending tin and zircon would address the marking problem but the pink colors of course remained because bec ause tin is so sensitive to chrome. Impossible as it may seem, they actually found an answer using ceramic chemistry. If you have ever worked with chrome tin stains you likely know that unless the chemistry of the host glaze is right the color does not develop. Getting the color to work can be a real challenge but in this case they actually wanted to sabotage it! Among warnings on stain manufacturers chrome-tin data sheets are mentions of the detrimental effects of zinc, raw alumina, magnesia and a lack of calcia. They reasoned that it should be possible to solve this problem by making the host glaze chemistry hostile to the development of chrome tin pinks. And that is what they did. Zinc, the most obvious choice, did kill the pink but it also imparted a yellow brown color, that would not do. The presence of adequate CaO is critical to the development of pink and MgO is detrimental. While both are fluxes a complete replacement was not practical. Glazes tolerate and usually benefit from relatively large amounts of CaO, but complete replacement with MgO or SrO (or even a mix) produces much different surfaces and less active melting. The critical factor, as implied above, is that if calcium is not present in a threshold minimum amount chrome tin pink colors can be completely absent. Thus the answer turned out to be a compromise: An MgO/SrO mix (with more MgO) replacing much of the CaO. This preserved the surface character and killed the pink. However the white color was compromised just a little so a final adjustment was done: a small amount of blue stain was added to brighten the white. There you have it, ceramic chemistry to the rescue again! However the story is not quite over, they still need to adjust things to better match the thermal expansion of the new glaze with the old.
Glaze Blisters Blisters are evident on the fired glaze surface as a 'moonscape' of craters, some with sharp edges and others rounded. These craters are the remnants of bubbles that have burst during final approach to temperature or early stages of cooling. In some cases there will be some unburst bubbles with a fragile 'dome' than can be broken. Blisters can vary in size and tend to be larger where the glaze is thicker.
Is the glaze fluid enough? Often glazes appear like the melt should have plenty of mobility to heal but this can be deceptive, a melt flow testing regimen is the only way to know for sure (melt flow testers have a reservoir at the top of a steep incline and the glaze runs down a calibrated runway). Generally a more fluid glaze will heal blisters much better (see section below on blisters occurring even after refire).
Are excessive gases generated during glaze fire? Significant amounts of gases can be generated within the glaze itself due to the decomposition of some materials after melting has started (i.e. dolomite, whiting, manganese dioxide, clays, carbonate colorants, etc). Substitute these materials for others that melt cleanly. For example, use frits, supply CaO from wollastonite instead of whiting or dolomite, use cleaner clay materials, or use stains instead of metallic carbonates. If you are using organic additives be aware that some of these can generate considerable gases during decomposition; do tests without them, use an inorganic substitute or find way to disperse them better into the slurry. You might be under estimating the amount of gases that are coming out. Are you holding the top temperature long enough? Perhaps a much longer than expected soak might be necessary (on very thick tile or sculptural pieces, for example, 24 hours might be needed). Could you do a test on a small piece to confirm this? It might also work to adjust the firing schedule to soak, decrease the temperature a little (so the glaze is still pretty fluid), hold it and then cool quickly for the next few hundred degrees to solidify the glaze.
Is the glaze recipe or chemistry the problem? The approaches to dealing with glaze chemistry issues differ in fast fire (e.g. tiles) and slow fire (studio pottery). In slow fire we want glazes that are mobile and can heal imperfections over a long soaking period. In fast fire we want glazes that remain unmelted until after 950C (gases from decomposition can occur up until this temperature) and then melt quickly after this. •
If you are firing fast then you need to use a fast-fire glaze formulation so the glaze does not begin to melt until after body gassing is complete (the whole modern whiteware and tile industries are built on this principle). In fast fire, matte glazes automatically have this property because the formulations to make a crystalline matte and a late melting glaze are the largely same. Glossy glazes, however require extra attention. Reduce zircon or alumina in the glaze melt to give it better flow properties. Or source them from a frit rather than raw materials. Reformulate the glaze to have more fluidity to heal imperfections (i.e. more flux or a lower alumina:silica ratio). Strontium carbonate can help smooth viscous zirconium glazes, small amounts of ZnO and Li2O can do miracles for glaze flow. Adjust the glaze so that it has a lower surface tension so that bubbles break more easily at the surface. 5
Does the recipe contain binders? When do these decompose to create gases (it might be higher than you think)? Boron can induce blistering, especially if its amount is quite high (check limit/target formulas for guidance). The reasons for this phenomenon are not because of gassing (this is demonstrated by the fact that high boron glazes often blister worse on a second firing). Boron is a glass like silica and it wants to form its own glass structures. High boron can thus cause phase separation (areas of discontinuous glass chemistry in the fired glaze, e.g. globules of a sodium borate glass in a calcium silicate glass matrix). Considering the important function of alumina in glass structure, the lack thereof would be an agravating factor in the separation. Phase boundary phenomenon and the differences in surface tension and melt fluidity of the phases could breed blisters. This process likely continues in a second firing fir ing (this accounts for blistering getting worse). Ferro Frit 3134, for example, has no alumina, lots of boron and plenty of CaO/Na2O, glazes high in it make ideal candidates for this phase separation.
Is the system is intolerant intolerant of of gases? gases? Gas release from decomposing materials in the body can continue until 950C. Many glazes begin melting long before this. •
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In the single fire process (i.e. tile) gases have to bubble up through the glaze if it melts too early. The most important factors in producing flawless glaze results in single fire ware are a dense properly pugged or pressed clay matrix that is not too thick, the use of fast-fire glazes specially formulated to melt as late as possible, a firing curve that recognizes the need for a slower rate-of-rise at glaze finish temperatures, and a body made from clean materials and containing a minimum or organics. Use a body of finer particle size so that gases are channelled to many more surface sites of lower volume and thus do not overwhelm the glaze if they have to bubble through it. Minimize techniques that roughen or remove fines from the leather hard or dry clay surface of bodies that contain coarser coars er particles. If necessary apply a fine particled slip to leather le ather hard or dry ware to filter internal body gases into finer bubbles during firing. Apply the glaze in a thinner layer to minimize its ability to contain large bubbles. Use clays not containing large gas generating particles (i.e. pyrites, sulphates) Some fluid glazes (i.e. rutile-blue) tend to be quite sensitive to thick application and fast firing and cooling and bubbling problems with them seem out-of-place. Experiment with firing curves to learn where heat-up or cool-down rates need to be slowed.
Is the glaze firing part part of of the the problem? •
Fire the glaze higher or adjust its formulation so that it melts better and more readily heals surface bubbles. In a slow-firing setting, you may need to soak the kiln longer at maturing temperature to give the glaze a chance to heal itself. In a fast-fire you need to do the opposite, soak only long enough to melt the glaze but not long enough to allow bubbles to grow. Fire the kiln slower during the approach to final temperature or down through transformation temperature. It is not easy to understand why very fluid glazes sometimes do not heal blisters well. Sometimes they are not as fluid as they appear, do flow testing to find out. It may be possible that they need to be cooled slower through the transformation process at which they begin to stiffen and solidify; this can be hundreds of degrees lower than the actual firing temperature if you are not using a fast-fire type glaze. Rather than trial and error firing tests to find a schedule that is sympathetic to your body-glaze combination have your body evaluated for TGA and DTA. Thermal Gravimetric Analysis 6
provides information on body weight loss during the whole firing curve so it tells you when gases are being generated. Differential Thermal Analysis shows where in the firing curve the body behaves endothermically and exothermically. exothermical ly. An expert can use information from these t hese tests and others to tune a firing schedule perfect for your situation. In the USA The Orton Ceramic Foundation can do this type of evaluation.
Is it it being being firing in a gas kiln? • •
Avoid very heavy reduction followed by periods peri ods of oxidation. It is best to start reduction one or two cones higher than the bisque temperature, this period in the glaze kiln can oxidize any remaining potential 'blister producing' volatiles that the bisque did not take care of. Avoid flame impingement on the ware. Make sure that stage one of the glaze fire is truly oxidizing to avoid buildup of internal carbon in the body. Watch the kiln to make sure there is plenty of oxygen present at all times.
An example of of how how calcium calcium carbonate can cause blistering as it it decomposes decomposes during ﬁ ring. ring. This is a cone 6 Frit 3249 Frit 3249 based based transparent transparent (G2867) (G2867) with 15% CaO added added (there (there is no blistering without without the the CaO).
An example of of how how a a carbonate can cause blistering. Carbonates Carbonates produce produce gases during t on. decomposi t on. This glaze (G2415B) contains 10% lithium carbonate.
Glaze Crawling Crawling is where the molten glaze withdraws into 'islands' leaving bare clay patches. The edges of the islands are thickened and smoothly rounded. In moderate cases there are only a few bare patches of clay, in severe cases the glaze forms beads on the clay surface and drips off onto the shelf. The problem is most prevalent in once-fire ware. 7
Is the glaze shrinking too much during drying? If the dried glaze forms cracks (or in serious cases flakes that peel and curl up at the edges) it is a sign that the glaze is shrinking too much. These fault lines provide places fo r the crawling to start. There are a number of possible contributors: •
If very fine-particled materials are present (i.e. zinc, bone ash, light magnesium carbonate) these will contribute to higher sh rinkage during drying. Try using calcined zinc, synthetic bone ash or another source of calcia, talc or dolomite to source magnesia instead of magnesium carbonate. It is normal to see 20% clays (ball clay, kaolin). If significantly more is present try using a less plastic clay (i.e. kaolin instead of ball clay, low plasticity kaolin instead of high plasticity kaolin, or a mix of calcined and raw kaolin). If you are using Gerstley borate, try a boron frit. You may need to do calculations to make these adjustments. Ultimately you need to tune the glaze's clay content to achieve a compromise of good hardness and minimal shrinkage. If a glaze has been ball milled for too long it may shrink excessively (for example, zircon opacified glazes can be ground more finely than tin ones). Highly ground glazes may produce a fluffy lay down. If a slurry has flocculated (due to changes in water, dry material additions like iron oxide, or addition of an acid, epsom salts, calcium chloride, etc) it will require more water to achieve the same flow and will therefore shrink more during drying and require a longer period to dry. Try using distilled water. Always measure the specific gravity to maintain solids content and use deflocculants/flocculants if necessary to thin/thicken the slurry (you can remove water from an existing glaze slurry by pouring some on a plaster batt, then mixing the water-reduced mass back in). Gerstley Borate is plastic and therefore contributes to glaze shrinkage, especially if the recipe already contains kaolin or ball clay. It also tends to gel glazes so they need excessive water. Use boron stains instead.
It is possible to create glaze slurries that gel and flow extremely well using the right kaolin (i.e. EPK) in adequate amounts. This requir es a glaze base whose other materials do not contribute too much Al2O3. We have a separate article on glaze slurry properties that deals with this (see links).
Is the glaze's dry-bond with the ware surface inadequate? •
The mechanism of the bond is simply one of physical contact, the roughness of the ware surface combined with the hardness of the glaze determines i ts ability to 'hang on'. Some surfaces can be very smooth (e.g. slip cast surfaces). To give the glaze better ability to hang on, there should be some clay in the glaze mix to both suspend the slurry and toughen the dried layer. If ware is also excessively powdery to handle this is a signal to incorporate more plastic clay, add a little bentonite, or add a hardener like gum. Add gum to glaze to bond better to bisque. If a glaze is deflocculated it may lack the necessary fluidity to run into tiny surface irregularities in the bisque and establish a firm foothold. Wetting agents are available and can be added to the slurry to improve bond.
Does application technique or handling compromise the fragile glaze-body bond? •
Make sure ware is clean and dust free, even oil from ones skin can affect glaze bond.
If glaze is applied too thickly the forces imposed by its shrinkage will overcome its ability to maintain a bond with the ware surface (especially inside corners or at sudden discontinuities). If a glaze can be applied more thinly, you should do so. Use a fountain glazing machine to do the insides of bowls and containers to achieve a thinner layer. If glaze needs to be applied in a thick layer, you can achieve a lower water content by deflocculating the glaze (i.e. with some sodium silicate or Darvan), however it may then tend to dry very slowly or form drips that crack and peel and instigate crawling. When applying the glaze in the normal layer thickness be careful to prevent drips that form thicker sections that can crack away during drying. It is practical to 'gel' the glaze slightly (i.e. with vinegar, Epsom salts) so that it 'stays put' after dipping or pouring. If a double-layer of glaze needs to be applied be careful that the second does not shrink excessively and pull at the first, compromising its bond with the body. If possible, the upper layer should have less clay and lower shrinkage and should dry quickly. It may be necessary to bisque each layer on before applying the next. Double-layering typical raw art and pottery glazes is difficult, special consideration must be given. If you have successfully done it in the past without any special attention then you may have simply simply been very lucky. When doing double-layer glazing be careful that the second layer is not flocculated (with an associated high water content). This will rewet the first layer and loosen it from the body. Adding iron oxide, for example, to a glaze will often flocculate it and require the addition of much more water to restore the same fluidity. Spraying glaze on in such a way that the glaze-body bond is repeatedly dried and rewetted could produce shrinkage-expansion cycle that compromises a glaze-bisque bond that could otherwise withstand one drying-shrink cycle. Force-drying of the ware can make the glaze visibly crack when it otherwise would not (slower shrinkage associated with slower drying gives it the glaze time to ease body interface tension by micro cracking). Preheating the bisque may cause escaping steam to rupture the bond with the ware. Rough handling of ware can compromise sections of the glaze body bond. Consider pouring a thin glaze slurry into the mold of a just-drained piece (perhaps a minute or two after the mold has been drained) and immediately pouring it out again. This base layer can be fired on in the bisque. It might be enough enough to prevent crawling when the piece is glazed later.
Is the glaze drying too slow? •
If the glaze dries too slowly the most fragile stages of adhesion are extended and cracks in the dried glaze layer can appear . Bubbles in the wet glaze layer can also form during the drying, these become areas of no bond with the underlying body and therefore can instigate crawling during melting. This can occur if ware is very thin, glaze has a high water content, or if ware is already wet when glaze is applied. To speed up drying try preheating the bisque (in a kiln to 150C or more if necessary), doing separate interior and exterior glazing, make ware thicker and better able to absorb water or apply the glaze in a thinner layer.
Is the ware once-fire? •
Once-fired ware is much more prone to crawling because the mechanical glaze-body bond is more difficult to achieve and maintain. If glaze is applied to leather hard ware it must shrink with the body. During the early stages of firing the ware also goes through volume changes and chemical changes that generate gases, these make it difficult for the glaze to hang on. When glaze is applied to leather hard ware you must be able to tune its shrinkage by adjusting the amounts and nature of the clays in the recipe (calculations may be needed). 9
Once-fire ware must not be fired too quickly, especially through the water-smoking period. Make sure ware is absolutely dry before firing. In damp conditions the powdery layer may reabsorb water from the air causing slight expansion and breaking of the adhesion.
Is the problem happening during firing? •
If glaze is applied over stains or oxides that lack flux (e.g. chrome pinks, manganese types, greens, cobalt aluminate) they will act to prevent bonding with the underlying body. Mix under-glaze stains with a flux medium so that over lying glazes can 'wet' them and form a glassy bond. If the glazed ware is put into the kiln wet and therefore dried quickly during the early stages of firing the glaze layer will tend to crack and curl and crawling can occur. If glazed ware is put into a kiln containing heavy damp ware such that early stages of firing occur in very high humidity conditions the glaze could be rewetted and forced through an expansion-shrinkage cycle that could affect its bond with the body. If a glaze contains significant organic materials (i.e. gums, binders) that gas off excessively during firing the glaze-body bond may be affected. Decomposition of materials like whiting can also generate significant amounts of gas within the glaze layer (try switching to wollastonite, it supplies SiO2 also and will allow you to reduce the s ilica content accordingly). Raw zinc oxide is very fine and tends to pull a glaze together during firing, use calcined zinc instead. If the glaze contains significant zircon opacifier, alumina, some stains, magnesium carbonate, the melt may be much 'stiffer' and flow less. This can affect its ability to resist crawling. Watch out for glazes with slightly soluble materials like Gerstley Borate or wood ash. With the former the partly soluble and the soluble portion tends to be the borate which will be absorbed into the bisque during application and then during firing creates a highly fluid layer between the body and the less developed glaze and thereby prevents adhesion of the glaze to the body (use frit to source boron instead). In addition soluble materials tend to flocculate (thicken) the slurry and attempts to thin them result in higher water content and therefore increased shrinkage. If the bisque firing is reduced or not adequately oxidized and excessive gases are generated during certain stages of the glaze firing, these can affect the glaze-body bond. If bisque ware is dense and non-absorbent (fired too high) it may not form a good bond with the glaze. The chemistry of glaze may be such that the surface tension of the melt encourages crawling (e.g. high alumina, high tin, significant chrome/manganese colorants, lack o f fluxes of low surface tension).
Is there a problem with the body? •
If the clay body co ntains soluble salts that come to the surface during drying, these can affect the fired melt's ability to form a glassy bond with the body. Precipitate these salts with a small addition of barium carbonate to t he body (for information on how this works search for Barium Carbonate in the materials section).
An noted above, if the if the body surface is too smooth, the glaze may not be able to adhere properly.
Glaze Crazing The fired glaze exhibits a network of fine cracks. These may be plainly visible after firing or may need enhancement with ink. Crazing may also appear after a period of time or after ware has been exposed to thermal shock. Fired strength (an thus functional ware quality) are directly related to crazing since ware strength is enhanced by having the glaze under slight compression whereas it is severely reduced (up to four times less) when the glaze is under tension. If the underlying clay matrix is porous and soaks up water then safety could be a concern with crazed ware since the cracks could be wide enough to provide a friendly breeding ground for colonies of bacteria. Containers used to store food are a special concern since a small colony in a crack can become a large culture in the food. If you have any doubt whether this is an important issue ask a commercial food service inspector about the subject.
Is the crazing a result result of of mistreatment mistreatment of of ware ware during use? If pieces must survive considerable thermal shock during use, then both ware and glaze need to have a low and linear thermal expansion curve and they must be compatible. This is difficult to achieve in low fire ware because little mullite or other low-expansion silicate minerals develop during firing. If your low fire body contains significant talc, reduce or eliminate it (also adjust glazes to have a lower expansion so they continue to fit the body). If your high fire body develops non-linear expanding cristobalite during firing, find a way to reduce this.
Is crazing a result result of of inappropriate inappropriate choice of of manufacturing manufacturing method or materials? High temperature firing is by far the best for the production of low-expansion ware. Many more minerals are available for both body and glaze mixes and higher temperatures produce low-expansion silicates and aluminates that give tough glaze and body matrixes capable of withstanding forces that might otherwise cause crazing. If ceramic ware is porous it can soak up water that causes the ware to expand, thereby putting tension on the glaze and crazing it.
Is crazing due to a simple thermal expansion mismatch between body and glaze? Fired ceramic expands and contracts as it is heated. If the fired glaze has a significantly higher coefficient of expansion than the body then no amount of careful firing or thin glazing will avoid the inevitable crazing. This is by far the most common cause of crazing and solution strategies are case studies of applying ceramic calculations to solve problems. If even only one piece crazes it is often a sign that all the other ware in that kiln will eventually craze. Such glazes usually need drastic changes since crazing is a visible manifestation of a fit problem that has already greatly reduced ware strength. Lower temperatures are far more sensitive in this respect in that there is a much narrower range within which a glaze and body will be compatible. To improve glaze fit adjust the clay body to give it higher expansion and thereby the greater contraction that compresses glazes to prevent crazing (i.e. increase silica for high temperature bodies, talc at low fire). You can also adjust the glaze to reduce its expansion. There are many ways to do this. For example, if the glaze is melting well and it is not a matte, try increasing the silica. Or try introducing boron at the expense of some of the flux since B 2O3 contributes to both glass development and melting. You can also introduce fluxing oxides of lower expansion at the expense of those with higher expansion in such a way that the fired properties are not changed too much; for example try adding CaO, MgO, or ZnO at the expense of Na 2O and K 2O (crazing is most serious with sodium and potassium glazes, to demonstrate mix nepheline syenite and water and apply as a glaze and fire at high temperature). If your glaze is opaque try using more low-expansion zirconium opacifier or use it instead of tin or titanium. Zirconium opacifiers are also useful in transparent glazes; they have a threshold amount under which they do not normally opacify. Thus it might be possible to add as much as 5% to make the glaze both more durable and reduce its expansion. Consider also the elasticity of the glaze as even relatively well fitted ones can craze if exposed to radical temperature changes. High levels of sodium, potassium and calcium can make the glaze more brittle (the former also increase thermal expansion). Boric oxide is known to improve elasticity. If the body expansion is too low (i.e. ovenware and flameware bodies) it can be very difficult to fit a glaze that has the desired visual characteristics. Lithium can dramatically reduce the thermal
expansion of glazes, but its use requires a lot of testing since its contribution is not linear and it introduces other dynamics that must be considered.
Could the Coloring Oxides in the Glaze be Involved? Generally increased additions of iron and copper oxide to a glaze will reduce crazing (if they are present in adequate amounts; beyond 1 or 2 percent). Cobalt could have a moderate lowering effect, but since so little is typically used in glazes it will not be significant.
Is the crazing a result result an an under fired body? Underfired bodies may contain uncombined alkali or alkaline earths than can react with water and swell the body. You can test this by putting a glazed sample in a pressure cooker for several hours or put a shard into an autoclave to see if crazing appears. Calcium carbonate is added to talc bodies to minimize moisture expansion.
Is the crazing a result result of of sloppy sloppy manufacture? Normally a glaze/body combination with compatible expansion characteristics will withstand considerable firing and usage abuse without displaying signs of crazing. However, in some cases, a glaze that otherwise 'fits' will craze if applied very thick. Also, if the kiln is cooled very quickly or unevenly, especially if ware is thicker, the severe stresses can produce crazing. However remember that a glaze's ability to withstand normal or even quick kiln cooling is an indicator of its ability to resist crazing in normal use.
Is ware crazing days or even months after firing? If you are cooling your kiln very slowly to prevent ware from crazing it is likely the glaze does not fit. While it may be true that slower firing seems to solve the problem, time will bring out the crazing that the kiln did not. In fact if you must slow cool to prevent crazing it is a virtual certainty that your glaze needs to have its thermal expansion reduced. Special Note: Solving crazing and shivering problems while retaining the visual character of a glaze is a classic problem for the application of ceramic chemistry calculations. There is a chapter in the lesson section of the INSIGHT manual on how to deal with this problem, it is a very practical approach.
Example of of crazing crazing in a glaze
Crazing in cone 10 reduct on on celadon glazes, especially on porcelain, is common because they are high in K 2O/Na2O. However this this problem problem can be solved solved by by increasing the SiO2 and and subs subst tu tut ng ng some of the KNaO for lower expansion lower expansion fluxes fluxes like CaO.
Example of a cone 10 transparent that is crazing badly. This is 10% calcium carbonate added to ravenscrag slip. 10% talc does not not craze. craze.
Glaze is Off-Color If your fired glaze is not the expected color here are some questions to ask.
Does the development development of of the the color depend on the chemistry of of the the glaze? In ceramics, color is about chemistry and melt dynamics, colors do not normally 'burn out'. The development of many colors requires that the host glaze's chemistry be sympathetic. For example chrome-tin pinks require glazes with minimum 10% CaO (calcium oxide) and B 2O3 (boric oxide) must be 1/3 or less the CaO content. Certain blues require the presence of BaO (barium oxide). The presence of ZnO (zinc oxide) is hostile to the development of many colors, as is MgO (magnesium oxide). Stain companies know all about this. Their websites and brochures have notations for many of the colors that tell you what chemistry the host needs and what conflicts to watch for. You might even consider phoning their technical staff.
Is there enough color in the glaze? Or too much? Metal oxide colorants or colorant blends darken glaze color as their proportion is increased. But the change is usually not linear and at some point maximum color is achieved and further additions will often begin to produce metallic, crystalline or matte effects (at this point the glaze can be unstable and leach metals into liquids and may even oxidize in air). The saturation point of a color may also be different in different host glazes.
Is the glaze opacity correct? The brightness of color also depends on host glaze opacity. Opaque glazes give flatter and lighter colors because you are only seeing the color on the surface, translucent and transparent bases enable you to see down into the glaze (thus the increased depth and vibrancy color).
Is the glaze developing micro-bubbles? Excessive bubble entrainment in the glaze matrix can alter color considerably. Micro-bubbled transparents become quite cloudy and colors will be subdued, especially if the glaze is transparent and lies over oxide decoration (which might be gassing to create the bubbles).
Is the glaze developing crystals? Does its color depend on the development of crystals? Crystals grow in some glazes during cooling of the kiln. Certain glaze chemistries and (mineralogies of ingredients) encourage crystal growth (i.e. low alumina, high zinc, too much flux). Cooling the kiln slowly during the period when the glaze is freezing promotes crystal growth. Many of the metal oxides freely participate in crystallization and the range of mineral crystal species they can form is amazing. A high-iron fluid glaze, for example, may fire glossy and almost black on quick cooling, but it may turn a muddy yellow on slow cooling (because the surface is covered with micro-crystals of iron).
Is it it a a reactive glaze? The character of a glaze can depend on additives that mottle and variegate the character of the color (i.e. titanium, rutile). Such additives may produce a melt of discontinuous fluidity (rivulets flowing around more viscous areas of the melt). These effects can combine with crystalization and variations in opacity to make stunning surfaces. Alas, such are troublesome. Materials like rutile can be variable and the effects they create are usually fragile. It is easy to predict consistency problems for such mechanisms. Potters can fiddle with reactive glazes, but industry generally stays away from them.
Do the results depend on a fragile melting mechanism? Is vigorous melting (and running) required to develop the color and character? As noted above, such glazes may not only be prone to color problems, but also running and blistering. Glossy rutile-blues are an example. Another thing to remember is that certain raw colors and stains volatilize (vaporize) above certain temperatures.
Kiln atmosphere, Ramp The mechanism of color development in a glaze may depend on kiln atmosphere (i.e. strong reduction, weak reduction, strong oxidation), or on the speed or curve of both the ramp up and down. Your kiln may have variations in the atmosphere or your electric kiln might be firing near reduction because of poor airflow combined with carbon burn-off. burn-off.
Has it it been been put put on on the right right side side of of the the glaze layer? The same metal oxide will develop different colors depending or whether it is painted under or over a glaze. If it is painted under, for example, glaze thickness, bubble population, crystal development and chemical interaction between glaze and color will shape the effect.
Would a stain be better? Achieving and maintaining an exact shade of color can be quite difficult with raw coloring oxides, especially if a blend is being used. For example, many people use cobalt, iron and manganese for black. However color shifts are common with this approach and it is usually not obvious which metal oxide should be increased or reduced to stabilize the color. Stain companies have invested considerable time to develop colors that are reliable and stable (often containing zircon, alumina, silica in addition to the metal oxides). Stains are more expensive, but the stain company assumes a burden that is often difficult for most companies or potters to handle.
Is the glaze the right right thickness? thickness? On the right right body? body? Many glazes develop deep color only if they are applied thickly enough. Others develop the desired effect when they are thin and the underlying body imposes some color. Light colored clay bodies foster the development of bright colors, iron bearing bodies subdue colors (especially when the glaze is thin). Many glazes will develop color of different character on refractory porous bodies compared to vitreous ones.
Water Contaminants It is standard practice to use filtered or distilled water for all glazes in industry. There are so many possible contaminants in water that companies cannot possibly deal with the kind of variation that can c an occur. Water can contain compounds of iron, sulphur, manganese and a host of sulfates and salts (and even particulates like coal dust). You might conclude that the proportions of these impurities is not sufficient to stain a body or glaze, however it is important to remember that they are soluble. That means that during drying, they are all transported to the surface by evaporating water and left concentrated there in a thin layer that will vary according to the thickness of that section of the piece. This is certainly enough to create a yellowish or brownish tinge, for example. In addition, soluble impurities in the water can and probably will affect the rheological (e.g. viscosity, thixotropy) properties of the glaze slurry. s lurry. This in turn can cause c ause thinning and settling settl ing and separation of the glaze glaz e suspension, crystallization of certain materials, thickening, etc. All of these will affect the chemical and physical homogeneity of the glaze laydown and its thickness, these of course, can effect the fired results (which include color).
Conclusion Try taking a cheap microscope and have a really close look at your glaze surface. You might be surprised at now much you learn about why the glaze looks the way it does. Understanding the mechanism of the color and surface will help you understand how to trouble-shoot problems. It does not take rocket science, anyone can note the transparency, micro-bubbles, crystalization, variegation in color and surface (phase differences), etc. And do not shy away from chemistry, in many cases you just need to know if an oxide is present or not and how much is i s there. Search for 'ceramic chemistry' on google, download a free trial of INSIGHT and work through the lessons section of the manual to learn how to enter a recipe and see its formula and analysis.
Glaze Marks or Scratches 'Cutlery Marking' occurs where metal instruments leave marks on glazed functional ware. This happens because the glaze is not smooth, it is abrasing microscopic particles of the metal. However if the marks left by these particles cannot be removed easily this is more than a cosmetic problem. It suggests that they are trapped in surface pores or irregularities (pores are a possible sign of under melting). This is a very different situation than if a sharp hard metal object can scratch the surface. Such a glaze is definitely soft and lacks resistance to wear (and has the potential of harboring bacteria). Even glossy glazes that appear hard can often be scratched easily. In general, the higher a glaze is fired, the better the potential to produce a hard and smooth surface. This is because high fire glazes require less flux and therefore have more silica and alumina. While a capable technician can produce a relatively hard glaze at any temperature range, a less knowledgeable or attentive person can make soft glazes in any range also. The chemistry principles of making a hard glaze are well known. Compare the glaze to a known hard glaze using a simple scratching test.
Use a concrete nail or the sharp corner of a file (these are about 6.5 hardness on the Mohs approximate scale of 1=talc, 2=gypsum, 3=calcite, 4=fluorite, 5=apatite, 6=orthoclase, 7=quartz, 8=topaz, 9=corundum (ruby or sapphire), 10=diamond). Another excellent hardness testing method is to direct a sandblast at the surface at a 45 degree angle. Microsurface optical or electron analysis can then be used to accurately rate abrasion resistance (equipment to do accurate surface plots is now quite common in many industries, search the internet or check with some labs or universities). 17
Is the surface smooth? Can you mark mark the the surface with a fork fork or or knife?
If a glaze surface has angular protrusions then it will be abrasive. This is often the case in glaze that feels silky to the touch. Microscopic sharp edges will cut away minute chunks of metal, possibly holding them in surface voids. •
Does the glaze contain zirconium opacifier?
Zirconium-silicate particles do not enter the melt and they are angular and can protrude from the glaze surface. If you can make a line even with a hard metal object this confirms that the surface obstructions are very hard. •
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You may need to ball mill finer, use a different or less opacifier, use a transparent overglaze, or employ a different base glaze that better envelopes the zircon. Use a microscope to check this. Does the glaze contain calcined alumina? As with zircon, you may need to use a finer size or mill the glaze more. Don't assume your ball mill is doing the job without testing particle size or surface area, a badly configured mill won't grind fine enough enough no matter how long it runs. Surface crystallization can produce an angular irregular abrasive surface. Islands of micro crystallization may be occurring even though the surface looks and feels smooth. Use a microscope. Check the glaze's chemistry to see if it is susceptible to crystal growth during cooling. Typically glazes low in alumina will devitrify (crystallize) during cooling. Increase the alumina to stiffen the melt and reduce the problem. Try cooling the kiln faster if other factors allow. Sometimes a slightly faster cooling cycle will not only reduce the crystals, but change their character to be less problematic. Is something nucleating the crystals (i.e. illmenite, wollastonite, titanium)? If the glaze is a crystalline matte you will need to rationalize it's appearance. Changes made to reduce or eliminate crystallization will affect the visual character. Sometimes smaller changes to glaze make-up to simply reduce devitrificaion are helpful. Or changes to the firing curve can be made to t o grow a finer crystal mesh. Consider switching to a high alumina al umina matte since they have smooth (although not flat) surface. Or you might consider employing a different crystalline mechanism. Are marks difficult to remove? Is the glaze mature? If the glaze is not fired high enough it will simply not melt adequately. The incompletely developed surface will be both abrasive (from undissolved abrasive particles) and lacking in hardness. Try firing the glaze higher to see if it improves. If it does, adjust your body to work at higher temperatures, or adjust glaze chemistry to melt lower. Sometimes only small additions of Li 2O or ZnO, for example, can give much better melts. Some soft glazes are volatile. If fired exactly right they are OK, but variations in the process result in problems with cutlery marking from time to time. Test your glaze at higher and lower temperatures to span variation typical in your kiln. Volatile glazes are typically unbalanced in their chemistry (one oxide will be very high or silica/alumina very low). Alumina is a key to glaze hardness, the more present the harder a glaze will be. Inadequate alumina will contribute to glaze solubility also. While it is true that matte glazes often have high alumina, glossy results are dominant and most glossy glazes can tolerate additional alumina without noticeable visual change. Higher temperature glazes or low to medium ones containing significant boron can often tolerate a higher than expected alumina increase, especially if you source it from a feldspar or frit. Thus you might even consider adding a little 18
boron to lower firing fi ring glazes glaze s so they t hey can accommodate more alumina. Although keep in mind that excessive alumina in a well-melted glaze can crystallize aluminates. Glazes lacking glass former SiO2 are likely to lack hardness. Check typical limits for the temperature range and type of glaze. If your glaze will tolerate more silica then put it in. If not then firing higher or adding some B 2O3 will enable the use of more SiO2. Better yet, use a finer grade of quartz (i.e. 15 micron, however make sure it is does not agglomerate during application). Zircon will improve hardness so use it as the opacifier (however remember that it can contribute to cutlery marking as outlined above). Although zirconium is considered an opacifier, many transparent glazes can tolerate 3-4% of a fine grade without loss of transparency (especially borate glazes). Put as much in as your glaze will tolerate. Source it from a zircon frit if necessary). Magnesia can reduce hardness so reduce it if you can. Magnesia holds thermal expansion down (and therefore tendency to craze) so consider carefully what to replace it with (perhaps one or more of SrO, Li 2O, CaO). If you are firing ware at low temperatures, consider using a fritted base or a commercially mixed powder. While durable ware can be made at lower temperatures, it is much more technically challenging. High borate glazes are often unbalanced and not only lack resistance to marking, but are leachable. Flux saturated reactive art ware or pottery glazes are often lacking in hardness. It is common to see high temperature glazes, for example, that contain 70% or more feldspar and little or no silica or kaolin. While they are visually pleasing, they lack the necessary silica and alumina to form a hard glass.
Glaze Pinholing, Pitting 'Pinholes' are small holes in the fired glaze surface penetrating down to the body below, often into a surface pore or opening. 'Pits' are smaller, they mar the surface but to not penetrate all the way down. Pinholes or pits are often no larger than the head of a pin. During firing bodies typically generate gases associated with the decomposition of organic materials and other minerals, escape of crystal water, etc. If ware is glazed these gases may need to bubble up through the glaze melt, depending on how early it begins to melt. The causes of pinholes can often be similar to those of blistering. Keep in mind also that larger pinholes may actually be crawling (see links to other articles). In the following I may confuse pinholing and pitting or may neglect to mention one or the other, I apologize for this. When pinholes or pits occur there are often more than one contributing factor. Generally a true pinhole is a problem with wit h the body that extends e xtends up into the glaze whereas a pit could be considered a problem with the glaze gla ze or the firing. Still Sti ll most strategies strate gies to eliminate these involve attack on several fronts: • • • • •
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Reducing burn-off by higher bisque or cleaner body (less lignite for example) Distributing body out-gassing by finer grinding Giving the gases more time to escape by slower firing or using a fast-fire glaze that melts later Giving the glaze time to heal by soaking or slower cooling Providing more kiln draft to oxidize and carry away products of decomposition coming from the body or glaze Making the glaze more fluid or altering its surface tension to enable it to better heal itself Selecting glaze materials that decompose to form less gases Being careful to apply a dense even lay down of glaze.
Hobby and small scale producers have the flexibility to do much longer firings and generally must do so for the lack of fast-fire equipment and materials. Industrial producers must find ways to fire quickly, often in an hour or less. Strangely, even though small scale producers fire much slower, they can have just as many problems with pitting and pinholing. Some are using prepared bodies and/or glazes and thus have less flexibility to change things. Keep this factor in mind as you read the material below, the world you are in will determine the validity of the comments being made. If a pitting or pinholing problem has started to happen and it has not occurred before do not assume that there is some new problem. If reading this article makes it clear that there are some things that you have been overlooking, then the success you have had up until now might be accidental. This may be an opportunity to make your your process better and more stable.
Is the body the problem? Are large particles or gas producing materials present?
Do a sieve analysis of the body to determine if large particles are present. Weigh, fire to cone 04, and re-weigh a sample of the coarse particle material to see if it loses significant weight (due to decomposition and associated gas generation). If the particles are volatile (i.e. lignite, sulfur compounds) they will generate high volumes of gases at individual sites, possibly overwhelming the glaze's ability to heal itself there. The most practical solution is to either remove the implicated material from the body batch in favor of a finer particle grade (to distribute gas generation to more sites of less volume) or use a cleaner alternative (by cleaner I mean low-lignite and low-sulphur ball clays). Are the particles melting vigorously?
Use a sieve to isolate some of the coarser particles and fire them to body temperature. Fire to see fi any of them are active melters. Examine pinholes under the microscope so see if a glassy pool exists at its base. If this is the case it is possible that a combination of vigorous melting activity and the resultant creation of a glass chemistry that resists pinhole healing could be occurring. In this case, the offending particles in the body must be eliminated or ground more finely. Troublesome materials in the body?
If you can see 'white spots' and dimples on the glaze surface this suggests that pinholes and imperfections existed but have healed incompletely (these may also suggest that the glaze melt does not flow as well as its glossy surface might suggest, more flux or later melting might be needed). Even fine particled bodies can gas badly, especially if they contain materials like talc, dolomite, or whiting that release high volumes of gas. It is common for some talc to be used as a flux in middle fire bodies (e.g. 2-5%) and there is not really a practical alternative that is as effective and inexpensive. That means that the firing curve must take the decomposition of talc into account slowing down the firing when this occurs. Are there soluble salts in the body?
Does the bare fired clay have a glassy film? Soluble salts within the body can move out to the surface during drying. If these are high in fluxing oxides they can act as a reactive intermediate layer between glaze and body. This can amplify existing pinhole contributors or produce glaze surface irregularities that are akin to pinholing. Add barium carbonate to the body mix to precipitate the solubles within the body or substitute implicated materials in the body batch. 20
Is the body too open?
What is the fired porosity of the body? Does it have an open porous structure resulting from many coarser particles or laminations and air pockets (e.g. (e .g. from poor pugging or sand, grog, shale, unground clay in the batch)? If pores are networked in a body that produces alot of gases on firing then these gases escaping from within are channeled into the network and converge at high volume surface vents (gas volume may be too large for the glaze to heal). Use a finer particled body or perhaps a fine slip between glaze and body. Is the body lacking maturity (not vitrifying)? For example, using a body intended for cone 10 used at cone 6 can actually impede the melt of the glaze since body silica and alumina can rob the glaze of some of its fluxes and therefore impede its ability to smooth out. Is the body bisque surface rough or irregular?
If the body surface is rough (because it contains grog or sand, or the ware has been mechanically trimmed during leather hard stage opening imperfections in the surface), pinholes often occur as the glaze dries on the body. This is a poor lay-down and these raw pinholes may turn out as fired pinholes. In addition, a rough surface exposes pore networks inside the body to larger volume 'exit vents' that produce pinholes in glazes. You can prevent this by using a finer body, smoothing the body surface in the leather hard state after trimming, or by applying a fine-grained slip. You can also wash bisque ware (do not soak it) prior to glazing, this will tend to make the wet glaze application fill surface irregularities rather than compress air into the voids then have it blow back out as a raw pinhole a few seconds later. Do you understand the gas evolution profile of of the the body?
There are many ways to study the characteristics of your body in this regard so that you can adjust your firing to slow down during the high gas evolution phases.
Is there a problem with the glaze recipe? Do you use binders?
Glaze binders have been known to produce serious pinholing and pitting problems. Some decompose at higher temperatures than you might think. Switch to another binder that decomposes at a lower temperature, eliminate it if there is adequate clay to harden the dry glaze layer, or reformulate the glaze to melt later and more quickly using a fast-fire frit. Once again I ask, do you really need a binder, or could bentonite do the same job? job? Are any glaze materials contributing to the problem?
Some glaze materials produce large volumes of gases as they decompose during firing (e.g. whiting, dolomite, talc, coloring carbonates like copper, cobalt). These materials can decompose as late at 1000C, if this is after the glaze has started to melt it means trouble. In serious cases the glaze may not just pit or pinhole, but it may blister, the problem can be reduced or eliminated by employing other sources of the needed oxides (i.e. wollastonite for CaO, frits for MgO, stains or coloring oxides for carbonates). Calculation will be required to make the substitution (so that the formula stays the same).
Do you need a fast -fire glaze
In industry the chemistry of fast-fire glazes is well understood (e.g. they have zinc and lower boron, this produces a later melt). If you are fast firing and are not using a glaze formulated for fast fire then you will almost certainly be having glaze pitting and surface imperfections. Is the glaze melt melt is is too viscous?
If the glaze melt is too thick it will resist flow, impede the passage of gas bubbles, tending to trap them in its matrix. Most often a glaze melt is viscous because it is not melting enough. However even well melting glazes can have a chemistry that makes them resist flow (i.e. high alumina content) or they may contain a material like Zirconium that stiffens the melt because it does not go into solution. Using melt flow testers to gauge the melt mobility of your glaze is a good idea, it is very difficult to detect melt flow changes by simple inspection of a glaze layer. You might think that the melt is fluid enough, but only a melt flow test will say for sure. Increasing flux content to produce a more fluid melt often works well to combat pinholes and pits. Sometimes very small additions of ZnO, SrO, or Li 2O can have a dramatic effect on glaze flow. Sourcing fluxes from frit or using a finer particle size material will improve the melt flow also. Or, you could simply fire higher. Likewise, a decrease in the Al2O3 content will make a glaze more fluid but could add unwanted gloss if you are using a matte. As already noted, if the glaze contains a melt stiffener like zircon, check to see if trading off some of it for tin oxide helps. It is possible that the glaze may be melting too much and blisters associated with glaze boiling may contribute to surface imperfections, however this is more likely to cause blisters or be associated with soluble salts from the body boiling below the glaze. Try adding Al 2O3 to the formulation and note an improvement to confirm this. Is the glaze melt melt and and sealing the surface too early?
Ideally the body should expel its gases before the glaze melts. Modern fast fire frits are specially formulated to melt much later. The modern whiteware industry is build on this premise and glaze formulations have been completely transformed in recent times. Fusion frit 300 is an example. If you are using early melting high boron frits reformulate your glaze to take advantage of fast fire formulations even if you don't fast fire.
Is there a problem with glaze application? If a glaze layer is too thin pinholes may be a product of a simple lack of glaze to heal them. Increasing the glaze thickness may dramatically reduce the pinhole population (of course your glaze must be stable enough not to run if applied thicker and it must fit well enough not to start crazing due to increased tension between it and the body). Keep in mind that what may appear to be pinholes may actually be blistering, this is often evident when increased glaze thickness reduces the pinhole count but reveals the remnants of many healed blister craters (dough nut shaped rounded bumps on the surface when viewed at an angle in the light). It is possible that improper application could contribute to pinhole formation. Such pinholes will usually be larger and possibly not be true pinholes, and they may be accompanied by crawling. To deal with this make sure your glaze slurry does not have too much water, that it lays down into a 22
dense layer on the body and that it bonds well to produce a homogeneous dried surface with minimum airspace. To encourage the production of a good surface during drying make sure ware is clean and dust free and that glaze does not form pinholes during drying (try prewetting the ware slightly if the latter happens). Many companies deflocculate their glazes to get a denser lay down.
Is the glaze contaminated? If pinholes are isolated and few in number it may be possible that a contaminant is getting into the glaze. Pour a sample through a fine screen to check. Do not underestimate the value of ball milling to improve fired glaze surface qualities, many a problem with pinholing and blistering has been solved this way. Many companies ball mill up to 12 hours for best results. Is the ware once-fire?
Once-fired ware is much more prone to crawling and pinholing because the glaze-body bond is more fragile after application and much more gas is generated during firing than for a body that has already be bisquit fired. Thus, while crawling is the most frequent complaint in once-fire glazed ware, pinholes are more common because of the significant out gassing associated with first-fire. If I f you add fast-fire to this mix sometimes it is a wonder that it is even possible to get a nice fired surface on a glaze! Try bisque firing to see if this eliminates the problem. If it does then the gases of firing a raw body are not being passed by your glaze; reassess the whole process to reduce all contributing factors fa ctors as much as possible. Use a fast-fire glaze. See the article on blisters for related information.
Is there problem with the glaze firing? If ware is fired too rapidly the glaze melt may not have a chance to smooth over. If thicker or protected sections of ware have more pinholes this is usually an indication that slower more even firing will improve the surface over the entire piece. Also, if glaze does not pit or pinhole in sections opposite an unglazed surface that it is clear that body gases are the problem and firing needs to be compensated at the right time (of the body needs to use cleaner materials). You need to consider both the needs of the glaze and body to determine where in the firing curve to fire more slowly. In most cases non-fast-fire settings fire slower toward the high end (i.e. an hour per cone at cone 6), soak if possible, and slow the initial cooling phase. If the glaze contains an early melting material (i.e. a high boron low alumina frit) you may need to slow the firing just before the frit begins to fuse to allow as much gas to vent as possible before continuing. Most frit suppliers supply melting or softening temperature information. Modern automatic kiln firing devices make it very easy to control the firing curve. Serious pinholing problems have often been completely eliminated after studying the gas evolution characteristics of body and glaze and employing a firing fir ing curve that slows down at appropriate times. Many engineers in industry specialize in the study of firing curves and the programming of automatic kilns. For an example of a TGA (thermal gravimetric analysis) curve, see Copper Carboante and Copper Oxide on this site). A very important factor to consider also is that modern industrial kilns supply a lot of airflow to the chamber and this carries away products of decomposition. If you are using a kiln without adequate ventilation then there may be not be enough oxygen available at the glaze surface to oxidize and carry away the carbon products of decomposition. Ventilation systems can be added to kilns but that does not mean they are adequate, the air may not be passing over all sections of the ware or at a great enough rate. Some industrial kilns have so much airflow that taller ware can actually blow over if it is 23
not set correctly! If you are doing fast-fire this is critical, a fast fire kiln absolutely must have good air flow. If you are using an electric kiln without airflow, then expect glaze imperfections unless you are firing very slowly. This is especially true if you are firing heavy masses of ware in an electric kiln, that ware may simply not be heating up as fast as your firing schedule might mislead you to believe; heat it up slower. Another factor to consider is that surface pitting can occur even on cool down (e.g. high sulphur bodies). Thus you may need to adjust the kiln firing program to cool more slowly until the glaze stiffens.
Is the problem with the bisque firing? Since most pinholes are the product of escaping gases, it is logical to bisque as high and as long as possible to eliminate the bulk of gases during that firing. The only disadvantage disa dvantage of bisquing higher is that ware will be less absorbent and thus may not be as easy to glaze. Find a good compromise temperature. Also, do not stack ware too tightly in the bisque and make sure there is good airflow in the kiln. It is important that the bisque fire be conducted in an oxidation atmosphere. If not Fe 2O3 within the body may be reduced to FeO, a strong flux. During the glaze firing an active glass will be formed within the body and the associated decomposition processes will generate gases that may cause bloating, blistering, or pinholing.
Is the problem spit -out? If the surface of the glaze is covered with minute broken blisters then the problem is probably spit-out, a condition caused by expulsion of trapped water vapor inside porous ceramics on refire for luster decoration. It is amazing how long it can take to drive off all the water in a fast firing, it may still be coming off past red heat! Make sure the ware you put in a glaze firing kiln is dry.
Pinholes in a glaze at at cone cone 10 reduct on on in an insu ﬃ ciently melted ciently melted glaze. glaze.
Pinholes and t ny blisters ny blisters in a cone 6 glossy glossy white white glaze
Testing for Testing for pinholes pinholes and and dimples dimples is often best best done done using a transparent transparent glaze glaze over over a a large surface and and looking looking at the at the surface in the light.
Right: Fired to cone 6 and soaked 15 minutes. Le f : Fired to cone 6, soaked 15 minutes, then cooled 100 degrees and soaked 45 minutes. Pinholes and dimples are gone, the clay is more mature, and the glaze is glossier and glossier and melted melted better. better.
Glaze Shivering Shivering is the opposite of glaze crazing, the fired glaze is under compression and wants to flake off the body, especially at edges. It it much less common because glazes tend to have a higher thermal expansion than bodies and because they can tolerate being under compression much better than being under tension. Of course, if a glaze is under compression on the inside of a vessel, the body will be under tension and this can cause failure of the piece. When the body-glaze interface is not well developed an overly compressed glaze will be able to release itself much more easily, especially on the edges of contours. This can be the case, not only with low fired ware, but where engobes or slips are being used under the glaze. If the engobe does not contain enough flux to firmly adhere it to the body and develop hardness, it will not be able to bond to the glaze well. It is important to recognize that the appearance of this issue is serious, a few shivered pieces coming out of the kiln could mean that all of them will shiver with time! Shivering is also serious in that razor sharp flakes of glaze could get into food or drink, you must make sure this can never happen. While many band-aid fixes to the issue are recommended, the base problem is a mismatch between the co-efficient of thermal expansion (COE) of body and glaze, nothing will properly fix it except raising the COE of the glaze (or lowering the body COE). Many many glazes have high expansion Na2O and K 2O that are more than they need to be (thus the prevalence of crazing), but here we actually need more of them (so this is an easy fix to do). However in fast-fire settings, Na 2O can cause bubbling (fast fire glazes have lower B2O3, higher ZnO and CaO, lower Na 2O and higher SiO 2, you must work within these guidelines). But you cannot just add soda feldspar or a high soda frit because they also contain other oxides. Using ceramic chemistry software (like Digitalfire Insight), you can figure out how to adjust the recipe so that the only change in the chemistry is an increase in the Na2O/K 2O. In the case of adding feldspar you would calculate how much to reduce the kaolin and silica in the recipe to compensate for the Al 2O3 and SiO2 also sourced from the feldspar. There are instructional videos at Digitalfire.com that demonstrate this. It may also be an idea of check your clay body. Has it changed? For example, if it is less mature its expansion could have increased and the glaze:body bond could have been degraded.
These mugs have experienced experienced very very bad bad shivering shivering (this is an an Albany Albany Slip Slip glaze with 10% lithium carbonate, it it is is known to have a very very low low thermal thermal expansion). expansion).
Example of of serious serious glaze shivering using G1215U low low expansion expansion glaze on a high silica body body at at cone cone 6.
Glaze Slurry is Difficult to Use We often tend to put so much effort into adjusting our glazes to fine-tune fired properties that we tolerate poor application properties. Such glazes are not only frustrating to use, but they often produce poor fired results. When a slurry is i s right it should 'gel' and 'hang on'. You should be able to dip your finger in and pull it out with an even coverage and no drips. In fact, a thixotropic glaze will resist shedding off ware covered with wax emulsion! And it will not settle out hard in the container! Yes, there is no reason to put up with a glaze that drips and drips, cracks on drying, dusts and does not produce an even layer (yes, these problems are all related). In industry, maintaining the 'rheology' (flow properties) of the glaze slurry through material, water quality, seasonal, and personnel changes is often the most difficult challenge a factory faces. Maintenance of the specific gravity in particular is a reference point, 'an anchor' around which all other adjustments hinge (if your glaze is working well, measure its specific gravity now). If your glazes application or drying properties are often problematic or they are always difficult to work with, read on, recipe change(s) might be most appropriate. Be careful about using glaze additives, try all the other approaches first. Gel Your Glaze, Glaze, Adjust Adjust its its Recipe to Gel
The glaze slurry must be thixotropic, it must "gel" so that the mechanism of its initial adherence to the ware is, to a considerable extent, a function of this property rather than absorption of water by porous bisque. While thixotropic behavior can be achieved by using glaze additives, most people lack the experience, knowledge, equipment and circumstances to use them properly. It is thus desirable to avoid additives if possible and try to select a kaolin or ball clay that contributes thixotropic properties. EPK (kaolin) is a good example. If your glaze does not contain adequate kaolin (15-20%) then use ceramic calculations (e.g. Digitalfire INSIGHT) to adjust it so that it does. How is this possible? Because ceramic chemistry sees materials as 'oxide contributors' and it is thus possible to supply a specific chemistry from different mixtures of materials. 27
It is possible to have 20% kaolin in cone 04 glazes if you use low alumina boron frits. At higher temperatures glazes have significantly more Al2O3 and SiO2 and so it is usually easy to achieve a 25% kaolin content (because it contributes Al 2O3 and SiO2). However there are many high temperature glazes that have large percentages of feldspar, sometimes 70%! In such the feldspar is supplying all of the needed Al2O3 and so there is little room for clay in the recipe. These glazes are evil and there is no need for this. The simplest way to fix this problem is use ceramic calculations to reduce the feldspar and supply the alkali oxides from other sources. This will enable you to increase the kaolin to supply the lost Al2O3 (from the feldspar reduction). Ferro Frit 3110 is a good example of a frit that is very similar to a feldspar in chemistry, but it has very low alumina. Consider What What Materials Materials You You Are Are Using
Different clays produce slurries of differing properties. Bentonite-like materials have the ability to gel in water in small amounts, they will help suspend the other particles better than any other material. However bentonites gel the water and hold onto it so well that using any more than 5% will cause glazes dry too slowly and shrink too much. Ball clay is better, 20% of it in a recipe can produce a nice slurry, and many people prefer its characteristics. However ball clay glazes do not necessarily gel well (and ball clay introduces more iron than you might want). If you have a kaolin that suspends well, it is the ideal material. In North American, EPK, for example, produces very nice slurries that suspend well and gel to help them hold on immediately after the dip. Experiment with the kaolins and ball clays available to you to find the best one. Some materials are soluble or partially soluble, this is even the case with some frits (which are of course not intended for glazes). When materials dissolve in the glaze they introduce electrolytes into the water which in turn can affect the viscosity of the glaze. For example, high nepheline syenite glazes can thicken over time and each time you add water to re thin the glaze shrinks and cracks more during drying on the ware. High boron materials are often soluble. Clays, especially raw and native clays, often contain soluble sulfates that can dramatically affect the slurry. These problems can be insidious because these materials often dissolve slowly overtime and thus the rheology will change accordingly. Admittedly, companies with a continuous production line can use slightly soluble materials since their glaze is used quickly and is not stored. Dense Bisque Ware
Bisque ware should not be too high in porosity. Variable porosity means variable thicknesses in the overlying glaze. Porous bisque ware demands that glaze slurries be thin and runny or the application will be too thick. If you are used to bisque firing from cone 010 to 06, go to 04 or higher if you can and use a more gelled glaze. Use Additives Use Additives Only if Absolutely Absolutely Necessary
Misuse of glaze additives is very common because they are not nearly as well understood as other materials. Often they are listed in recipes in which they are not really needed. They should be avoided if possible, because they often have detrimental side effects. Remember, although you might think your glaze needs them, does it really? Only all-fritted glazes with very low kaolin normally need additives in typical traditional ceramic applications (an exception is crystalline glazes that require a low alumina content). Many people use additives that actually worsen the application properties of their glazes. In these cases, often a recipe adjustment to increase clay content (by sourcing the same chemistry from a different set of materials) or a simple bentonite addition would be much better (i.e. gum additions may give a thinner applying, slower drying, 'drippy' glaze). Again, do not use an additive if it is not needed, additives are not a substitute for a good glaze recipe. 28
Individual additives often defy easy classification because they claim to impart suspending, adhesive and flow properties. Thus picking the right one is a matter of discerning the need and using the additive that 'emphasizes' the needed slurry property and gives the fewest side effects (i.e. color change, slow drying rate, biodegradation, film formation). I might add that it is also common to use much more of an additive than is needed, normally completely ruining glaze slurry properties. While gum does form a gel to suspend particles, it is usually more useful in making the slurry 'sticky', and acts as a temporary glue to cement otherwise loosely adhered particles; thus it is referred to as a 'binder', 'hardener', 'adhesive'. Remember that the mechanism of glaze adherence is normally simply contact, it 'hangs on' to irregularities in the surface by virtue of its own strength. Thus a harder dry glaze layer will adhere better. Note also that clays can impart both dry hardness plus suspension and gelling properties to the slurry, whereas gums usually only harden it. Starches usually act as hardeners and may thicken the slurry (therefore suspending it better). Cellulose ethers are used like gum and starch to harden and thicken, they are said to be more consistent and easier to control. Claylike plasticizers (like Veegum) can impart similar claylike properties to a slurry, but remember that the beneficial properties of kaolin, for example, come largely from having alot of it in the recipe. Bentonite clay, likewise, can be beneficial but only in amounts small enough that prevent it from slowing down the drying significantly. So generally clay-like additives have these same limitations. Other additives include wetting agents, foam control agents and sealers. People who know how to use these materials can do things with glaze that others might think impossible. Likewise, those of us who do not know how to use them can create a real mess. Manufacturers usually have instructions so do not buy these materials without good instructions. Flocculants, Deflocculants
Electrolytes change the pH of the suspension and affect the charge of particles (this changing slurry viscosity); a few drops can make a thick slurry very runny and thin (deflocculating it), or make a thin one gel (flocculating it). Thus deflocculants/deflocculants can be used to adjust otherwise variable flow properties. But this cannot be done by a novice. The amounts required are generally extremely small and must be tuned to the specific batch by careful measurements (a few drops too much can literally turn your glaze into jelly or make it settle like a rock). It is amazing how much a small amount of a flocculant, such as calcium chloride, epsom salts, or vinegar can gel a glaze (so it makes sense to test on a small amount before adding it to a whole batch). At the risk of being repetitive, please consider: if you need to use these materials is it possible that adjusting the recipe to increase the clay or remove soluble materials (e.g. boric acid, nepheline syenite, lithium carbonate) would be a better approach. One real downside of these materials is i s they can c an put a glaze batch on a roller rolle r coaster viscosity ride, even with powerful mixing equipment to try stabilize their action. Do you really want that? Mixing
If you are storing your glaze slurries it is very beneficial to have a mixer that can put alot of energy into the slurry to thoroughly wet the surfaces of all particles during primary mixing. After this, final adjustments with water content and possible additives can be done to establish the final rheological properties. When this is done the glaze slurry will be more stable for a longer period of time. Conclusion
My general advice is this: If your glaze is not suspending, hardening, gelling or applying properly, then, if possible reformulate it to have more clay, especially kaolin. If it still needs help then add bentonite (up ( up to 3%). If the glaze still needs nee ds extra help, then use an additive, but beware. If there ther e are ar e 29
still problems, then, heaven forbid, use a flocculant or deflocculant or study up on other more exotic additives!
Powdering, Cracking and Settling Glazes When glazes 'powder' onto your hands and create dust during handling it can be more than just aggravating. The causes of dusting generally contribute to other problems (slurries settle quickly and lay-down varies in thickness). By contrast, when glazes shrink excessively and crack and fall off during drying it is totally frustrating. These two glaze problems are actually closely related, that is, they have a common cause as we shall see. I've seen normally impatient people demonstrate a remarkable tolerance for these situations. After all that glaze recipe 'came from the Gods and we can't mess with it'. Right? On the contrary, this situation is one that can be dealt with logically. It might seem that the chemistry of the glaze could not possibly have anything to do with problems like this. But think again. This is exactly the kind of problem where it really shines. Why? Because many of the solutions involve altering the glaze recipe without changing its overall chemistry. There are lots of examples of doing this in the tutorial videos you can watch at digitalfire.com. First, what causes dusting? The answer is lack of particle binding (a binder is needed to 'glue' the particles together). What about glaze shrinkage and cracking? Too much particle binding and associated shrinkage. Let us consider a little background. Glaze slurries are suspensions of mineral powders (a bunch of microscopic rocks floating in water). What makes them float? The same thing that hardens the glaze powder: Clay (e.g. kaolin, ball clay, bentonite). Clay particles particle s are thin and flat and very small. One gram gra m of clay has an unbelievably large total surface area compared to other minerals used in ceramics. Clay particles have a curious surface chemistry that produces opposite electrical charges on the faces and edges. This results an affinity for water on the faces, this is what produces plasticity in clay bodies, the water glues together yet lubricates movement of the particle faces one against the other. In high water systems, like glaze slurries, suspended clay particles hang on to each other directly (edges against faces) and indirectly (faces against faces) using water as the glue. This is often referred to as 'a house-of-cards arrangement' and it can accommodate large amounts of other mineral particles within the matrix and still exhibit the same properties (to a lesser degree of course). Conceptually the other mineral powders are just 'dead microscopic rocks' along for the ride! The mechanism of the 'bonding' that takes place during dewatering (drying) is not commonly understood. As interparticle water is removed during drying, clay particles move closer together (and pull others with them). The packing results in shrinkage of the entire matrix. Large particle clays (like kaolins) shrink 5% or less from plastic to dry whereas really fine particled clays might shrink 25% or more (shrinkage is more complex than simply particle size, but for our purposes we will not get into that). However mere particle proximity does not in itself create a bond. The chemistry on the surface results in the migration of some chemical species across the boundary. While this creates a very weak bond, the fact that there are billions of particles bonded together in such a fashion creates a clay surface that we perceive to be a fairly hard product. The finer the clay particles the harder it will be. However clay bodies and glazes also contain all kinds of other particles in the mix that do not bond, and as noted, they reduce the number of clay-to-clay bonds (which is bad) but also reduce the drying shrinkage (which is good). Therefore a dried matrix, whether clay body or glaze layer, is a bunch of rock particles held together by billions of weakly bonded clay particles. Now, the question is: What bonds a dry glaze layer to a piece of bisque ware? Well there is no obvious dry adhesion mechanism or boundary chemical reaction. The mechanism of the bond relates 30
to the sticky nature of the wet glaze and the microscopically rough surface of the bisque ware. During hardening the glaze layer loses its wet adhesion and simple mechanical contact is the only microscopic bond. The layer stays on because all the minute surface cracks and pores give it places to hang on to. As you can imagine, this bond is weak at best. Since all glazes shrink during drying, it is not clear how the weak bond with the bisque is able to withstand the pulling forces associated with the shrinkage. Some glazes hardly shrink at all because they lack clay content and that is, of course, why they dust off excessively. However glazes that harden properly during drying always crack, you just do not see the cracks. Micro-cracks must develop to relieve the stress. However when there is too much shrinkage they become visible cracks. With even more stress the glaze cracks to form 'islands' with curled up edges (like a dried up lake bottom). You can see this effect clearly if you watch a slurry of pure kaolin or ball clay dry on a bisque surface. As you can see, we want a glaze to have enough clay so that it forms a hard dry layer but not so clay that it shrinks excessively and cracks off the bisque. There are a number of strategies you can employ if your glaze is powdering on one extreme or shrinking and cracking off on the other. It follows that a powdering glaze needs either more clay or a finer more plastic clay whereas wherea s a glaze that is shrinking s hrinking and cracking needs less clay or less plastic clay. Typically pottery glazes need a minimum of 15% kaolin to harden adequately. Ball clays and bentonites can be used, as well as other clay materials, but kaolin works the best to gel the slurry. It might seem that because ball clay is much more plastic than kaolin, you could use a lot less, but in practice, 15% ball is also needed. The same can be said for bentonite, while 5% bentonite might plasticize a body as much as 15% kaolin, this alone is not enough to harden a glaze well, the bulk is needed. -If your cracking and shrinking glaze employs a relatively plastic kaolin (like #6 Tile or Sapphire), try switching to a less plastic one like EPK or Pioneer. This will not affect glaze chemistry much. A similar switch of one ball clay for another is not as likely to work since pretty well all common ball clays are very plastic. If your glaze is powdering then switch from the less plastic material to a more plastic one. However I must say that if your glaze has a problem, this one change is not likel y going to solve it, more will be needed. -Add some bentonite for powdering glazes (e.g. 3%), remove it from cracking glazes if it is there. Bentonite is super fine and super plastic and therefore dries very hard and shrinks a lot. The small amount of bentonite does not affect the glaze chemistry too much. Remember you can't add bentonite to an existing slurry, it agglomerates into balls that even a propeller mixer won't break up; you need to shake it up with the powder in a new batch to separate the particles). -Add CMC gum to powdering glazes. Like bentonite, it needs to be added during dry mixing. Gum is very sticky and it hardens, using it is a way of 'gluing' a glaze on the ware. Strangely gum also helps suspend, but I have no idea why. Gum burns away so it has no effect on glaze chemistry (although the decomposition can produce glaze faults like blistering and pinholing). One problem: gummed glazes dry slower and drip-drip-drip after glaze dipping pull-out. Experiment with the amount, try 0.5% to start. Add it as a gum solution, not as a powder. Do not use gum unless you need it. -Use kaolin instead of ball clay for cracking glazes (and vice versa for powdering ones). Since kaolin has less silica you will need to use ceramic chemistry to figure out how to compensate for the change in alumina and silica. No big deal? Think again, the amount of SiO 2 and Al2O3 is the primary determining factor for many fired glaze properties and kaolin is the number one source of Al 2O3. Matte glazes are more likely to over-shrink because they have more clay to supply the Al 2O3, but they also have a more critical chemistry balance, the substitution needs to be done correctly. 31
-Check the specific gravity of your glaze (its weight per cc). If it is too low (below 1.4) then it is gelling and there is too much water in the slurry. Perhaps your water supply contains electrolytes that are flocculating the mix, that is, thickening it. Try using distilled water. Also, look out for slightly soluble materials in your glaze, they might be the source of electrolytes. High nepheline syenite glazes can do this. If you have a big container of flocculated glaze there is not much you can do with it except throw it out. You might try adding a small amount of deflocculant like Darvan or Sodium Silicate (e.g. 0.1%) to thin it but then you still have to figure out how to get all the water out and it might be thick again next week! One last note about powdering glazes: Because they generally lack clay they settle out also. Often a layer of water forms at the surface only a minute or two after stirring (generally not easily seen). Although an adequately thick layer may still build up on the piece during dip, on pull-out the water layer may wash glaze off on the last-to-leave sections (usually the rim). The principles mentioned above apply, if you don't want to stir it every minute then the glaze needs reformulation so the chemistry stays the same but more plastic materials are used to source alumina. A classic source of this problem is too much feldspar: Some glazes have 60%, this is way too much. Using ceramic chemistry you can reduce the feldspar drastically and source the lost Al 2O3 from kaolin, the SiO 2 from silica and the alkalies from a frit (e.g. Ferro Frit 3110). Another note about glaze bonding: If you fire your bisque too high it might not be absorbent enough to build up a good layer of glaze on dipping and still dry out quickly. If a glaze needs to dry over a long period on water logged ware, then it will usually crack. Likewise, if your ware has very thin walls then there simply will not be enough porosity in the matrix to pull the water out of the glaze quickly enough (normally a glaze should lose its wet sheen within 30 seconds, many do in less than 10 seconds) to form the mechanical bond with the ware. One solution is to heat the bisque and dip it hot into the glaze using dipping tongs (of course that is not an option if ware is delicate). Alternatively, you could heat and then spray the glaze onto it. Also, remember that smooth porcelain surfaces do not provide as many imperfections for the glaze to hang on to so you need even more control of the drying shrinkage. The moral of the story is that you need to understand the purpose of each material in the glaze (and even the water) to fix problems. And glaze chemistry is pretty hard to avoid, here again it is one of the most valuable tools to solve the problem. You can download Digitalfire Insight for a free two month trial or sign up for Insight-live.com and do your work online. It is alot better to control what your glaze is doing that to switch to another that just has a different set of problems.
An example of of how how a a glaze that that contains contains too much much plastic plastic clay clay can can shrink shrink and and crack crack during during drying. This is raw Alberta Slip applied applied to to non‐bisqued bisqued samples. samples.