ADA Tests for Elastomers

No 19 for Non-Aqueous, Elastomeric Dental Impression Materials

REPORTS OF COUNCILS AND BUREAUS Revised American Dental Association Specification No. 19 far Non-Aqueous, Elastomeric Dental Impression Materials. Council on Dental Material. and Device. The revised American Dental Association Specification No. 19 for Non-Aqueous Elastomeric Dental Impression Materials has been approved by the Council on Dental Materials and Devices of the American Dental Association. The formulation of this and other specifications for dental materials and devices is being carried on through subcommittees of the American National Standards Committee MD-156 for Dental Materials and Devices. The Council on Dental Materials and Devices acts as the administrative sponsor of mat committee, which has representation from all interests in the United States in the standardization of materials, equipment, and instruments in dentistry. The Council has adopted the specifications showing professional recognition of their usefulness in dentistry and has forwarded them to the American National Standards Institute with a recommendation that the specification be approved as American National Standards. Approval of the revised ADA Specification No. 19 as an American National Standard was !ranted by the American National Standards Institute on Oct 4, 1976. The Council acknowledges, with thanks, the work of the subcommittee members who formulated the standard: Mr. Emery Dougherty (chairman), L. D. Caulk Co., Milford, Del; Dr. Ramon Baez, Deerfield, Ill; Dr. Lawrence Gettleman, Harvard University, Boston; Mrs. Carole Groh, Kerr Mfg. Co., Romulus, Mich; Dr. George E. Myers, School of Dentistry, University of Michigan, Ann Arbar, Mich; Mr. Michael A. Pellico, Cadco, Los Angeles; Dr. Brahma D. Sharma, Coe Laboratories, Inc., Chicago; Mr. Kenneth H. Strader, Lactona-Surgident, Richmond, Va; Dr. James A. Stackhouse, Jr., New Jersey College of Medicine and Dentistry, Jersey City, NJ. Non-Aqueous Elastomeric Dental Impression Materials are classified as Type I, II, or according to certain of their' elastic properties and dimensional change after "setting." Each type is further classified according to its apparent viscosity and intended use. Measured from the beginning of the mix, "working time" is defined as the time that the apparent viscosity. increases to a level as defined in the text. "Setting time" is defined as the transitional time at which plastic properties which permit molding and impression taking are lost, and elastic properties permitting removal of the impression material over undercuts are acquired. No such single time occurs for present materials and no direct test is included. Rather, the development of suitable elastic properties at the time recommended by the manufacturer for removal of the impression (3.3.10g) is evaluated by a test “compression set." A requirement for dimensional change has became written so that the component resulting from polymerization shrinkage is included together with changes occurring after removal of the impression from the mouth. ANSI/ADA Specification No. 18 Approved Oct 4. 1878 American National' Standard for Non. Aqueous EIastomeric

Dental Impression Materials.

1. Scope. Definitions and classification 1.1 Scope. This specification is for elastomeric dental impression materials based, for example, on polysulfides, polysiloxanes, polyethers, or other non. aqueous material s capable of reacting to form a rubberlike material which can be used for taking impressions. 1.2 Definitions. For the purpose of this specification, the following definitions apply: 1.2.1 Mixing time.. The elapsed time required to make a uniformly colored and homogeneous mixture of the components. 1.2.2 Working time. The time, measured from the beginning of the mix when the apparent viscosity increases to a level as defined by the procedures of this standard. 1.2.3 Setting time. The transitional time, measured from the beginn1ng of the mix, at which plastic properties that permit molding and impression taking are lost, and elastic properties permitting removal of the impression material over undercuts are acquired. No such single "setting" time occurs for present materials and no direct test is included. Rather, the development of suitable elastic properties at the time recommended by the manufacturer for removal of the impression (3.3.10g) is evaluated by a test for compression set (4.3.5). 1.3 Types. The materials described in this specification shall be classed as type I, II, or III according to certain of their properties after setting. These properties ref1ect (a) the elastic recovery of the materials after a compressive strain (compression set); (b) the resistance. to distortion under a small compressive stress (flow); and (c) the total linear dimensional change of a specimen after a minimum of 24 hours in an unstressed condition (24-hour dimensional change). Limits for these properties are given in Table l. 1.4 Viscosity description. Each typo of material shall be further c1assified according to its viscosity and intended purpose. Limits for applicable test methods are found in Table 2. Table 1 physical property requirements part 1

Table 2 physical property requirements part 2

2. Applicable specifications 2.1 Specifications. The following specification forms a part of this specification: ANSI//American Dental Association Specification No. 25 for Dental Gypsum Products. 3.Requirements

3.1 Components. The impression material shall consist of any number of components which, when used in accordance with the manufacturer's instructions, will produce an elastomer suitable for taking oral impressions of teeth and soft tissues. All components shall be supplied in sufficiently contrasting colors to give a c1ear indication when a uniform mix is achieved. Components shall not show gross separation; and when supplied in a tube. shall be capable of being extruded by hand pressure at room temperature (18-25°C). 3.2 Toxicity. (see 6.1) The manufacturer shall submit a statement that the impression material, used according to the directions supplied with the material, has no known toxic effect on the normal healthy person. 3.3 Physical property requirements. 3.3.1 Requirements. The requirements for mixing time, working time, consistency disk, compression set. detail reproduction, compatibility with gypsum, flow, dimensional change, and strain in compression are as listed in table 1 and 2. 3.3.2 Mixing time. The mixing time determined according to 4.3.2 shall not exceed 1.0 minute and shall be within the manufacturer's stated mixing time (3.3.l0g). 3.3.3 Working time. The working time as determined in accordance with 4.3.3 shall be greater than or equal to the minimum value found in Table 2 for the stated viscosity description and type of material. The test value also shall be equal to or greater than the minimum working time as stated by the manufacturer (3.3.l0g). 33.4 Minimum time for removal from the mouth. The minimum time for removal of the impression from the mouth, measured from the beginning of the mix, shall be stated by the manufacturer in this instructions for use (3.3.l0g). In no' case shall this value be greater than ten minutes. ' 3.3.5 Detail reproduction.. Materials tested according to 4.3.7 shall comply with the requirements for detail reproduction found in Table 2. The respective lines shall be considered satisfactory if they are continuous for the full distance between the crosslines. 3.3.6 Maximum dimensional change. The maximum negative dimensional' change determined according to 4.3.9 for each type of material shall be as listed in Table 1. If a manufacturer provides in this directions for a maximum recommended time for pouring a gypsum cast of the impression (3.3.l0i) which is greater than 24 hours, the value for dimensional change measured at that recommended time shall be as listed in Table 1. 3.3.7 Compatibility with gypsum. (See 6.2) The material shall impart a smooth surface free of gross pitting to an' separate cleanly from a gypsum cast poured against the material in accordance with 4.3.8. The gypsum shall reproduce the appropriate line widths as listed in Table 2. The reproduction shall be considered satisfactory if it is continuous for the fun distance between the cross lines. If the manufacturer provides in this directions a recommended time for pouring a gypsum cast which is greater than ten minutes the impression material also shall meet the requirements of this paragraph when evaluated according to 4.3.8 after the test specimens have been aged at the recommended conditions for the maximum recommended time (3.3.10i). 3.3.8 Compatibility with metallizing bath. The manufacturer's instructions shall specify the type of plating bath to be used, if any (3.3.l0h). If plating is not recommended, or if plating is not applicable, the manufacturer shall make a statement to that effect. If the manufacturer indicates the material may be metal plated, it shall be compatible with either acid copper plating baths or alkaline silver plating baths when tested by 4.3.10. 3.3.9 Deterioration. .Materials that have been stored at 6O+-2 °C for seven days in accordance with 4.3.11 shall comply with the compression set requirements of Table 1 when tested by the method outlined in 4.3.5. After storage, materials shall not show gross separation. 3.3.10 Instructions for use. Instructions for use shall include: a. Chemical nature of system: for example, either silicone, polysulfide, polyether, etc.

b. The type and viscosity description of the material, either very high, high, medium, or low, as defined by this standard. c. Any precautions or special instructions for handling or storing components. Directions for very high viscosity materials shall include a statement that these material s are not suitable for detailed impression-taking procedures when used alone. d. Directions for proportioning the components, by weight (grams) and volume (milliliters). e. The mixing apparatus and/or technique used. f. Ambient mixing conditions. g. The mixing time, minimum working time, and the minimum time for removing the impression from the mouth. h. The type of metallizing bath to be used. if any. i. The maximum recommended time for pouring a gypsum dental stone cast of the impression. j. Recommended storage conditions (temperature and humidity) for the components. k. The maximum shelf life under those storage conditions. 4. Sampling, Inspection, and testing procedures. 4.1 Sampling. A sample consisting of five packages shall be procured for testing for compliance with this standard. 4.2 Visual inspection. Compliance with the requirements of3.1, 3.3.10, and 5 shall be determined by visual inspection. 4.3 Physical tests. 4.3.1 Standard, test condition All physical tests shall be made under uniform atmospheric conditions of 23.0+-1.0°C and 50+-5% relative humidity unless specifically stated otherwise in this specification. Materials and test equipment shall be conditioned in the test room for not less than ten hours before testing. The materials shall be mixed according to the manufacturer's instructions. All test mixes shall be proportioned by weight with a weighing tolerance of +-0.1 g. 4.3.2 Mixing time. Sufficient weight of material to form a minimum volume of 15 mi shall be mixed according to the manufacturer's directions. The time required to obtain a uniform, essentially streakfree mixture shall be less than the manufacturer's stated mixing tif!1e (3.3.10g). 4.3.3 Working time. A sufficient quantity of material shall be mixed according to 4.3.1 and placed in a stainless steel mold (Fig 1, top) mounted on a flat stainless steel plate 3.0 mm thick (Fig 1, bottom). Each cavity shall be 3.0 mm deep and 25.4 mm in diameter. The material shall be leveled with a spatula. Do not remove material below the level of the tray, but a slight excess of up to 1 mm is not significant. Serially numbered disks made of unplasticized polymethyl methacrylate, nylon, or Delrin, 16.0 mm in diameter and 10 mm high, shall be placed on the material at half minute intervals according to the schedule found in Table 3. The required load (Table 4) shall be immediately applied axially for 15 seconds using a suitable apparatus (Fig 1, right) equipped with a dial gauge graduated in 0.02mm or less divisions. The load, shall then be quickly removed but the disk shall be allowed to remain undisturbed. After the minimum time recommended for removal from the mouth (3.3.10g) the height of each disk, in place on the material, shall be read to the nearest 0.02 mm on the dial gauge using a total load of 125 g, and recorded as reading B. The material shall be stripped from the mold and the disks returned to their original relative, positions in the empty mold cavities. The height of each disk shall be read on the dial gauge and recorded as reading A. B-A=H. The first time at which H becomes equal or greater than (a) 0.20 mm or (b) twice its value at 1.5 minutes, whichever is greater, is the working time. The material shall fail if the original value of H at 1.5 minutes is 1.20 mm or more. The average of three determinations shall be reported to the nearest half minute. If individual de ter-

minations vary by more than 0.5 minutes from the average, the test shall be repeated.

Fig 1 top mold for working time test Table 3 schedule for placement of disks in working time procedure

4.3.4 Consistency. A volume of 0.50 ml of a mix of material shall be deposited on a polyethylene- or cellophane-covered glass plate. One and five-tenths minutes after the start of the mix, the 0.5 ml of material shall be covered with another cellophane or polyethylene sheet and a glass plate (weighing 75+-5 g), plus a 500g weight applied axially to the specimen. At 12 minutes after the start of the mix, the load (glass plate plus 500-g weight) shall be removed, and the major and minor diameters of the slumped mass of material shall be measured. The value for the consistency disk shall be the average of three determinations and shall be recorded to the nearest millimeter.

Fig 2 Split mold assembly for strain in compression and compression set tests. Two halves of “B” fit within “A” and are retained by it to form the mold assembly. 4.3.5 Compression set. 4.3.5.1 A split, stainless steel mold, 12.5 mm inside diameter and 20 mm high, shall be retained by a tapered metal ring 35 mm in outside diameter and 16 mm high, as shown in Figure 2. The mold may be lubricated with a silicone grease (high vacuum type) to prevent the impression material from adhering. A sufficient quantity of material s hall be mixed according to 4.3.1 and placed into the mold by means of a spatula. A sheet of polyethylene followed by a square metal plate shall be brought in contact with each end of the mold using a clamp, thus exuding excess material from the mold. 4.3.5.2 Test procedure. At 1.5 minutes from the start of mix, the mold and accompanying plates shall be immersed in a 32+-1°C water bath The mold and plates shall remain in the water bath for the minimum time recommended by the manufacturer for removing the impression material from the mouth (3.3.9g). One minute after removal of the specimen from the water bath, the specimen shall be placed in an instrument consisting essentially of a dial gauge graduated in 0.02 mm or less mounted to a steady base and equipped with a screw, positioned in such a manner that sufficient force can be applied to the specimen to produce the required amount of strain (Fig 3). One minute after removal from the water bath, a plate weighing 2.5+-0.5g shall be placed on top of the specimen and the foot of the dial gauge shall be brought into contact with the plate. The weight of the plate plus the force of the spring in the gauge shall be 50+-5 g. The dial gauge shall be read 30 seconds after the foot of the gauge contacts the plate and the value recorded as reading A. The dial gauge foot shall be lowered 2.4 mm by means of the screw and shall be maintained in this position on the specimen for 30 seconds; then the foot of the dial gauge hall be raised and the specimen allowed to rest under no load (except that of the lightweight plate) for 30 seconds. The dial gauge foot shall again be brought into contact with the place on the specimen for 30 seconds and a second reading shall be taken. This value shall be recorded as reading B. After the readings have been taken, the specimens shall be sectioned lengthwise, and the readings for any of the specimens found to contain gross bubbles or voids shall be discarded. The difference between readings A and B, divided by the original length of the specimen and multiplied by 100, shall be considered to be the percentage of compression set The height of the mold (20.0 mm) shall be considered to be the original length of the specimen. The value for compression ser shall be the average of three determinations and shall be recorded to the nearest 0.1% (A-

B)/ 20x 100)=% compression set.

Fig 3 An apparatus for the determination of compression set. A, screw B dial indicator C specimen D ratcher E specimen mold. 4.3.6 Strain in compression. 4.3.6.1 Preparation of" specimens. Specimens shall be prepared as described in 4.3.5.1 (compression set). 4.3.6.2 Test procedure. Six minutes after removal from the water bath, the specimen shall be placed in a suitable instrument (Fig 4) and shall be subjected to a load calculated to produce a stress of 100 g/cm2. Thirty seconds later, the dial indicator, graduated in 0.02 mm, or less, shall be read. This value shall be reading A. Sixty seconds after application of a stress of 100 g/cm2, an additional load calculated to produce a total stress on the specimen of 1,000 g/cm2 shall be gradually applied during an interval of ten seconds. Thirty seconds after initiation of the stress of 1.000 g/cm2, a reading of the dial indicator shall be taken. This value shall be reading B. The difference between readings A and B, divided by the original length of the specimen and multiplied by 100, shall be recorded as the percentage of strain between the stresses of 100 and 1,000 g/cm2. The height of the mold (20.0 mm) shall be considered to be the original length of the specimen. The value for strain in compression shall be the average of three determinations and shall be recorded to the nearest 0.1%.

Fig 4 An apparatus for determination of strain compression. A specimen loading rack weighing 125g; B specimen; C collar weigh weighing 1, 125g. 4.3.7 Detail reproduction. 4.3.7.1 Equipment:" The apparatus required to prepare the specimens consists of a ruled block (part AA) and mold (part BB) as described in the detailed drawings (Fig 5). The mold shall be lubricated with a quantity of mold release agent such as high vacuum silicone grease. Do not lubricate the ruled block. Clean the ruled block with a solvent before use. If the test material adheres to the block, the block may be lightly dusted with talcum powder and the excess talcum powder blown away as an aid to separation.

Fig 5 Apparatus for detail reproduction, compatibility with gypsum, and dimensional stability. 4.3.7.2 Test procedure. Place the mold (BB) on the test block (AA). The impression material to be tested shall be mixed according to 4.3.1 and placed into the mold by means of a spatula 1.5 minutes after the start of the mix. The mold shall be immediately covered with a thin sheet of polyethylene followed by a rigid, flat, metal plate, and a sufficient force shall be applied to seat the plate firmly against the mold. The assembly shall be immediately transferred to a water bath at 32+-2°C. Three minutes after the minimum time recommended by the manufacturer for removal of the impression from the mouth (3.3.10g), the mold and test block shall be separated. The reproduction shall be satisfactory if the appropriate line is reproduced continuously for the full 25 mm between cross lines in at least two of three specimens prepared. Observation shall be made immediately after separation under low-angle illumination without magnification. 4.3.8 compatibility with gypsum. 4.3.8.1 Test procedure. After the separation of the test specimen formed and tested in 4.3.7.2. the mold BB retains the specimen. Place BB over riser CC and gently press clown to force the specimen up so that the ruled surface of the specimen is level with the top of mold BB. Part DD shall be fitted to BB. Part DD may be lubricated with a quantity of mold release agent such as silicone high vacuum grease 10 facilitate removal of the poured impression. Ten minutes later. a gypsum slurry shall be poured against the impression with gentle vibration so as to completely fill the mold. The gypsum shall be unmodified alpha calcium sulfate hemihydrate. adjusted to give a setting time of 10+-3 minutes by the addition of calcium sulfate dihydrate. The setting time shall be periodically determined in accordance with ADA Specification No. 25 for Dental Gypsum Products. This type of gypsum usually requires about 100 g of powder in 30 mi of water to produce a mix of workable consistency. The gypsum shall be capable of reproducing the 0.020mm line when poured against the ruled test block (Fig 5), and mar be mixed under vacuum and/or colored with a suitable dye for better definition. The poured impression shall be stored in air at 23.0+-2.0°C and 50+-10% relative humidity for 30 minutes. The gypsum cast shall be separated from the impression material and examined under low-angle illumination with up to ten times magnification. The reproduction shall be

considered satisfactory if the required line is reproduced continuously for the full 25 mm between cross lines in at least two of three specimens prepared. 4.3.9 Dimensional change. 4.3.9.1 Equipment. The apparatus used shall consist of a ruled block (part AA). mold (BB). and riser (CC) as shown in Figure 5. The mold shall be lubricated with a quantity of mold release agent such as a high vacuum silicone grease. Do not lubricate the ruled block/Clean the ruled block with a solvent before use. If the test. material adheres to the block. the block may be lightly dusted with talcum powder and the excess talcum powder blown away as an aid to separation. A measuring microscope. having a micrometer stage with an accuracy of 0.005 mm. shall be used. 4.3.9.2 Test procedure. Place the mold (BB) on the test block (AA). The impression material to be tested shall be mixed according to 4.3.1 and placed into the mold by means of a spatula 1.5 minutes after the start of the mix. The mold shall be immediately covered with a thin sheet of polyethylene followed by a rigid, flat ,metal plate. Sufficient force shall be applied to seat the plat firmly against the mold. The assembly shall be immediately transferred to a water bath at 32+-2°C. Three minutes after the minimum time recommended by the manufacturer for removal of the impression material from the mouth (3.3.10g). the assembly shall be removed from the water bath and the mold and test block shall be separated. The impression shall be pressed out of the mold using the riser (CC). The side opposite the reference marks is dusted with talcum powder and transferred impression side up to a flat plate also dusted with talcum powder. The distance between the crosslines cd and c'd', on the ruled metal block (Fig 5). shall be measured to the nearest 0.005 mm and recorded as reading A. Twenty-four hours after the impressions have been prepared. the distance between the crosslines cd and c'd', reproduced in the impression. shall be measured and recorded as reading B. The measurements shall be made with the aid of the edges of the crosslines and shall be performed each time in the same way, measuring the same distance. for example, from the left edge of one line to the leer edge of the other line. Dimensional change shall be calculated as follows: Dimensional change %=(A-B)/Ax 100. The average result of three determinations shall be reported to the nearest 0.05%. 4.3.10 Compatibility with metallizing baths. 4.3.10.1. Metallizing bath preparation. If the manufacturer specifies an acid copper plating bath. the following test solution shall be used: Copper plating bath: copper sulfate. 125 g; sulfuric acid. 37.5 g; phenol, 90%, 5 ml ethyl alcohol, 25 ml; and distilled water. 500 ml. If the manufacturer specifies an alka1ine silver plating bath the following test solution shall be used: silver plating bath; silver cyanide. 28 g; potassium cyanide, 53 g; potassium carbonate. 42 g; and distilled water 950 g. 4.3.10.2 Metallizing bath compatibility test procedure. A specimen prepared in accordance with 4.3.9 shall be suspended in the metallizing bath 30 minutes after mixing. Approximately half the specimen shall be in the solution and half out. After immersion for 16 hours. the disk shall be removed. rinsed in water. blown dry with a gentle air blast. and inspected. Discoloration and slight softening shall be acceptable, but the material shall be rejected if blistering. wrinkling, grass softening, etching, sloughing, or swelling occurs. After the inspection, a mix of unmodified alpha calcium sulfate hemihydrate (4.3.8.1) shall be poured over the test specimen to a height of at tease 6 mm. One half hour later, the impression material shall be stripped from the ser stone. A slight color transfer to the stone shall be acceptable, but the material shall be rejected if a gross transfer of material, adhering to the gypsum surface, occurs. 4.3.11 Deterioration. (shelf life). Components of the material shall be stored in their original containers at 6O+-2°C for seven days. Compression ser tests as described in 4.3.5 shall then be conducted on these materials.

4.3.12 Flow. 4.3.12.1 Preparation of specimens. Specimens shall be prepared as described in 4.3.5 (compression set), except that on removal from the mold, the specimens shall be stored at standard conditions (4.3.1) until 1.0 hour has elapsed from the start of mix. 4.3.12.2 Test procedure. The specimens shall be placed in an instrument consisting essentially of a dial gauge graduated in 0.02 mm, mounted to a steady base. At one hour from the start of mix. a plate weighing 2.5+-0.5 g shall be placed on top of the specimen, and the foot of the dial gauge shall be brought into contact with the plate. The force exerted by the dial gauge, plus the weight of the place, shall be 100 g. The dial gauge shall be read 30 seconds after the foot of the gauge contacts the plate and the value recorded as reading A.A second reading shall be taken 15.0 minutes after the foot of the dial gauge contacts the plate and the value recorded as reading B. The difference between reading A and reading B, divided by the original length of the specimen and multiplied by 100, shall be considered to be the percentage of flow. The height of the mold (20.0 mm) shall be considered to be the original length of the specimen. The value for flow shall be the average of three determinations and shall be recorded to the nearest 0.1 %. 5. Preparation for delivery. 5.1 Packaging. 5.1.1 Immediate containers. The immediate container is the bottle. tube, jar, vial, or other container complete with liner, stopper, coating, and so on that hold and are in direct contact with the components of the impression material. 5.1.1.1 Compatibility with immediate container. The immediate container shall not interact chemically with the components of the impression material. Their quality, purity, and appearance shall not be affected by the immediate container, nor shall the immediate container be altered in any way by the contents. 5.1.1.2 Cleanliness.: The immediate container and contents shall be.1ree from dirt. foreign matter, or other contaminates. 5.1.1.3 Leakage. The immediate container with closures in place shall not leak or lose contents during ordinary shipping, handling, and storage. Tubes shall not rupture during extrusion of the components. 5.1.2 Unit package. The Unit package consists of at least one immediate container of each component necessary to the production of the elastometric impression material. It shall protect the immediate containers from damage and breakage during shipment. If the unit package contains material of more than one type or viscosity description, each material shall separately comply with the applicable requirements of this standard. Instructions for use (3.3.9) shall be found in each unit package. 5.1.2.1 Proportioning. The unit package shall include suitable means for dispensing and proportioning the components. 5.2 Marking. 5.2.1 Lot numbers. Each immediate and unit container of material shall be marked with a serial number or a combination of letters and numbers that refer to the manufacturer's records for that particular lot or batch. 5.2.2 Date of manufacture. The date of manufacture (year and month) shall be given on each immediate and unit container of material as a separate item or as a par! of the lot number (5.2.1). 5.2.3 Net volume. The minimum net volume in milliliters shall be given in legible type on the immediate container. 5.2.4 Mixed volume. The minimum volume after mixing the components in the proper ratio according to the manufacturer's instructions (3.3.I0d) shall be given in legible type on each unit package of the material. Excess of one component after that required to produce a product in the stated ratio (3.3.10d) shall not be included in the statement of minimum volume after mixing. 5.2.5 Type and viscosity description. The type (1.3), viscosity description (1.4), and generic name

(polysulfide, silicone, polysiloxane, polyether, and so on) of the impression material shall be stated on the unit package and on the base material's immediate container. 5.2.6 Storage conditions. Each unit and exterior package shall clearly state the recommended range. of temperatures and other necessary information for successful storage of the product. 6. Notes (These notes are for information only and are not requirements of the specification.) 6.1 Toxicity. The American Dental Association has published tentative test methods for toxicity and biologic suitability. Paragraphs 3.2.4.1 and 3.2.4.2 of “ Recommended standard practices for biological evaluation of dental materials," (JADA 84:382 Feb 1972) have application with respect to elastomeric impression material. It is highly recommended that each manufacturer certifying his product to comply ;with this specification conduct these tests and provide the resulting data with pertinent comment to the American Dental Association. After a period of time has confirmed the practicability of these test methods, it is envisioned they will become pan of this specification or its successor. 6.2 liberation of gases. Certain impression material formulations have been shown to cause pitting in gypsum casts of impressions (Reisbick, M.H. "The accuracy of highly f1lled elastomeric impression materials." J Prosthet Dent 33:67, 1975) as a result of evolved gas during the polymerization reaction. Paragraph 3.3.6 intends to deny such products approval under this standard. ' 6.3 Source of unmodified alpha calcium sulfate hemihydrate A quantity of unmodified alpha calcium sulfate hemihydrate is available in the United States from the American Dental Association, 211 E Chicago Ave, Chicago, 60611. A nominal handling fee and shipping costs are to be paid by the certifying manufacturer 6.4 Information on equipment and testing procedures. Information on equipment and testing procedures may be obtained from the Council on Dental Materials and Devices of the American Dental Association. 211 E Chicago Ave, Chicago, 60611. 6..5 Ruled test block and auxiliary equipment. A list of commercial sources that manufacture or have expressed interest in manufacturing this equipment is available from the American Dental Association, 211 E Chicago Ave, Chicago, 60611. 6.6 Conversion factors: 25.4 mm = 1 inch (°C x 1.8) + 32= °F 453.59 g = 1 pound