- Email: [email protected]
Inlay Casting*
1760
T h e Journal of the American D ental Association
dentists can possibly give. T h is fact being recognized, there is before the dental societies throughout the country an addi-
tional task— th at of sponsoring a movement whereby an adequate dental staff shall be provided each institution.
INLAY CASTING* By HERBERT D. COY, D. D. S., Hamburg, Iowa A C C O R D I N G to authorities, inlays T h e Research Commission of the of glass and porcelain were made American D ental Association therefore more than 100 years ago. made an appropriation, arranging with In his “ H istory of D ental Surgery,” the N ational Bureau of Standards at the late C. R. E. Koch states that gold W ashington, D . C., for an impartial was used as a filling for carious teeth as survey and investigation, hoping to early as 1450. standardize dental materials and to es T h e first gold inlay is supposed to tablish definite specifications. have been made by a gold foil operator T h e furnishing of scientific data on the who, when he dislodged a filling in fin physical characteristics of materials ishing and polishing it, cemented it into rather than the working out of methods place, and thus gave us the gold inlay. of procedure or some certain technic is From the ill luck of this gold foil opera the aim of the Bureau of Standards. tor, as we are led to believe, through many M any of the dental materials on the variations, extensions and experiments, market were of very low quality, and the we have produced a scientific technic for dentist could determine their usefulness the casting of a gold inlay. only by experimenting on his patients; Research into the various casting which resulted in injury to many teeth technics prevalent today has revealed to and many mouths. A great expenditure of money has the careful reader of dental literature the fact th at many, or shall I say all ? of our been made in the past by the dental pro current casting technics have been based fession in vain attempts to solve the prob more largely upon personal opinion than lems of dental casting, although scientific on a sound foundation of scientific facts. study of methods or materials was prac M any conflicting opinions and state tically unknown. W e have purchased ments were formerly in existence. T h e equipment and many special devices on knowledge available regarding the phys the recommendation of this clinician or ical characteristics of the materials used that, paying a generous fee for an inten in dental casting was very indefinite, sive personal demonstration, only to these conditions making it necessary for learn th at he has not based his operation some agency to search for the truth. on scientific facts, the results obtained leaving much to be desired. *Read before the Section on Operative Den T h is accounts for the present coopera tistry at the M idw inter Clinic of the Chicago Dental Society, Jan. 21, 1932. tion of a number of widely scattered den Jour. A. D. A., October, 1932
C oy— In la y Castings
tists w ith the Bureau. A comparison of the various casting methods and materials can be made im partially by these workers, who are not affiliated w ith any com mercial interests. Although a few of the manufacturers have employed trained workers, who have improved their employers’ products, personal endorsement is still being stressed by salesmen, rather than physical properties of the materials, and technics are still being urged on us which have no scientific foundation. T h e cooperative research program on dental inlay materials and technics was undertaken by the Bureau of Standards and a cooperative committee of the A m er ican D ental Association, w ith several objects in view when they set to work in April, 1929. T h e combined groups over the country have about 200 members at the present time, who make up the cooperative com mittee. T h e N ational Bureau of Standards is an im partial body. W e all know some thing of the incomparable facilities at its command for the carrying on of its work, from a machine th at w ill measure a mil lionth of an inch to another as large as a house. W hile we as taxpayers are sup porting it and while commercial interests generally have made use of its many facilities, the dental profession has been very slow to take advantage of its m ar velous and unbounded resources. I t has nothing to sell and nothing to gain be cause of its activities. Its sole object is to establish the facts as it finds them. I t has been very free to commend the parts of the various casting procedures which it has been able to verify, and equally ready to condemn that which it has proved to be false; an attitude caus ing enmity in certain quarters. T h e results of this research have been
1761
published from time to time in T h e J o u r n a l .1
I would urge each of you to give these articles careful consideration. A fifth specification to cover dental inlay golds was prepared by the Bureau and presented to the profession at the annual meeting of the American D ental Association at Memphis. T h e specifications for investments are covered by the following requirements as given in an article w ritten by N . O. T aylor, research associate for the Am er ican D ental Association and the National Bureau of Standards of W ashington, D. C .2 A satisfactory investment should have su f ficient thermal expansion to compensate for a considerable portion of the casting shrink age of inlay golds. It should not contract dur ing the setting period and should not show marked irregularities in volume change in heating. 1. T h e setting expansion shall be 0.05 per cent or over, at the end of twenty-four hours. 2. T he thermal expansion shall be 0.7 per cent or over, when heated from room tempera ture to 1300 F. A thermal expansion curve or. sufficient data shall be supplied to enable the purchaser to make proper use of the mate rial. 3. During the preceding test, the specimen shall not, at any temperature above 400 F., show a length shorter than the original length at room temperature, and shall not at any higher temperature show a shrinkage or de crease in length of more than 0.15 per cent of the maximum length at any lower tempera ture. 4. T he compressive strength shall not be less than 300 pounds per square inch. 1. T aylor, N. O.: Specifications for Dental Am algam Alloys, J. A. D . A., 17:112 (Jan.) 1930. Taylor, N. O .: Paffenbarger, G. C., and Sweeney, W . T . : Dental Inlay Casting In vestments, J. A. D. A., 17:2266 (Dec.) 1930: Specification for Dental Impression Com pound, ibid., 18:53 (Jan.) 1931; Specification of Inlay Casting W ax, ibid., 18:40 (Jan.) 1931. 2. Taylor, N. O .: Standards for Dental M a terials, Iow a D. Bull., 17:94 (A u g.) 1931.
1762
T he Journal of the American D ental Association
5. T h e time of set shall not be less than five nor more than thirty minutes and shall be indicated on the package. T h e time of set shall not vary more than 20 per cent from the manufacturers’ stated value. Rough castings may result from the use of coarse investments. Some manufacturers ad vocate a fine inner investm ent and a coarse outer one, but laboratory tests gave no basis for this recommendation and the requirements are based upon a single type of investment whose fineness shall be as follows. 6. Eighty-five per cent shall pass a No. 200 sieve, 95 per cent shall pass No. 100 sieve and 100 per cent shall pass a No. 30 sieve. The general requirements for investment are as follow s: 1. T he material shall not crack in heating. 2. The material shall not contaminate the alloy cast into it and shall not cause pitting, roughness of surfaces or voids in the alloy. 3. W hen heated, the material shall not give off poisonous or offensive odors.
2. Softening point. T he softening point of the materials shall be between 100 and 108 F. 3. W orking range. T he material shall soften sharply and a working plasticity shall be obtained at or below 112 F. 4. Therm al expansion. A curve or nu merical data shall be supplied with the mate rial which w ill enable the purchaser to make the proper use of the material in technic re quiring w ax expansion. T hese requirements, if follow ed, w ill serve to eliminate the large number of excessively soft w axes on the market as w ell as a few which are too hard. T he soft waxes are the more dangerous variety, since an inlay technic can give results no better than the original w ax pattern. If it is made of soft material and is warped or distorted in withdraw al from the cavity, poorly fitting castings w ill be the inevitable result.
T h e group of workers with whom I have been associated is under the able di rection of Charles E. W oodbury, Council TEN TA TIV E SPEC IFICA TIO N S FOR DENTAL Bluffs, Iowa. T h e findings and the IN L A Y WAX technic which I shall present to you are 1. T h é waxes should soften without be the combined results of our cooperative coming flaky or laminated. efforts. 2. T hey should be sufficiently plastic at T h e method by which we have obtained temperatures slightly above mouth tempera the best results gives consideration to both ture to permit forcing them into all the details the therm al expansion of waxes and the of the cavity walls. setting and therm al expansion of invest 3. T hey should harden sufficiently at mouth temperature to permit w ithdraw al from the ments. cavity without distortion. I t has been proved by investigation, 4. T hey should carve without chipping and both by the Bureau and other investiga flaking. tors, that the shrinkage of gold from the 5. T he color should be such that it facili molten state to the point of crystallization tates the carving of the pattern in the mouth is approximately 1.25 per cent per volume through contrast with the hard and soft tissues of the gold. of the mouth. Extensive experiments have proved 6. T he thermal expansion characteristics should be known to insure correct use of the that the shrinkage of gold is practically materials in any technic requiring w ax ex constant when the tem perature varies pansion. from the lowest degree at which it can be Flow tests w ere developed for the determ cast to the maximum tem perature obtain ination of rigidity and working range. T her mal expansion determinations were specified able w ith a good blowpipe, using hy to provide other necessary data. These de drogen and compressed a ir.. tail requirements are: T h e properties of wax patterns are 1. Flow at mouth temperature. Samples such that it seems far more desirable to 1 cm. in diameter shall not shorten more than obtain the m ajor portion of the compen 1 per cent in length w hen subjected to a load sation by the expansion of the investment of 2,000 gm. for ten minutes at 99 F.
Coy— Inlay Castings rather than to risk distortion of the thin and plastic wax pattern. T h e ideal investment would be one in which the setting and therm al expansion combined would equalize the deficiency caused by the shrinkage of gold. Experiments are now being conducted looking tow ard the perfection of such a product. Casting w ill then be greatly simplified and an inlay that fits w ill be come a certainty rather than an accident. A t the present time, there is no invest ment th at has enough expansion to com pensate or offset the shrinkage of gold in all cases. T h is condition makes it neces sary to utilize the therm al expansion of wax to some extent, although this should be confined to a minimum. T h e investments that have the greatest amount of therm al expansion have been found to be those of high silica and low plastic content. Silica is an expanding material ; plaster, a contracting material. A ll the investments listed by the Bureau of Standards as those most nearly meeting their requirements have a high silica and low plaster content. T hey all have a progressive expansion. By this, I mean that the higher the tem perature to which they are heated, the greater the expansion, reaching the maximum at red heat. If an investment is to be desirable for use in a w ax expansion method, it must also be slow setting, so that the heat used in expanding may have time to penetrate to the pattern before the initial set of the material. Shrinking investments are those of a high plaster content. T h e use of these w ith w ax expansion seems al ways to result in undersize castings. No scientific evidence has ever been presented that would w arran t the use of this type of investment. N either does there seem to be any advantage to be derived from the use of a double investment.
1763
N othing original is claimed for the technic herewith presented. Scientific facts taken from the above-mentioned articles, furnished by the Bureau of Standards and verified by many castings made independently in the laboratories of the cooperative committee, have fur nished a basis for the method. Preparation of all cavities, where an inlay restoration is indicated, should be made by the method as outlined by G. V. Black in his book on operative dentistry. I t should be so formed that all the walls will be slightly diverging and free from undercuts, as from this type of cavity the wax pattern may be readily removed. T h e walls must be smoothed w ith stones and chisels and the cavosurface angles ac curately beveled and finished. T h e preparation of a well-finished cavity is absolutely essential in the pro duction of an ideal wax pattern. T his preparation is made easier by the applica tion of the rubber dam. I t is my opinion that the making of a true wax pattern is the most difficult part in the procedure of making an inlay. Since wax is a plastic substance and changes form whenever subjected to change in temperature, it is essential that whenever wax is placed in a cavity in the mouth, it should not be chilled by the use of cold w ater or air, but it should be held under pressure until sufficient time has elapsed for it to assume mouth tem perature. T h e pattern should be carved to form. W hen the carving is completed, the proxi mal surfaces of the pattern should be pol ished w ith linen tape. A solvent such as eucalyptus or oil of cajuput is not to be used. T h e completed pattern should be removed from the cavity whenever pos sible by attaching the sprue and then gently applying force rather than by the
1764
T he Journal o f the American D ental Association
use of an explorer. I t should then be washed w ith a solution of equal parts of hydrogen peroxid and green soap, rinsed w ith room tem perature w ater and dried w ith a slight application of air from a syringe. T h e pattern is now ready for investing. Any of the investments which meet the tentative specifications of the Bureau as previously outlined may be choosen.3 T h e greater variation w ill be found in the amount of w ater necessary in the mix, and in the setting time. As a m atter of convenience, I shall choose Ransom and Randolph’s green in vestment. T h e inlay ring to be used should be lined w ith a thin layer of sheet asbestos and the ring sealed to a small metal slab w ith beeswax. T h e lining should be moistened w ith w ater, so that the w ater content of the investment will not be changed when poured into the ring. T h e investment w ill then be mixed, 16 gm. of w ater to 50 gm. of investment being used, the amounts being doubled if necessary to fill the ring. I f w ax expansion is necessary, the w ater used in the mix should be warm enough that the finished mix of invest ment is approximately the same tempera ture as the expansion bath. I t should first be spatulated by hand, then by the use of a mechanical device such as the W hip-m ix or K err for 100 revolu tions of the handle, or about 30 seconds. I t is then vigorously vibrated on the lathe to remove any remaining air bubbles. T h e ring is then vibrated full of investment, more of which is formed on the pattern by a camel’s hair brush, care being taken not to trap any air. T h e pattern is now gently vibrated into the investmentfilled ring until the sprue form er base is 3. Footnote 1, second reference.
seated. A clamp is now adjusted to hold the base and the ring securely together. Plaster bowl and spatula should be dry and clean when the mix is begun. T h e investment is placed in the w ater bath for tw enty minutes, or until set. A fter removal of the metal base, the sprue former base and the sprue pin, the invested pattern is placed in a furnace for fifteen minutes at a heat of approximately 500 F. T h e heat is then raised to 900 F. for fifteen minutes, then to red heat, at which point the casting should be made, the investment reaching its maximum ex pansion at that heat. T h e type of casting machine to be used makes little difference and is not con sidered a variable. T h e amount of w ater used in the mix may be varied to suit the convenience of the operator, but it should be continually kept in mind th at the smaller the amount of w ater used, the greater the therm al ex pansion of the investment and the smooth er the finished casting. Rough castings are usually caused by one of the follow ing: Excessively vibrating. Use of an unclean pattern. A ir being incorporated into the invest ment. T oo high a proportion of w ater in the mix. Burning out too rapidly. T h e various individuals and the groups of the cooperating committee presented clinics at the M emphis meeting of the American D ental Association under the direction and w ith the assistance of the Bureau of Standards and perhaps the many ideas and technics may be harmon ized into one ideal method of casting and adopted by the profession as a stan dardized technic.
T h e Journal of the American D ental Association
dentists can possibly give. T h is fact being recognized, there is before the dental societies throughout the country an addi-
tional task— th at of sponsoring a movement whereby an adequate dental staff shall be provided each institution.
INLAY CASTING* By HERBERT D. COY, D. D. S., Hamburg, Iowa A C C O R D I N G to authorities, inlays T h e Research Commission of the of glass and porcelain were made American D ental Association therefore more than 100 years ago. made an appropriation, arranging with In his “ H istory of D ental Surgery,” the N ational Bureau of Standards at the late C. R. E. Koch states that gold W ashington, D . C., for an impartial was used as a filling for carious teeth as survey and investigation, hoping to early as 1450. standardize dental materials and to es T h e first gold inlay is supposed to tablish definite specifications. have been made by a gold foil operator T h e furnishing of scientific data on the who, when he dislodged a filling in fin physical characteristics of materials ishing and polishing it, cemented it into rather than the working out of methods place, and thus gave us the gold inlay. of procedure or some certain technic is From the ill luck of this gold foil opera the aim of the Bureau of Standards. tor, as we are led to believe, through many M any of the dental materials on the variations, extensions and experiments, market were of very low quality, and the we have produced a scientific technic for dentist could determine their usefulness the casting of a gold inlay. only by experimenting on his patients; Research into the various casting which resulted in injury to many teeth technics prevalent today has revealed to and many mouths. A great expenditure of money has the careful reader of dental literature the fact th at many, or shall I say all ? of our been made in the past by the dental pro current casting technics have been based fession in vain attempts to solve the prob more largely upon personal opinion than lems of dental casting, although scientific on a sound foundation of scientific facts. study of methods or materials was prac M any conflicting opinions and state tically unknown. W e have purchased ments were formerly in existence. T h e equipment and many special devices on knowledge available regarding the phys the recommendation of this clinician or ical characteristics of the materials used that, paying a generous fee for an inten in dental casting was very indefinite, sive personal demonstration, only to these conditions making it necessary for learn th at he has not based his operation some agency to search for the truth. on scientific facts, the results obtained leaving much to be desired. *Read before the Section on Operative Den T h is accounts for the present coopera tistry at the M idw inter Clinic of the Chicago Dental Society, Jan. 21, 1932. tion of a number of widely scattered den Jour. A. D. A., October, 1932
C oy— In la y Castings
tists w ith the Bureau. A comparison of the various casting methods and materials can be made im partially by these workers, who are not affiliated w ith any com mercial interests. Although a few of the manufacturers have employed trained workers, who have improved their employers’ products, personal endorsement is still being stressed by salesmen, rather than physical properties of the materials, and technics are still being urged on us which have no scientific foundation. T h e cooperative research program on dental inlay materials and technics was undertaken by the Bureau of Standards and a cooperative committee of the A m er ican D ental Association, w ith several objects in view when they set to work in April, 1929. T h e combined groups over the country have about 200 members at the present time, who make up the cooperative com mittee. T h e N ational Bureau of Standards is an im partial body. W e all know some thing of the incomparable facilities at its command for the carrying on of its work, from a machine th at w ill measure a mil lionth of an inch to another as large as a house. W hile we as taxpayers are sup porting it and while commercial interests generally have made use of its many facilities, the dental profession has been very slow to take advantage of its m ar velous and unbounded resources. I t has nothing to sell and nothing to gain be cause of its activities. Its sole object is to establish the facts as it finds them. I t has been very free to commend the parts of the various casting procedures which it has been able to verify, and equally ready to condemn that which it has proved to be false; an attitude caus ing enmity in certain quarters. T h e results of this research have been
1761
published from time to time in T h e J o u r n a l .1
I would urge each of you to give these articles careful consideration. A fifth specification to cover dental inlay golds was prepared by the Bureau and presented to the profession at the annual meeting of the American D ental Association at Memphis. T h e specifications for investments are covered by the following requirements as given in an article w ritten by N . O. T aylor, research associate for the Am er ican D ental Association and the National Bureau of Standards of W ashington, D. C .2 A satisfactory investment should have su f ficient thermal expansion to compensate for a considerable portion of the casting shrink age of inlay golds. It should not contract dur ing the setting period and should not show marked irregularities in volume change in heating. 1. T h e setting expansion shall be 0.05 per cent or over, at the end of twenty-four hours. 2. T he thermal expansion shall be 0.7 per cent or over, when heated from room tempera ture to 1300 F. A thermal expansion curve or. sufficient data shall be supplied to enable the purchaser to make proper use of the mate rial. 3. During the preceding test, the specimen shall not, at any temperature above 400 F., show a length shorter than the original length at room temperature, and shall not at any higher temperature show a shrinkage or de crease in length of more than 0.15 per cent of the maximum length at any lower tempera ture. 4. T he compressive strength shall not be less than 300 pounds per square inch. 1. T aylor, N. O.: Specifications for Dental Am algam Alloys, J. A. D . A., 17:112 (Jan.) 1930. Taylor, N. O .: Paffenbarger, G. C., and Sweeney, W . T . : Dental Inlay Casting In vestments, J. A. D. A., 17:2266 (Dec.) 1930: Specification for Dental Impression Com pound, ibid., 18:53 (Jan.) 1931; Specification of Inlay Casting W ax, ibid., 18:40 (Jan.) 1931. 2. Taylor, N. O .: Standards for Dental M a terials, Iow a D. Bull., 17:94 (A u g.) 1931.
1762
T he Journal of the American D ental Association
5. T h e time of set shall not be less than five nor more than thirty minutes and shall be indicated on the package. T h e time of set shall not vary more than 20 per cent from the manufacturers’ stated value. Rough castings may result from the use of coarse investments. Some manufacturers ad vocate a fine inner investm ent and a coarse outer one, but laboratory tests gave no basis for this recommendation and the requirements are based upon a single type of investment whose fineness shall be as follows. 6. Eighty-five per cent shall pass a No. 200 sieve, 95 per cent shall pass No. 100 sieve and 100 per cent shall pass a No. 30 sieve. The general requirements for investment are as follow s: 1. T he material shall not crack in heating. 2. The material shall not contaminate the alloy cast into it and shall not cause pitting, roughness of surfaces or voids in the alloy. 3. W hen heated, the material shall not give off poisonous or offensive odors.
2. Softening point. T he softening point of the materials shall be between 100 and 108 F. 3. W orking range. T he material shall soften sharply and a working plasticity shall be obtained at or below 112 F. 4. Therm al expansion. A curve or nu merical data shall be supplied with the mate rial which w ill enable the purchaser to make the proper use of the material in technic re quiring w ax expansion. T hese requirements, if follow ed, w ill serve to eliminate the large number of excessively soft w axes on the market as w ell as a few which are too hard. T he soft waxes are the more dangerous variety, since an inlay technic can give results no better than the original w ax pattern. If it is made of soft material and is warped or distorted in withdraw al from the cavity, poorly fitting castings w ill be the inevitable result.
T h e group of workers with whom I have been associated is under the able di rection of Charles E. W oodbury, Council TEN TA TIV E SPEC IFICA TIO N S FOR DENTAL Bluffs, Iowa. T h e findings and the IN L A Y WAX technic which I shall present to you are 1. T h é waxes should soften without be the combined results of our cooperative coming flaky or laminated. efforts. 2. T hey should be sufficiently plastic at T h e method by which we have obtained temperatures slightly above mouth tempera the best results gives consideration to both ture to permit forcing them into all the details the therm al expansion of waxes and the of the cavity walls. setting and therm al expansion of invest 3. T hey should harden sufficiently at mouth temperature to permit w ithdraw al from the ments. cavity without distortion. I t has been proved by investigation, 4. T hey should carve without chipping and both by the Bureau and other investiga flaking. tors, that the shrinkage of gold from the 5. T he color should be such that it facili molten state to the point of crystallization tates the carving of the pattern in the mouth is approximately 1.25 per cent per volume through contrast with the hard and soft tissues of the gold. of the mouth. Extensive experiments have proved 6. T he thermal expansion characteristics should be known to insure correct use of the that the shrinkage of gold is practically materials in any technic requiring w ax ex constant when the tem perature varies pansion. from the lowest degree at which it can be Flow tests w ere developed for the determ cast to the maximum tem perature obtain ination of rigidity and working range. T her mal expansion determinations were specified able w ith a good blowpipe, using hy to provide other necessary data. These de drogen and compressed a ir.. tail requirements are: T h e properties of wax patterns are 1. Flow at mouth temperature. Samples such that it seems far more desirable to 1 cm. in diameter shall not shorten more than obtain the m ajor portion of the compen 1 per cent in length w hen subjected to a load sation by the expansion of the investment of 2,000 gm. for ten minutes at 99 F.
Coy— Inlay Castings rather than to risk distortion of the thin and plastic wax pattern. T h e ideal investment would be one in which the setting and therm al expansion combined would equalize the deficiency caused by the shrinkage of gold. Experiments are now being conducted looking tow ard the perfection of such a product. Casting w ill then be greatly simplified and an inlay that fits w ill be come a certainty rather than an accident. A t the present time, there is no invest ment th at has enough expansion to com pensate or offset the shrinkage of gold in all cases. T h is condition makes it neces sary to utilize the therm al expansion of wax to some extent, although this should be confined to a minimum. T h e investments that have the greatest amount of therm al expansion have been found to be those of high silica and low plastic content. Silica is an expanding material ; plaster, a contracting material. A ll the investments listed by the Bureau of Standards as those most nearly meeting their requirements have a high silica and low plaster content. T hey all have a progressive expansion. By this, I mean that the higher the tem perature to which they are heated, the greater the expansion, reaching the maximum at red heat. If an investment is to be desirable for use in a w ax expansion method, it must also be slow setting, so that the heat used in expanding may have time to penetrate to the pattern before the initial set of the material. Shrinking investments are those of a high plaster content. T h e use of these w ith w ax expansion seems al ways to result in undersize castings. No scientific evidence has ever been presented that would w arran t the use of this type of investment. N either does there seem to be any advantage to be derived from the use of a double investment.
1763
N othing original is claimed for the technic herewith presented. Scientific facts taken from the above-mentioned articles, furnished by the Bureau of Standards and verified by many castings made independently in the laboratories of the cooperative committee, have fur nished a basis for the method. Preparation of all cavities, where an inlay restoration is indicated, should be made by the method as outlined by G. V. Black in his book on operative dentistry. I t should be so formed that all the walls will be slightly diverging and free from undercuts, as from this type of cavity the wax pattern may be readily removed. T h e walls must be smoothed w ith stones and chisels and the cavosurface angles ac curately beveled and finished. T h e preparation of a well-finished cavity is absolutely essential in the pro duction of an ideal wax pattern. T his preparation is made easier by the applica tion of the rubber dam. I t is my opinion that the making of a true wax pattern is the most difficult part in the procedure of making an inlay. Since wax is a plastic substance and changes form whenever subjected to change in temperature, it is essential that whenever wax is placed in a cavity in the mouth, it should not be chilled by the use of cold w ater or air, but it should be held under pressure until sufficient time has elapsed for it to assume mouth tem perature. T h e pattern should be carved to form. W hen the carving is completed, the proxi mal surfaces of the pattern should be pol ished w ith linen tape. A solvent such as eucalyptus or oil of cajuput is not to be used. T h e completed pattern should be removed from the cavity whenever pos sible by attaching the sprue and then gently applying force rather than by the
1764
T he Journal o f the American D ental Association
use of an explorer. I t should then be washed w ith a solution of equal parts of hydrogen peroxid and green soap, rinsed w ith room tem perature w ater and dried w ith a slight application of air from a syringe. T h e pattern is now ready for investing. Any of the investments which meet the tentative specifications of the Bureau as previously outlined may be choosen.3 T h e greater variation w ill be found in the amount of w ater necessary in the mix, and in the setting time. As a m atter of convenience, I shall choose Ransom and Randolph’s green in vestment. T h e inlay ring to be used should be lined w ith a thin layer of sheet asbestos and the ring sealed to a small metal slab w ith beeswax. T h e lining should be moistened w ith w ater, so that the w ater content of the investment will not be changed when poured into the ring. T h e investment w ill then be mixed, 16 gm. of w ater to 50 gm. of investment being used, the amounts being doubled if necessary to fill the ring. I f w ax expansion is necessary, the w ater used in the mix should be warm enough that the finished mix of invest ment is approximately the same tempera ture as the expansion bath. I t should first be spatulated by hand, then by the use of a mechanical device such as the W hip-m ix or K err for 100 revolu tions of the handle, or about 30 seconds. I t is then vigorously vibrated on the lathe to remove any remaining air bubbles. T h e ring is then vibrated full of investment, more of which is formed on the pattern by a camel’s hair brush, care being taken not to trap any air. T h e pattern is now gently vibrated into the investmentfilled ring until the sprue form er base is 3. Footnote 1, second reference.
seated. A clamp is now adjusted to hold the base and the ring securely together. Plaster bowl and spatula should be dry and clean when the mix is begun. T h e investment is placed in the w ater bath for tw enty minutes, or until set. A fter removal of the metal base, the sprue former base and the sprue pin, the invested pattern is placed in a furnace for fifteen minutes at a heat of approximately 500 F. T h e heat is then raised to 900 F. for fifteen minutes, then to red heat, at which point the casting should be made, the investment reaching its maximum ex pansion at that heat. T h e type of casting machine to be used makes little difference and is not con sidered a variable. T h e amount of w ater used in the mix may be varied to suit the convenience of the operator, but it should be continually kept in mind th at the smaller the amount of w ater used, the greater the therm al ex pansion of the investment and the smooth er the finished casting. Rough castings are usually caused by one of the follow ing: Excessively vibrating. Use of an unclean pattern. A ir being incorporated into the invest ment. T oo high a proportion of w ater in the mix. Burning out too rapidly. T h e various individuals and the groups of the cooperating committee presented clinics at the M emphis meeting of the American D ental Association under the direction and w ith the assistance of the Bureau of Standards and perhaps the many ideas and technics may be harmon ized into one ideal method of casting and adopted by the profession as a stan dardized technic.