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The Casting Process
E s t e s — C a s t in g P r o c e s s
tinued to give abnorm al values in the repeat analysis. T h e results regarding red-cell sedi mentation rate are given in T able 2. O nly two control cases, out o f twentyfour, or 8 p er cent, gave values above n orm al; whereas, in the periodontoclasia group, twenty-seven out o f fifty-one cases tested, or 53 p er cent, showed a high rate. "T h e percentage o f deviations in our control g ro u p does not differ appreciably from that previously re ported by Haskins and his collabora tors.30 Conclusions Fifty-five out o f seventy patients with advanced periodontoclasia showed one or m ore deviations from normal blood ranges. These deviations were, in gen eral, slight, in cluding calcium, phos phatase, sugar, cholesterol, and vitamin C content. N o differences were found between the group from 13 to 33 years (predom inantly fem ale) and that be tween 34 and 60 years. N o clear dis tinction was fou n d between the group with m oderate o r heavy calculus and that with slight o r n o deposits. In the control group o f twenty-two persons who were free from periodontoclasia, the
1377
Table 2 .— Cell Sedimentation Rate Percent Number age of Number of Cases Cases of Above Above Cases Normal Normal Control group Periodontoclasia group Normal group (from literature)
24 51
2 27
8 53
295
15
5
ranges o f values were normal with the exception o f three low vitamin C values. These results suggest that some systemic condition was one o f the contributing factors in' periodontoclasia in m any o f the subjects studied. This view is strengthened by the results obtained when the b lood was analyzed a second time, after the teeth were extracted, since o f twenty-one cases that showed deviations from the norm al in the first analysis, twelve continued to give abnor mal values in the repeat analysis. T h e red-cell sedimentation rate, which is an indication o f infection, inflamma tion and malignancy, was high in about one-half o f the periodontoclasia cases tested.
THE CASTING PROCESS
George D. Estes, D.D., In the casting process, the investment o f the wax pattern is an im portant step. Attention to this operation will d o m uch toward assuring a pattern chamber from w hich a casting may be obtained and which is sm ooth and is free from nodules and w ill reproduce the fine markings in the original w ax model. This problem has received a great deal Jour. A .D .A., Vol. 3a, Nov. i-Dec. 1, 1945
Minneapolis, Minn. o f attention from dentists and other in vestigators ever since the casting process was first given to the profession. R . L . Colem an,1 and later Souder and Paffenbarger,2 in their study o f the physical 1. Coleman, R. L.: Physical Properties of Dental Materials, Gold Alloys and Accessory Materials. Bureau of Standards Research Paper 32, 1928.
13 78
T h e J o u r n a l o f t h e A m e r ic a n D e n t a l A s s o c ia t io n
properties of dental materials, gave con siderable attention to the causes of sur face roughness on castings, reaching the conclusion that this roughness was pro duced in two w ays: (1) by water clinging to the pattern and weakening the invest ment forming the surface of the mold and (2), more important, by the pres ence of air trapped in the investment and forming bubbles that cling to the wax pattern during the process of in vesting. Tw o methods were suggested to over come this difficulty, at least partially: (1) careful hand or mechanical spatulation of the mixture of water and invest ment to eliminate air bubbles as far as possible and great care in applying the mixture to the pattern by brushing and painting; and (2) m ixing of the invest ment in a vacuum , for which Dr. Cole man devised an experimental apparatus. M ixing of the investment in a vacuum was found to give a mold surface exceed ingly smooth and dense and entirely free from air bubbles; but, apparently, the apparatus was thought too complicated for use in the average dental office. I wish to present to the profession a new method for investing the w ax pat tern in a vacuum . By this method, the water and investment material are mixed in the ordinary manner and then placed in a vacuum chamber from which all air has been removed. T h e investment of the pattern is completed in the vacuum chamber in the total absence of air. There is thus produced a mold, having an exceedingly smooth and dense surface, from w hich a casting may be secured that is entirely free from nodules caused by air contained in the investment mix ture or trapped during the process of investing the pattern. T h e resulting cast ings are not only smooth, but also re produce the finest details in the wax pattern.
T h e idea of investing wax patterns in a vacuum is not altogether new. E. R. Granger,3 among others, devised ap paratus for this purpose and published a description of his method in an article on casting platinum. T h e process about to be described depends on an entirely different principle. It is not original with me, having been discovered by C. H. Brace, of the General Refineries, M in neapolis. M r. Brace devised his method for the m anufacturing jeweler, to be used in making multiple castings for rings and other jewelry. Thinking that his method m ight be of use to dentists in their work, he experimented in that field also and developed an apparatus for dental use. I had an opportunity to spend some time with M r. Brace in working out several of the special prob lems of adapting the process to the mak-
Fig. I.— Bell-jar ( A ) , with rubber collar (B ), in position on ground glass slab (C ), with sprue base and rings in place in readi ness to exhaust air.
2. Souder, Wilmer, and Paffenbarger, G. C .: 3. Granger, E. R .: Platinum-Iridium Physical Properties of Dental Materials, Casting, New Concept of Casting Process. Circular C 433, 1942. J.A.D.A., 2 7 :1718, November 1940.
E s t e s — C a s t in g P r o c e s s
>379
adopted the process for investing wax patterns in my office and have been using it routinely for three or four years. A number of men to whom I have demon strated the method report equally satis factory results. It is as a method that has been tried and proved in practice that I am offering it here.
Fig. 2.— Filter pump with coupling ( A ) , which fits any water faucet, and hose leading to vacuum chamber.
ing of dental castings. I am indebted to M r. Brace and other members of the research staff at General Refineries for the demonstration of the process and for the free use of their facilities in experi menting with it. T h e results obtained were so gratifying that I immediately
Briefly, the process consists in placing the w ax pattern, mounted on a sprue base and surrounded by a casting ring filled with freshly mixed investment, in a vacuum chamber and exhausting the air. A fter all air has been removed from the investment material in the vacuum chamber, the air is again admitted and the investment material, now completely freed from all trapped and dissolved air, is forced down into the casting ring and against the w ax pattern. By the elimina tion of all air, the particles of invest ment material are brought into the closest possible contact w ith the wax pattern and with each other. T h e resulting mold, after the investment has set and the w ax has been eliminated, presents an extremely smooth dense surface and the castings obtained are uniformly smooth and reproduce the finest markings on the w ax pattern. The apparatus employed, which is fully illustrated in the photographs accom
Fig. 3.— Casting ring ( A ) , with rubber collar in place; rubber sprue base with pattern in position (B ) , and second casting ring that fits onto first one (C).
1380
T h e J o u r n a l o f t h e A m e r ic a n D e n t a l A s s o c ia t io n
panying this article, consists first of a vacuum chamber made of glass or other transparent material so that the process m ay be observed, and a pump for ex hausting the air. As originally designed, the vacuum chamber was a bell-jar, pro vided with a T fitting at the top, having a connection with the air pump by means of a noncollapsible rubber tube and a stopcock for admitting air. The bottom of the bell-jar is provided with a soft rubber collar and rests on a piece of plate glass, ground on one side, so that when the jar with its rubber collar is placed on the ground surface of the slab, an air tight joint is formed. As the air is exhausted from the bell-jar, the pressure of the air on the outside forces the bell-jar against the slab so tightly as to exclude all air. T h e pump for exhausting the air is a filter type or air pump of the water jet type such as is sold by makers of chemical apparatus. It operates by water pressure and has a rubber connection to fit any cold water faucet. It is necessary to have a water pressure of about 45 pounds per square inch to operate the pump efficiently. A t this pressure, the
Fig. 4.— Sprue base, rings and collar assem bled, ready to be placed in vacuum chamber.
pump will produce a vacuum of 5 or 6 mm. of mercury, which is sufficient for the purpose. O ther types of air pumps are available and m ay be sub stituted when water pressure is not suffi cient to operate the filter pump just described. This type of pump is in expensive and will last indefinitely with out requiring any attention. T h e only other special requirements are a sprue base, preferably of rubber, into which the sprue pin holding the pattern will fit tightly and over which the casting ring will fit snugly, so that neither the ring nor the pattern will be displaced during the investment proc ess. T w o casting rings are needed. One fits over the sprue base in the usual manner and the other is attached to the top of the first one by means of a rubber collar so as to double the length of the casting ring during the process of in vestment. T h e method of using the above-described apparatus for investing a wax pattern is as follows: T h e w ax pattern is sprued in the usual manner and mounted on the sprue base, into which it fits tightly. T h e casting ring, which fits the base snugly, is then placed in position and the second ring is placed on top of the first one and is held in place by the rubber collar so that the height of the casting ring is about doubled. T h e manner of assembling is shown in Figure 4. T h e investment material and water are then weighed or measured, placed in the clean plaster bowl and spatulated until the water and the powder are thor oughly mixed. N o special care is needed to avoid mixing in air at this point. This m ixture of investment material is then poured into the top of the casting ring and allowed to flow down over the w ax pattern. T h e amount should be sufficient to fill the first ring and extend up a short distance into the second ring. T h e entire assemblage is then placed on
E s t e s— C a s t in g P r o c e s s
the glass slab and the bell-jar placed in position over it. T h e stopcock shown at D in Figure 1 is then closed. T h e filter pump, which is connected with the belljar by a rubber hose, is placed in position on the water faucet and the valve fully opened. In a few seconds, the investment material will be seen to rise to the top of the second ring and flow over the top slightly, giving off numerous bubbles. The bubbling proceeds violently for a few seconds and then begins to subside, and the investment material will be seen to sag slightly and tend to settle back into the casting ring. In a few seconds more, the stopcock may be opened and th e air allowed to enter the vacuum chamber. The bubbling ceases instantly and the inrushing air forces the investment material down into the casting ring and causes it to adapt itself closely to the w ax pat tern. T he water m ay now be shut off and the bell-jar removed from the slab. T h e extension ring with its rubber collar is then removed from the casting ring and the investment in the first ring leveled off with a spatula. This completes the investment of the w ax pattern. The entire process takes from two to three minutes. It will be seen that the sprue pin, sprue base and casting ring must fit to gether snugly so that they will not be displaced as the investment boils up in the ring. T h e function of the second ring, which is placed over the regular casting ring, is to confine the invest ment material so that too much will not flow over the top of the ring, leaving insufficient investment to fully cover the w ax pattern. I have modified the original apparatus somewhat in order to m ake its use in the small space available in the usual dental office more convenient. A pint fruit jar with a screw cover is substituted for the bell-jar and the necessary connections
1381
Fig. 5.— Apparatus as modified by author. Sprue base with casting ring and radiator hose extension in position are shown at A. The fruit jar vacuum chamber is shown at D with top provided with T connection hav ing a stop-cock at B and connection for hose to air pump at C.
are soldered to the metal cover, which is reinforced by soldering a disk of brass or copper in place of the glass disk usually found there. T h e cover is made to fit the jar tightly by means of the rubber washer supplied for the purpose. T h e smaller chamber speeds up the proc ess as there is less air to be exhausted. A short length of automobile radiator hose is used to slip over the casting ring to increase its length temporarily, in stead of the metal ring and rubber collar. This still farther simplifies the apparatus. T h e principle on which the process de pends is the well-known physical law that the boiling point of water, or the tem perature at which it turns to vapor, is lowered as the atmospheric pressure is reduced. W ater boils at 100 C., or 212 F., at the usual pressure of 760 mm. of mercury. A t
1 382
T h e J o u r n a l o f t h e A m e r ic a n D e n t a l A s s o c ia t io n
Fig. 6.— Process in operation in vacuum chamber. The investment is bubbling over the top of the ring.
a pressure of 6.5 mm., such as is m ain tained in the vacuum chamber described above, the boiling or vapor point is 5 C. W hen the mixture of water and invest ment is subjected to this low pressure, the water in the investment begins to vaporize rapidly. The bubbles of water vapor rise to the top, carrying the par ticles of investment along with them. W hile this boiling is taking place, all the air that may have been trapped in the in vestment mixture, together with any air dissolved in the water, is freed and carried away by the pump. This boiling of the investment material in the casting ring is a key to the success of the proc ess. T h e boiling is necessary in order to free the air bubbles throughout the mass of investment material and allow them to rise to the top and escape. W hen the stopcock is opened and the air again ad mitted to the vacuum chamber, the
investment material, now entirely free of air, is forced against the pattern. T h e w ax pattern is now completely sur rounded by a ring full of investment material that is absolutely free of air. W hen this material sets and the w ax is burned out, there results a mold or pattern chamber with a dense smooth surface into which the gold is cast. T w o principal advantages in the use of this process may be m entioned: 1. The castings are smooth and accurate in the reproduction of the finest detail in the wax pattern. T h e smoothness and ab sence of all nodules on the cavity sur face of the casting saves a great deal of time in seating and finishing the casting. No time need be wasted in removing the annoying nodules, small and large, so often present in castings m ade by other methods. W hen desired, time may be saved and equally good results secured by mounting several patterns on one base and investing them in one ring, or by placing several rings filled with invest ment under the bell-jar and completing the investment simultaneously. 2. Not least in importance, the investing of the w ax pattern is no longer a process re quiring practice, special training or any manual dexterity for satisfactory results. In the use of this process, the personal equation is practically eliminated and the process of investing w ax patterns becomes a simple mechanical procedure requiring no special ability or training and well within the capabilities of the average dental assistant. T h e results of the novice, after observing one or two demonstrations, can equal those of the expert. As the investment of the w ax pattern is a process usually relegated to the assistant or technician, this method should have a special appeal to the busy dentist. 1546 Medical Arts Building.
tinued to give abnorm al values in the repeat analysis. T h e results regarding red-cell sedi mentation rate are given in T able 2. O nly two control cases, out o f twentyfour, or 8 p er cent, gave values above n orm al; whereas, in the periodontoclasia group, twenty-seven out o f fifty-one cases tested, or 53 p er cent, showed a high rate. "T h e percentage o f deviations in our control g ro u p does not differ appreciably from that previously re ported by Haskins and his collabora tors.30 Conclusions Fifty-five out o f seventy patients with advanced periodontoclasia showed one or m ore deviations from normal blood ranges. These deviations were, in gen eral, slight, in cluding calcium, phos phatase, sugar, cholesterol, and vitamin C content. N o differences were found between the group from 13 to 33 years (predom inantly fem ale) and that be tween 34 and 60 years. N o clear dis tinction was fou n d between the group with m oderate o r heavy calculus and that with slight o r n o deposits. In the control group o f twenty-two persons who were free from periodontoclasia, the
1377
Table 2 .— Cell Sedimentation Rate Percent Number age of Number of Cases Cases of Above Above Cases Normal Normal Control group Periodontoclasia group Normal group (from literature)
24 51
2 27
8 53
295
15
5
ranges o f values were normal with the exception o f three low vitamin C values. These results suggest that some systemic condition was one o f the contributing factors in' periodontoclasia in m any o f the subjects studied. This view is strengthened by the results obtained when the b lood was analyzed a second time, after the teeth were extracted, since o f twenty-one cases that showed deviations from the norm al in the first analysis, twelve continued to give abnor mal values in the repeat analysis. T h e red-cell sedimentation rate, which is an indication o f infection, inflamma tion and malignancy, was high in about one-half o f the periodontoclasia cases tested.
THE CASTING PROCESS
George D. Estes, D.D., In the casting process, the investment o f the wax pattern is an im portant step. Attention to this operation will d o m uch toward assuring a pattern chamber from w hich a casting may be obtained and which is sm ooth and is free from nodules and w ill reproduce the fine markings in the original w ax model. This problem has received a great deal Jour. A .D .A., Vol. 3a, Nov. i-Dec. 1, 1945
Minneapolis, Minn. o f attention from dentists and other in vestigators ever since the casting process was first given to the profession. R . L . Colem an,1 and later Souder and Paffenbarger,2 in their study o f the physical 1. Coleman, R. L.: Physical Properties of Dental Materials, Gold Alloys and Accessory Materials. Bureau of Standards Research Paper 32, 1928.
13 78
T h e J o u r n a l o f t h e A m e r ic a n D e n t a l A s s o c ia t io n
properties of dental materials, gave con siderable attention to the causes of sur face roughness on castings, reaching the conclusion that this roughness was pro duced in two w ays: (1) by water clinging to the pattern and weakening the invest ment forming the surface of the mold and (2), more important, by the pres ence of air trapped in the investment and forming bubbles that cling to the wax pattern during the process of in vesting. Tw o methods were suggested to over come this difficulty, at least partially: (1) careful hand or mechanical spatulation of the mixture of water and invest ment to eliminate air bubbles as far as possible and great care in applying the mixture to the pattern by brushing and painting; and (2) m ixing of the invest ment in a vacuum , for which Dr. Cole man devised an experimental apparatus. M ixing of the investment in a vacuum was found to give a mold surface exceed ingly smooth and dense and entirely free from air bubbles; but, apparently, the apparatus was thought too complicated for use in the average dental office. I wish to present to the profession a new method for investing the w ax pat tern in a vacuum . By this method, the water and investment material are mixed in the ordinary manner and then placed in a vacuum chamber from which all air has been removed. T h e investment of the pattern is completed in the vacuum chamber in the total absence of air. There is thus produced a mold, having an exceedingly smooth and dense surface, from w hich a casting may be secured that is entirely free from nodules caused by air contained in the investment mix ture or trapped during the process of investing the pattern. T h e resulting cast ings are not only smooth, but also re produce the finest details in the wax pattern.
T h e idea of investing wax patterns in a vacuum is not altogether new. E. R. Granger,3 among others, devised ap paratus for this purpose and published a description of his method in an article on casting platinum. T h e process about to be described depends on an entirely different principle. It is not original with me, having been discovered by C. H. Brace, of the General Refineries, M in neapolis. M r. Brace devised his method for the m anufacturing jeweler, to be used in making multiple castings for rings and other jewelry. Thinking that his method m ight be of use to dentists in their work, he experimented in that field also and developed an apparatus for dental use. I had an opportunity to spend some time with M r. Brace in working out several of the special prob lems of adapting the process to the mak-
Fig. I.— Bell-jar ( A ) , with rubber collar (B ), in position on ground glass slab (C ), with sprue base and rings in place in readi ness to exhaust air.
2. Souder, Wilmer, and Paffenbarger, G. C .: 3. Granger, E. R .: Platinum-Iridium Physical Properties of Dental Materials, Casting, New Concept of Casting Process. Circular C 433, 1942. J.A.D.A., 2 7 :1718, November 1940.
E s t e s — C a s t in g P r o c e s s
>379
adopted the process for investing wax patterns in my office and have been using it routinely for three or four years. A number of men to whom I have demon strated the method report equally satis factory results. It is as a method that has been tried and proved in practice that I am offering it here.
Fig. 2.— Filter pump with coupling ( A ) , which fits any water faucet, and hose leading to vacuum chamber.
ing of dental castings. I am indebted to M r. Brace and other members of the research staff at General Refineries for the demonstration of the process and for the free use of their facilities in experi menting with it. T h e results obtained were so gratifying that I immediately
Briefly, the process consists in placing the w ax pattern, mounted on a sprue base and surrounded by a casting ring filled with freshly mixed investment, in a vacuum chamber and exhausting the air. A fter all air has been removed from the investment material in the vacuum chamber, the air is again admitted and the investment material, now completely freed from all trapped and dissolved air, is forced down into the casting ring and against the w ax pattern. By the elimina tion of all air, the particles of invest ment material are brought into the closest possible contact w ith the wax pattern and with each other. T h e resulting mold, after the investment has set and the w ax has been eliminated, presents an extremely smooth dense surface and the castings obtained are uniformly smooth and reproduce the finest markings on the w ax pattern. The apparatus employed, which is fully illustrated in the photographs accom
Fig. 3.— Casting ring ( A ) , with rubber collar in place; rubber sprue base with pattern in position (B ) , and second casting ring that fits onto first one (C).
1380
T h e J o u r n a l o f t h e A m e r ic a n D e n t a l A s s o c ia t io n
panying this article, consists first of a vacuum chamber made of glass or other transparent material so that the process m ay be observed, and a pump for ex hausting the air. As originally designed, the vacuum chamber was a bell-jar, pro vided with a T fitting at the top, having a connection with the air pump by means of a noncollapsible rubber tube and a stopcock for admitting air. The bottom of the bell-jar is provided with a soft rubber collar and rests on a piece of plate glass, ground on one side, so that when the jar with its rubber collar is placed on the ground surface of the slab, an air tight joint is formed. As the air is exhausted from the bell-jar, the pressure of the air on the outside forces the bell-jar against the slab so tightly as to exclude all air. T h e pump for exhausting the air is a filter type or air pump of the water jet type such as is sold by makers of chemical apparatus. It operates by water pressure and has a rubber connection to fit any cold water faucet. It is necessary to have a water pressure of about 45 pounds per square inch to operate the pump efficiently. A t this pressure, the
Fig. 4.— Sprue base, rings and collar assem bled, ready to be placed in vacuum chamber.
pump will produce a vacuum of 5 or 6 mm. of mercury, which is sufficient for the purpose. O ther types of air pumps are available and m ay be sub stituted when water pressure is not suffi cient to operate the filter pump just described. This type of pump is in expensive and will last indefinitely with out requiring any attention. T h e only other special requirements are a sprue base, preferably of rubber, into which the sprue pin holding the pattern will fit tightly and over which the casting ring will fit snugly, so that neither the ring nor the pattern will be displaced during the investment proc ess. T w o casting rings are needed. One fits over the sprue base in the usual manner and the other is attached to the top of the first one by means of a rubber collar so as to double the length of the casting ring during the process of in vestment. T h e method of using the above-described apparatus for investing a wax pattern is as follows: T h e w ax pattern is sprued in the usual manner and mounted on the sprue base, into which it fits tightly. T h e casting ring, which fits the base snugly, is then placed in position and the second ring is placed on top of the first one and is held in place by the rubber collar so that the height of the casting ring is about doubled. T h e manner of assembling is shown in Figure 4. T h e investment material and water are then weighed or measured, placed in the clean plaster bowl and spatulated until the water and the powder are thor oughly mixed. N o special care is needed to avoid mixing in air at this point. This m ixture of investment material is then poured into the top of the casting ring and allowed to flow down over the w ax pattern. T h e amount should be sufficient to fill the first ring and extend up a short distance into the second ring. T h e entire assemblage is then placed on
E s t e s— C a s t in g P r o c e s s
the glass slab and the bell-jar placed in position over it. T h e stopcock shown at D in Figure 1 is then closed. T h e filter pump, which is connected with the belljar by a rubber hose, is placed in position on the water faucet and the valve fully opened. In a few seconds, the investment material will be seen to rise to the top of the second ring and flow over the top slightly, giving off numerous bubbles. The bubbling proceeds violently for a few seconds and then begins to subside, and the investment material will be seen to sag slightly and tend to settle back into the casting ring. In a few seconds more, the stopcock may be opened and th e air allowed to enter the vacuum chamber. The bubbling ceases instantly and the inrushing air forces the investment material down into the casting ring and causes it to adapt itself closely to the w ax pat tern. T he water m ay now be shut off and the bell-jar removed from the slab. T h e extension ring with its rubber collar is then removed from the casting ring and the investment in the first ring leveled off with a spatula. This completes the investment of the w ax pattern. The entire process takes from two to three minutes. It will be seen that the sprue pin, sprue base and casting ring must fit to gether snugly so that they will not be displaced as the investment boils up in the ring. T h e function of the second ring, which is placed over the regular casting ring, is to confine the invest ment material so that too much will not flow over the top of the ring, leaving insufficient investment to fully cover the w ax pattern. I have modified the original apparatus somewhat in order to m ake its use in the small space available in the usual dental office more convenient. A pint fruit jar with a screw cover is substituted for the bell-jar and the necessary connections
1381
Fig. 5.— Apparatus as modified by author. Sprue base with casting ring and radiator hose extension in position are shown at A. The fruit jar vacuum chamber is shown at D with top provided with T connection hav ing a stop-cock at B and connection for hose to air pump at C.
are soldered to the metal cover, which is reinforced by soldering a disk of brass or copper in place of the glass disk usually found there. T h e cover is made to fit the jar tightly by means of the rubber washer supplied for the purpose. T h e smaller chamber speeds up the proc ess as there is less air to be exhausted. A short length of automobile radiator hose is used to slip over the casting ring to increase its length temporarily, in stead of the metal ring and rubber collar. This still farther simplifies the apparatus. T h e principle on which the process de pends is the well-known physical law that the boiling point of water, or the tem perature at which it turns to vapor, is lowered as the atmospheric pressure is reduced. W ater boils at 100 C., or 212 F., at the usual pressure of 760 mm. of mercury. A t
1 382
T h e J o u r n a l o f t h e A m e r ic a n D e n t a l A s s o c ia t io n
Fig. 6.— Process in operation in vacuum chamber. The investment is bubbling over the top of the ring.
a pressure of 6.5 mm., such as is m ain tained in the vacuum chamber described above, the boiling or vapor point is 5 C. W hen the mixture of water and invest ment is subjected to this low pressure, the water in the investment begins to vaporize rapidly. The bubbles of water vapor rise to the top, carrying the par ticles of investment along with them. W hile this boiling is taking place, all the air that may have been trapped in the in vestment mixture, together with any air dissolved in the water, is freed and carried away by the pump. This boiling of the investment material in the casting ring is a key to the success of the proc ess. T h e boiling is necessary in order to free the air bubbles throughout the mass of investment material and allow them to rise to the top and escape. W hen the stopcock is opened and the air again ad mitted to the vacuum chamber, the
investment material, now entirely free of air, is forced against the pattern. T h e w ax pattern is now completely sur rounded by a ring full of investment material that is absolutely free of air. W hen this material sets and the w ax is burned out, there results a mold or pattern chamber with a dense smooth surface into which the gold is cast. T w o principal advantages in the use of this process may be m entioned: 1. The castings are smooth and accurate in the reproduction of the finest detail in the wax pattern. T h e smoothness and ab sence of all nodules on the cavity sur face of the casting saves a great deal of time in seating and finishing the casting. No time need be wasted in removing the annoying nodules, small and large, so often present in castings m ade by other methods. W hen desired, time may be saved and equally good results secured by mounting several patterns on one base and investing them in one ring, or by placing several rings filled with invest ment under the bell-jar and completing the investment simultaneously. 2. Not least in importance, the investing of the w ax pattern is no longer a process re quiring practice, special training or any manual dexterity for satisfactory results. In the use of this process, the personal equation is practically eliminated and the process of investing w ax patterns becomes a simple mechanical procedure requiring no special ability or training and well within the capabilities of the average dental assistant. T h e results of the novice, after observing one or two demonstrations, can equal those of the expert. As the investment of the w ax pattern is a process usually relegated to the assistant or technician, this method should have a special appeal to the busy dentist. 1546 Medical Arts Building.