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Investment Materials*
INVESTMENT MATERIALS*
By WILLIAM H. TAGGART, D.D.S., Chicago, Illinois T
H E R E has been a good deal o f controversy the last few years about investm ent m aterials, and my talk to you tonight w ill consist rath er in explaining the basic principles o f in vestm ent materials and the method o f handling them than in trying to convert you to using any one kind o f investment. O ne o f the principal ingredients in an investm ent m aterial is plaster o f Paris, which comes from mines, being chipped and blasted out just as ore is, in the form o f m edium hard rock, called gypsum or calcium sulphate. T h is ore is crushed to any degree o f fineness de sired, and is put in large cauldrons, where, by constant stirring and rotating o f these cauldrons, it is brought up to a high degree o f heat in order to drive off the w ater o f crystallization. T h e gypsum is ground to all grades o f fineness needed, and these different grades are segregated by sifting. Even in the coarser grades o f plaster, there is a large percentage o f fine and superfine particles o f plaster. T h e re fo re, the degree o f fineness o f plaster in an investm ent is not so much o f an elem ent in allow ing the air to escape from a m old as some have tried to imply, as the high percentage o f fine or, as it is called in the cem ent trade, “ neat” m aterial is sufficient to fill the interstices between the coarser grains, this tending to make a m aterial just * R e a d b e fo re th e C h ica g o D e n ta l Society, O ct. 19, 1926. J o u r . A . D . A . , J u ly , 1927
as impervious to air as though it were all made o f fine m aterial. T h e reason the coarser grades o f plaster are not more in use in investment m aterials is that they do not lend themselves to m anipulation so readily as the finer grades do. I f we mix a batch o f coarse plaster thin enough to pour on a glass surface and another batch o f fine plaster and pour on a surface, we find that one surface has about the same degree o f smoothness and imperviousness to air as the other; in other words, the “ neat” m aterial has a tendency to settle to the surface which it touches. W h en this plaster is first burned and cooled, it sets very rapidly; in fact, too rapidly for any dental use. T h is tendency is con trolled in the factories by taking certain known quantities o f plaster that has already been set and grinding it w ithout fu rth e r burning. T his, in a crude way, controls the time in which the plaster sets. W hen this new plaster has w ater mixed w ith it, the w ater o f crystallization which the original gypsum rock had is restored, and it is brought back to a crystalline form , but is never so hard as the original gypsum rock was. T h e other principal ingredient in an invest m ent m aterial is silex. T h e principle here is the same as regards the d if fe re n t grades o f fineness as in plaster. T h ere are all grades o f fineness when this is crushed. I may explain this a little 1190
T a g g a r t — I n v e s t m e n t M a t e r ia l s more plainly by saying that recently I had some m aterial which I bought fo r 90 mesh, which only means th at the largest particles are 90 mesh, the rest going through all different grades down to impalpable powder. I found that out o f this 90 mesh m aterial, 50 per cent would go through a 200 mesh sieve; and on e-h alf o f the 50 per cent, or onefourth o f the original am ount, w ould go through a 600 mesh, and there were all grades o f fineness up to possibly the thousandths. T hese are the tw o prin cipal ingredients in any investm ent m aterial. N ow these m aterials, a fte r being m anufactured, are quite hydro scopic; th a t is, they have a tendency to absorb moisture, and in just the pro portion as the m aterial is allow ed to absorb moisture, it w ill change its setting qualities, or it w ill change the length o f tim e which it takes to set. Sometimes plaster and silex, being crude materials, are allow ed to stand on the railway p latfo rm in the drenching rain, and the barrels become w ater soaked, the plaster eventually absorbing more or less o f this moisture, which accounts, in a great measure, fo r the fact that two shipments o f plaster from the same original batch seldom w ork exactly alike, as you have all noticed, and as all dealers have noticed when the dentist comes back claim ing th at the plaster does not set the same as the last that he bought. Any other ingredients mixed w ith these two m aterials are usually trade secrets. In speaking o f these different investment materials, and the principle o f their use, it is hard fo r me, as a m anufacturer of investm ent m aterial, to keep from using the personal pronoun, as though I were trying to persuade you to use my m ate rial. Such is not the intention, and when I allude to my m aterial or any other m aterial, I shall mention the names
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w ithout an idea o f criticizing the m a terial o f others or advertising my own. I hope you w ill understand this thor oughly. W einstein, in one o f his ex haustive and not very scientific articles published in one o f the dental journals, made the rem ark that he and his as sociates had found that graphite is not o f any use in an investment material. H e did not mention my name, but m ine is the only investm ent that has graphite in it. In talking to a profes sional audience on a professional subject and as one o f the profession, certain ethics are required, but as all o f these m en are now, regardless o f their titles, m anufacturers, I may as w ell speak openly as to the qualities o f the different m aterials. As W einstein has made the rem ark that he has found graphite o f no use in an investment m aterial, I shall explain to you why I have put it in mine. W h en I was a boy, from the time I was 8 until I was 18 years o f age, I was always “ hanging around” factories and machine shops. L iving in a com paratively small tow n, being acquainted w ith practically all o f the factory men and machinists, and not m aking myself a nuisance, I was allow ed to help w ith little things about the work, and one o f my self-imposed duties was to take a chunk o f charcoal, place it in a loose cotton bag, and w ith a board or ham m er, crush it into a fine powder. Some o f it w ould be exceedingly fine. As I have told you before, any crushed m aterial w ill have all grades o f fineness. W hen the m old was open and the pattern taken out, this bag o f crushed charcoal w ould be shaken and the m old would be sprinkled all over w ith it, so as to make a smooth casting, as it seemed to fill up the meshes in the grains o f sand and also made a good surface against which the hot m etal w ould run. T h is was on
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coarse castings. Y ou possibly have noticed that, on stove work, the lids and all parts o f stove castings are very smooth and fined grained. In the case o f these stove castings, instead o f using charcoal, which was only used fo r the coarser grade o f work, they sub stituted fine graphite, sprinkling it on the surfaces o f the mold. O n surfaces they could reach w ith their trowels, the workers w ould w ork this fine graph ite into the sand surface. T h is is why the stove castings are the smoothest of all castings that we see. W h en the time came fo r me to develop my investing m aterial to carry out my ideas in casting gold, the only m aterials we had at hand were not m anufactured products but self-m ade m aterials, consisting in some cases o f fine sand and plaster; in others, o f marble dust and plaster, and in others, o f pumice stone and plaster. These m aterials were all right fo r the purpose fo r which they were intended, except that they were made by rule o f thumb. T h e m anufacturer w ould take some plaster and some sand, or some pumice stone or some m arble dust, never caring much about the proportion, and would use it fo r the solder work. O f course, no gold was supposed to T>e^ molded against this kind o f surface, so that it answered all practical purposes. W h en it came to developing an investment fo r casting, a different surface had to be found. T h is was done by using a high grade o f plaster, and fine, very fine, silex. Even this did not give the proper surface, because plaster came in direct contact w ith the wax. I tried to line my molds w ith graphite, taking my cue from my early experience in w orking about factories. I did this by trying to spray it with a syringe into the inside o f the m old; but this did not answer, fo r the reason that the delicate corners
would be filled up w ith too much graph ite and it w ould be practically impos sible to get it out again. W h en I conceived the idea o f m ixing a certain proportion o f graphite with the plaster and silex, I found that I could get much more satisfactory castings, because this exceedingly fine graphite in sm all quan tities has a tendency to flow to the sur face and cover the surface before the other m aterials set. T his, you can see, w ill give a u n ifo rm surface o f graphite to the inside o f the mold where it comes in contact with the polished wax. T h e graphite, being unburnable at very high temperatures, does not disintegrate under high heat as does the plaster con ten t; and the plaster proper, being pro tected by a thin layer o f graphite, was in a better condition to receive the high tem perature gold w ithout burning the plaster. N ow my investment has another grade o f graphite put into it, a very coarse grade, which answers the follow ing purpose: These particles of coarse graphite are exceedingly thin, so very thin, in fact, that it is almost impossible to measure them. T h e size and circum ference is about one six teenth inch on the surface, and these coarse grains o f graphite overlap each other, acting like shingles distributed u niform ly through the plaster base. W h ile this graphite has no tendency to make the plaster harder, it does make it tougher, because this shingling effect, running through the whole mass, binds it together and makes the m aterial much tougher, but no harder. Hardness is not the only quality an investment needs. Toughness is o f more importance O th er ingredients are put into this com pound fo r certain specific purposes, and have no particular bearing on the scien tific side. A fte r this investment was developed, I found that, taking into
T a g g a r t — I n v e s t m e n t M a te r ia ls consideration all o f the m inute and acute angles that a w ax pattern m ight have, it was alm ost impossible to get a thick m ix to go into these sharp angles, so I finally made a compound which could be mixed to the consistency of very thick cream and w ould pour and, o f its own accord, run into these fine angles, which always should be painted first with some o f this fine graphite m ix ture that comes to the surface in the mixing bowl. T h is puts a very stable coating o f graphite, plaster and silex on the surface next to the pattern, which makes an ideal surface to cast against. A fte r getting these ingredients properly proportioned, I found that there would be no two batches o f the consist ency obtained by the old methods. So, in order to make this process as scientific as possible, I substituted the weighing device, by means o f which the proportion o f investm ent and w ater were the same each time, instead of w eighing by rule o f thum b, by putting in a little w ater and a little more plaster, etc., since the length o f time required to get through this process in mixing would finally reach a stage at which the setting qualities o f the plaster would mislead the dentist as to the quantity of w ater. By w eighing the ingredients, I got m ore and more u n ifo rm results. W e w ill take up, first, some o f the points that I w ould call intensive salesmanship in praising the virtues o f certain so-called processes. T ak e, fo r instance, an invest m ent that has come out w ithin the last year or so and is designated as “ Knapp’s process,” which consists in putting a coating o f finer m aterial around the wax pattern and letting that set to a certain extent and then surrounding this w ith a coarser investment. W h y it should be called Knapp’s process is a mystery to me. I w ill give D r. Knapp
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credit fo r, possibly, not know ing about w hat I am going to tell you now, and not having any ulterior motive in calling it K napp’s process. I f you w ill look in D r. Goslee’s book on crown and bridge w ork, you w ill see this identical proc ess described in the 1907 and 1910 edi tions. You w ill see the different steps that I took in the early stages o f my w ork. You w ill notice the primary coating put around the wax pattern and the secondary coating surrounding it in the flask. T h is is part o f the so-called Knapp technic. A long with this, D r. Knapp advocates boiling out the wax by fo rm in g steam, which is form ed in the excess w ater that is in the mixture. W hen this gets to a point where it is sufficiently hot to m elt the w ax pattern on the inside o f the mold, by turning the flask upside down, the wax is sup posed to run out with the steam. A good portion o f it does run out, but when the wax pattern is brought up to the tem perature o f this steam, which is at least 212 degrees (tem perature at which w ater boils), that wax is thin enough also to soak into the investment compound. A fte r this, it is dried out to get rid o f the last vestige o f w ater that is in the investment, which is not sufficient to form a visible steam. N ow it seems that the inside o f the mold is perfectly free from wax, but it is not. T h e detrim ental w ax is still inside of the mold, and an excess o f heat will be required to volatilize it. T his, D r. Knapp does not explain; but when he has the moisture driven off by this low heat, he commences to m elt his gold, and by the time his gold is m elted, he has put enough more heat on this investment to burn out the wax and to that extent has driven off the w ater o f crystal lization in the plaster, just as though he had volatilized that wax in the first place.
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O f course, this sounds like a quicker process, but it is not. A few years ago, at one o f our dental clinics, a young man was dow n on the program to show his process fo r m aking gold inlays, in which he said there was no necessity for burning out the w ax; but he uncon sciously did that very thing by the length o f tim e he had to keep his blowpipe on the gold, and before it could be m elted, he had burned out the w ax just as had to be done in the use o f all other invest ments. N ow , as to the advantage o f using coarse m aterial fo r the outside coating which is placed over this pri m ary investm ent, so that it w ill be more porous fo r the air to get out o f the mold! I f this w ax has been volatilized before the gold has been throw n into the m old, the am ount o f air that has to be displaced to let this gold into the mold is so infinitesim ally small that no m atter how fine the investm ent m aterial may be, it is still porous enough to let the air out. F o r instance, when you com mence the m elting o f the gold, the mold is fu ll o f air which is o f room tem per ature. As soon as the heat comes on, this sm all quantity o f air in the mold cavity proper is rarefied, and, in this condition, there is not, possibly, onetenth as much as the original am ount o f air to be got rid o f, and under the pressure o f casting, the air w ill get out. T h is gets rid o f w hat is called air bub bles. T h e gas bubbles that form fro m the residue o f wax are the ones we have to contend w ith. T hese gas bub bles result because the w ax has not been thoroughly volatilized, and w hen the hot gold strikes the rem aining wax, it im m ediately form s into gas pockets. T h e re fo re, leaving the least bit o f wax in the m old by failure to burn it out thoroughly is w hat does the h arm ; not the fa c t th at the air cannot get out of
the mold. By the time the heat has reached the point necessary to volatilize this wax, the plaster o f Paris, which is called the binder, has been disintegrated, and is practically the poorest m aterial that is in the investment m aterial; and this holds good fo r any investment that is made w ith a plaster o f Paris binder. N ow the proportions o f plaster and silex also have a great bearing on the non shrinking qualities o f a good investment. D r. Knapp, in one o f his printed articles, has said that his outer investment con sists o f a coarse grade o f silex mixed w ith fifty per cent plaster. A 50-50 proportion o f plaster and silex, which is the nonshrinkable portion o f an invest m ent, w ill invariably mean shrinkage u nder the heat necessary to m elt the gold. T h e plaster should be as small a pro portion o f the investment as can pos sibly be used and still act as a binder to help hold the' particles o f silex in place. T hese basic principles which I have given you hold good with any in vestment having a plaster o f Paris con tent. C om ing back to the mixing o f these different ingredients, my theory, as possibly you know, is to mix the invest m ent thin enough that it w ill pour, because chemicals have a peculiar habit, which is inherent in all o f them , when the chemical affinities are satisfied; th at is, w hen the powder has all the w ater it needs to make it set, the excess w ater is not utilized in hardening. W henever a liquid and a powder such as we are speaking o f are brought into contact w ith each other, a chemical and mechanical reaction takes place, and a chemical reaction always evolves a gas. N ow , in m ixing the plaster o f Paris in a thick way, these m inute quantities o f chemical gases and the air th at is contained in the w ater and the air th at is contained in the powder are all mixed to
T a g g a r t — I n v e s t m e n t M a te r ia ls gether, and these small bubbles o f gas have not sufficient bouyancy to rise out o f the m ixture, but when the m ixture is made thin and jarred and jolted, you w ill find, by closely w atching the surface o f your m ixture w ith the m ag nifying glass, that the m inute bubbles rise to the surface. T h e jolting o f the investm ent bowl helps to make them rise and also, w hen they come to the surface, it explodes them just as, in our childhood days, w hen someone stamped on the floor, the ja r w ould burst our soap bubbles. N ow these gas bubbles are all in there in a thick m ixture and cannot get out; and, in a thin m ixture, we must take the tim e necessary to get them to the surface, where they will explode, so that there w ill be no gas bubbles w hatever in the investment be fore the flask is filled. W h ile these gas bubbles are rising through the investment m aterial, they are very clannish, and, as soon as one touches the other, they coalesce, and, by the time they reach the surface o f the w ax pattern, they have form ed into quite a sizable gas bubble, which accounts fo r the nodules o f gold to be seen on the surface of the m ajority o f gold inlays. T h e d if ference in time required to get an in vestment m aterial into the proper w orking condition is tim e w ell spent, if our tim e is w orth $5, $10 or $20 an hour, because, i f we have these gold nodules on the inlay, it w ill take a quarter o f an hour or more to get rid o f the nodules on the surface of the casting; whereas, by m ixing our m aterial properly, and avoiding jarrin g our investm ent a fte r it is in the flask, these gold nodules w ill not form . These gold nodules also have a dis agreeable way o f form ing in some vital angle in the filling, and, under these circumstances, we have no guide as to
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how much to bur off in order to get rid o f them and allow the inlay to seat itself properly as the wax did. T h e lack o f fit o f an inlay at the gingival m argin, which occurs in so many fillings, is due not to the fact th at the gold has shrunk or that the wax has contracted as m uch as that the cavity has been poorly prepared and the w ax has never reached the gingival m argin. W hen it comes to the m elting o f the gold that is to be forced into the cavity mold, a fte r the wax has been burned out, the flask should be allow ed to come back to room tem perature, and then the gold should be melted as rapidly as possible. T o do this, I would advise you to get some kind o f a high heat blowpipe and not use a compressed air or blast blow pipe because this again brings the flask up to a high temperature and a high tem perature means expansion. But use these high tem perature flames in such a way that the flame w ill be a soft rather than the fast and furious one o f city gas, acetylene or oxygen. These unreasonably high tem peratures burn out the alloys in alloyed gold .which cost from 25 to 50 cents per penny w eight more than pure gold. H igh tem peratures burn out these alloys, and when the casting is finished, we have not the alloy fo r which we paid such a high price. In other words, dentists are spending hundreds of thou sands o f dollars fo r alloys that never get into the filling because they are volatilized into the air. I think the theory that the wax contracts or, as some say, expands is largely exploded. I w ill adm it that wax in a molten state, w hen poured into a m old, shrinks m a terially, but that does not signify that, w hen it comes back to the body tem pera ture, it w ill shrink, as the harder in gredients in a w ax act as a binder,
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holding the wax and keeping it from changing its form . W a x should be used at as low a tem perature as we can pos sibly use it and still get it into its place under firm and steady pressure, not let ting up on this pressure at any stage, putting the finger or thum b on a second or th ird time, as some do, in other words trying to knead it into place, because each tim e the pressure is taken oif and then renewed, the w ax is pushed into a new position, and w hen we finally get through w ith the pressure and have trim m ed and burnished the wax to the m argins as it should be, we have only a m arginal fit, as the in side o f the pattern has been shifted from one side to the other and can never make a true frictional fit; and yet the m arginal fit may look all right. W e should be a little bit shy o f men who have tw o or three columns o f statistics showing experiments under different temperatures. These statistics are meaningless, fo r the reason that the tem peratures given are beyond anything required in practice, and they have no bearing w hatever on the subject. In the early days o f casting, when I advo cated burning out the w ax at as low a tem perature as possible, which, at that time, I fe lt was not over from 250 to 280 degrees, W eston A. Price came out w ith a long and, apparently, exhaustive article, containing many statistics, and he based his conclusion as to my invest m ent m aterial on how it behaved a t a tem perature o f 1,000 degrees. A few years a fte r D r. Price had w ritten his article on investments, he came out trying to prove in a scientific way that wax w ould shrink in m aking patterns. T a k in g the statistics as they looked on paper and not as real facts, it looked as though he were m aking a point, and it did impress a great many dentists, and
numberless articles have been w ritten on the subject o f “ C ontraction in W a x .” T w o years a fte r these articles had been printed, D r. Price came to my office and told me that, a fte r fu rth er investi gation, he had found that the shrinkage o f w ax in inlay work was not w ell founded and was o f no importance. I w ant to make the point that these false scientific articles are very detrim ental to progress in art, especially when they come from prom inent men, fo r the rea son that the dentist, finding that his in lays do not fit, falls back on w hat he thinks is the broad theory that w ax changes its form so much, and thus ac counts fo r his poorly fitting inlays and, in this way, excuses him self, or at least salves his conscience into the belief that his fillings are as good as can be made anyhow by this process. W hereas, if he were encouraged by constructive science, there would be a better personal equation in his work. I f one man can do this w ork perfectly w ith any o f these invest ments on the m arket, any other man can do the same if he w ill do it the same way and take the same care in his technic. I have adm ired D r. Price fo r being m an enough to come out and tell me that he had made a mistake. These pseudoscientific papers are ridiculous. A m aterial should be tested under the conditions in which it is to be used and not in any outlandish way, with ex pectation that it w ill stand up under conditions which never exist in the real use o f the materials. W h a t I am trying to emphasize as thoroughly as I can is that one must first learn one’s business thoroughly. W h en these materials are used, they must be used absolutely ac cording to the directions that accom pany them and under no conditions m ust these be varied. O nce having learned this, one should strike out boldly
P a te r s o n — E s ta b li s h i n g th e P r o s th e tic C u r v e s o f O c c lu s io n fo r self-expression; that is, try to im prove on method and m aterial, because the more we know o f many things, the better we are able to appreciate every separate thing. T o know only a few things, even though we know these w ell, keeps one in a very small circle. O th er thoughts which I could in corporate in a paper may be brought out to better advantage in an open discus sion, and there has been enough m ate rial given here to bring out an exhaustive discussion. M y adm onition to you is not
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to w ander away from the subject m at ter. Discussions are quite apt to lead the discussers into other fields which have nothing to do with the papers that have been read. I rem em ber at one o f our early meetings in the Illinois State D en tal Society, a paper was read on the subject o f operative dentistry. I t was a good and instructive paper, but before the discussion was over, it wound up w ith a debate on the best kind o f an articulator to be used in making fu ll dentures.
ESTABLISHING THE PROSTHETIC CURVES OF OCCLUSION ON THE BITE BLOCKS IN FULL DENTURE CONSTRUCTION*
By ALEXANDER HORN PATERSON, D.D.S., F.A.C.D., Baltimore, Maryland T
O study any subject scientifically, it is necessary to analyze all o f the parts that constitute the whole, and, from the analysis, determ ine the basis upon which the subject is built. H aving determ ined the basis, each part in the reconstruction w ill assume a de finite place and the rearrangem ent o f the parts w ill be the method by which an intim ate knowledge o f the construc tion is obtained. In dentistry, as w ell as in other allied professions, the foregoing statem ent would apply only to a sm all group, classified as investigators, whose minds would dem and, in the study o f any mechanism or subject, a detailed know l edge. T h e great mass o f individuals do * R e ad b e fo re th e Section on F u l l D e n tu re P ro sth esis a t th e Seventh I n te rn a tio n a l D e n ta l C o n g ress, P h ila d e lp h ia , P a ., A u g . 24, 1926.
not possess minds curious enough to stim ulate the desire necessary to investigate but are satisfied to accept the fundam en tals established by the scientist in his investigation. T h is is as it should b e; all cannot be investigators because the talent or m ental vision necessary to make out a successful investigator is given to few ; nor is it necessary that the dentist be an investigator, provided the mechanism or subject presented fo r his consideration by the investigator is easily compre hended and simple to apply. T he den tist, ordinarily, is not concerned in the theory relative to the construction o f any o f the appliances used, but is vitally interested in the results obtained. In developing the fundam entals necessary fo r a complete understanding o f the method by which the curves o f occlusion are established and presented
By WILLIAM H. TAGGART, D.D.S., Chicago, Illinois T
H E R E has been a good deal o f controversy the last few years about investm ent m aterials, and my talk to you tonight w ill consist rath er in explaining the basic principles o f in vestm ent materials and the method o f handling them than in trying to convert you to using any one kind o f investment. O ne o f the principal ingredients in an investm ent m aterial is plaster o f Paris, which comes from mines, being chipped and blasted out just as ore is, in the form o f m edium hard rock, called gypsum or calcium sulphate. T h is ore is crushed to any degree o f fineness de sired, and is put in large cauldrons, where, by constant stirring and rotating o f these cauldrons, it is brought up to a high degree o f heat in order to drive off the w ater o f crystallization. T h e gypsum is ground to all grades o f fineness needed, and these different grades are segregated by sifting. Even in the coarser grades o f plaster, there is a large percentage o f fine and superfine particles o f plaster. T h e re fo re, the degree o f fineness o f plaster in an investm ent is not so much o f an elem ent in allow ing the air to escape from a m old as some have tried to imply, as the high percentage o f fine or, as it is called in the cem ent trade, “ neat” m aterial is sufficient to fill the interstices between the coarser grains, this tending to make a m aterial just * R e a d b e fo re th e C h ica g o D e n ta l Society, O ct. 19, 1926. J o u r . A . D . A . , J u ly , 1927
as impervious to air as though it were all made o f fine m aterial. T h e reason the coarser grades o f plaster are not more in use in investment m aterials is that they do not lend themselves to m anipulation so readily as the finer grades do. I f we mix a batch o f coarse plaster thin enough to pour on a glass surface and another batch o f fine plaster and pour on a surface, we find that one surface has about the same degree o f smoothness and imperviousness to air as the other; in other words, the “ neat” m aterial has a tendency to settle to the surface which it touches. W h en this plaster is first burned and cooled, it sets very rapidly; in fact, too rapidly for any dental use. T h is tendency is con trolled in the factories by taking certain known quantities o f plaster that has already been set and grinding it w ithout fu rth e r burning. T his, in a crude way, controls the time in which the plaster sets. W hen this new plaster has w ater mixed w ith it, the w ater o f crystallization which the original gypsum rock had is restored, and it is brought back to a crystalline form , but is never so hard as the original gypsum rock was. T h e other principal ingredient in an invest m ent m aterial is silex. T h e principle here is the same as regards the d if fe re n t grades o f fineness as in plaster. T h ere are all grades o f fineness when this is crushed. I may explain this a little 1190
T a g g a r t — I n v e s t m e n t M a t e r ia l s more plainly by saying that recently I had some m aterial which I bought fo r 90 mesh, which only means th at the largest particles are 90 mesh, the rest going through all different grades down to impalpable powder. I found that out o f this 90 mesh m aterial, 50 per cent would go through a 200 mesh sieve; and on e-h alf o f the 50 per cent, or onefourth o f the original am ount, w ould go through a 600 mesh, and there were all grades o f fineness up to possibly the thousandths. T hese are the tw o prin cipal ingredients in any investm ent m aterial. N ow these m aterials, a fte r being m anufactured, are quite hydro scopic; th a t is, they have a tendency to absorb moisture, and in just the pro portion as the m aterial is allow ed to absorb moisture, it w ill change its setting qualities, or it w ill change the length o f tim e which it takes to set. Sometimes plaster and silex, being crude materials, are allow ed to stand on the railway p latfo rm in the drenching rain, and the barrels become w ater soaked, the plaster eventually absorbing more or less o f this moisture, which accounts, in a great measure, fo r the fact that two shipments o f plaster from the same original batch seldom w ork exactly alike, as you have all noticed, and as all dealers have noticed when the dentist comes back claim ing th at the plaster does not set the same as the last that he bought. Any other ingredients mixed w ith these two m aterials are usually trade secrets. In speaking o f these different investment materials, and the principle o f their use, it is hard fo r me, as a m anufacturer of investm ent m aterial, to keep from using the personal pronoun, as though I were trying to persuade you to use my m ate rial. Such is not the intention, and when I allude to my m aterial or any other m aterial, I shall mention the names
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w ithout an idea o f criticizing the m a terial o f others or advertising my own. I hope you w ill understand this thor oughly. W einstein, in one o f his ex haustive and not very scientific articles published in one o f the dental journals, made the rem ark that he and his as sociates had found that graphite is not o f any use in an investment material. H e did not mention my name, but m ine is the only investm ent that has graphite in it. In talking to a profes sional audience on a professional subject and as one o f the profession, certain ethics are required, but as all o f these m en are now, regardless o f their titles, m anufacturers, I may as w ell speak openly as to the qualities o f the different m aterials. As W einstein has made the rem ark that he has found graphite o f no use in an investment m aterial, I shall explain to you why I have put it in mine. W h en I was a boy, from the time I was 8 until I was 18 years o f age, I was always “ hanging around” factories and machine shops. L iving in a com paratively small tow n, being acquainted w ith practically all o f the factory men and machinists, and not m aking myself a nuisance, I was allow ed to help w ith little things about the work, and one o f my self-imposed duties was to take a chunk o f charcoal, place it in a loose cotton bag, and w ith a board or ham m er, crush it into a fine powder. Some o f it w ould be exceedingly fine. As I have told you before, any crushed m aterial w ill have all grades o f fineness. W hen the m old was open and the pattern taken out, this bag o f crushed charcoal w ould be shaken and the m old would be sprinkled all over w ith it, so as to make a smooth casting, as it seemed to fill up the meshes in the grains o f sand and also made a good surface against which the hot m etal w ould run. T h is was on
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coarse castings. Y ou possibly have noticed that, on stove work, the lids and all parts o f stove castings are very smooth and fined grained. In the case o f these stove castings, instead o f using charcoal, which was only used fo r the coarser grade o f work, they sub stituted fine graphite, sprinkling it on the surfaces o f the mold. O n surfaces they could reach w ith their trowels, the workers w ould w ork this fine graph ite into the sand surface. T h is is why the stove castings are the smoothest of all castings that we see. W h en the time came fo r me to develop my investing m aterial to carry out my ideas in casting gold, the only m aterials we had at hand were not m anufactured products but self-m ade m aterials, consisting in some cases o f fine sand and plaster; in others, o f marble dust and plaster, and in others, o f pumice stone and plaster. These m aterials were all right fo r the purpose fo r which they were intended, except that they were made by rule o f thumb. T h e m anufacturer w ould take some plaster and some sand, or some pumice stone or some m arble dust, never caring much about the proportion, and would use it fo r the solder work. O f course, no gold was supposed to T>e^ molded against this kind o f surface, so that it answered all practical purposes. W h en it came to developing an investment fo r casting, a different surface had to be found. T h is was done by using a high grade o f plaster, and fine, very fine, silex. Even this did not give the proper surface, because plaster came in direct contact w ith the wax. I tried to line my molds w ith graphite, taking my cue from my early experience in w orking about factories. I did this by trying to spray it with a syringe into the inside o f the m old; but this did not answer, fo r the reason that the delicate corners
would be filled up w ith too much graph ite and it w ould be practically impos sible to get it out again. W h en I conceived the idea o f m ixing a certain proportion o f graphite with the plaster and silex, I found that I could get much more satisfactory castings, because this exceedingly fine graphite in sm all quan tities has a tendency to flow to the sur face and cover the surface before the other m aterials set. T his, you can see, w ill give a u n ifo rm surface o f graphite to the inside o f the mold where it comes in contact with the polished wax. T h e graphite, being unburnable at very high temperatures, does not disintegrate under high heat as does the plaster con ten t; and the plaster proper, being pro tected by a thin layer o f graphite, was in a better condition to receive the high tem perature gold w ithout burning the plaster. N ow my investment has another grade o f graphite put into it, a very coarse grade, which answers the follow ing purpose: These particles of coarse graphite are exceedingly thin, so very thin, in fact, that it is almost impossible to measure them. T h e size and circum ference is about one six teenth inch on the surface, and these coarse grains o f graphite overlap each other, acting like shingles distributed u niform ly through the plaster base. W h ile this graphite has no tendency to make the plaster harder, it does make it tougher, because this shingling effect, running through the whole mass, binds it together and makes the m aterial much tougher, but no harder. Hardness is not the only quality an investment needs. Toughness is o f more importance O th er ingredients are put into this com pound fo r certain specific purposes, and have no particular bearing on the scien tific side. A fte r this investment was developed, I found that, taking into
T a g g a r t — I n v e s t m e n t M a te r ia ls consideration all o f the m inute and acute angles that a w ax pattern m ight have, it was alm ost impossible to get a thick m ix to go into these sharp angles, so I finally made a compound which could be mixed to the consistency of very thick cream and w ould pour and, o f its own accord, run into these fine angles, which always should be painted first with some o f this fine graphite m ix ture that comes to the surface in the mixing bowl. T h is puts a very stable coating o f graphite, plaster and silex on the surface next to the pattern, which makes an ideal surface to cast against. A fte r getting these ingredients properly proportioned, I found that there would be no two batches o f the consist ency obtained by the old methods. So, in order to make this process as scientific as possible, I substituted the weighing device, by means o f which the proportion o f investm ent and w ater were the same each time, instead of w eighing by rule o f thum b, by putting in a little w ater and a little more plaster, etc., since the length o f time required to get through this process in mixing would finally reach a stage at which the setting qualities o f the plaster would mislead the dentist as to the quantity of w ater. By w eighing the ingredients, I got m ore and more u n ifo rm results. W e w ill take up, first, some o f the points that I w ould call intensive salesmanship in praising the virtues o f certain so-called processes. T ak e, fo r instance, an invest m ent that has come out w ithin the last year or so and is designated as “ Knapp’s process,” which consists in putting a coating o f finer m aterial around the wax pattern and letting that set to a certain extent and then surrounding this w ith a coarser investment. W h y it should be called Knapp’s process is a mystery to me. I w ill give D r. Knapp
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credit fo r, possibly, not know ing about w hat I am going to tell you now, and not having any ulterior motive in calling it K napp’s process. I f you w ill look in D r. Goslee’s book on crown and bridge w ork, you w ill see this identical proc ess described in the 1907 and 1910 edi tions. You w ill see the different steps that I took in the early stages o f my w ork. You w ill notice the primary coating put around the wax pattern and the secondary coating surrounding it in the flask. T h is is part o f the so-called Knapp technic. A long with this, D r. Knapp advocates boiling out the wax by fo rm in g steam, which is form ed in the excess w ater that is in the mixture. W hen this gets to a point where it is sufficiently hot to m elt the w ax pattern on the inside o f the mold, by turning the flask upside down, the wax is sup posed to run out with the steam. A good portion o f it does run out, but when the wax pattern is brought up to the tem perature o f this steam, which is at least 212 degrees (tem perature at which w ater boils), that wax is thin enough also to soak into the investment compound. A fte r this, it is dried out to get rid o f the last vestige o f w ater that is in the investment, which is not sufficient to form a visible steam. N ow it seems that the inside o f the mold is perfectly free from wax, but it is not. T h e detrim ental w ax is still inside of the mold, and an excess o f heat will be required to volatilize it. T his, D r. Knapp does not explain; but when he has the moisture driven off by this low heat, he commences to m elt his gold, and by the time his gold is m elted, he has put enough more heat on this investment to burn out the wax and to that extent has driven off the w ater o f crystal lization in the plaster, just as though he had volatilized that wax in the first place.
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O f course, this sounds like a quicker process, but it is not. A few years ago, at one o f our dental clinics, a young man was dow n on the program to show his process fo r m aking gold inlays, in which he said there was no necessity for burning out the w ax; but he uncon sciously did that very thing by the length o f tim e he had to keep his blowpipe on the gold, and before it could be m elted, he had burned out the w ax just as had to be done in the use o f all other invest ments. N ow , as to the advantage o f using coarse m aterial fo r the outside coating which is placed over this pri m ary investm ent, so that it w ill be more porous fo r the air to get out o f the mold! I f this w ax has been volatilized before the gold has been throw n into the m old, the am ount o f air that has to be displaced to let this gold into the mold is so infinitesim ally small that no m atter how fine the investm ent m aterial may be, it is still porous enough to let the air out. F o r instance, when you com mence the m elting o f the gold, the mold is fu ll o f air which is o f room tem per ature. As soon as the heat comes on, this sm all quantity o f air in the mold cavity proper is rarefied, and, in this condition, there is not, possibly, onetenth as much as the original am ount o f air to be got rid o f, and under the pressure o f casting, the air w ill get out. T h is gets rid o f w hat is called air bub bles. T h e gas bubbles that form fro m the residue o f wax are the ones we have to contend w ith. T hese gas bub bles result because the w ax has not been thoroughly volatilized, and w hen the hot gold strikes the rem aining wax, it im m ediately form s into gas pockets. T h e re fo re, leaving the least bit o f wax in the m old by failure to burn it out thoroughly is w hat does the h arm ; not the fa c t th at the air cannot get out of
the mold. By the time the heat has reached the point necessary to volatilize this wax, the plaster o f Paris, which is called the binder, has been disintegrated, and is practically the poorest m aterial that is in the investment m aterial; and this holds good fo r any investment that is made w ith a plaster o f Paris binder. N ow the proportions o f plaster and silex also have a great bearing on the non shrinking qualities o f a good investment. D r. Knapp, in one o f his printed articles, has said that his outer investment con sists o f a coarse grade o f silex mixed w ith fifty per cent plaster. A 50-50 proportion o f plaster and silex, which is the nonshrinkable portion o f an invest m ent, w ill invariably mean shrinkage u nder the heat necessary to m elt the gold. T h e plaster should be as small a pro portion o f the investment as can pos sibly be used and still act as a binder to help hold the' particles o f silex in place. T hese basic principles which I have given you hold good with any in vestment having a plaster o f Paris con tent. C om ing back to the mixing o f these different ingredients, my theory, as possibly you know, is to mix the invest m ent thin enough that it w ill pour, because chemicals have a peculiar habit, which is inherent in all o f them , when the chemical affinities are satisfied; th at is, w hen the powder has all the w ater it needs to make it set, the excess w ater is not utilized in hardening. W henever a liquid and a powder such as we are speaking o f are brought into contact w ith each other, a chemical and mechanical reaction takes place, and a chemical reaction always evolves a gas. N ow , in m ixing the plaster o f Paris in a thick way, these m inute quantities o f chemical gases and the air th at is contained in the w ater and the air th at is contained in the powder are all mixed to
T a g g a r t — I n v e s t m e n t M a te r ia ls gether, and these small bubbles o f gas have not sufficient bouyancy to rise out o f the m ixture, but when the m ixture is made thin and jarred and jolted, you w ill find, by closely w atching the surface o f your m ixture w ith the m ag nifying glass, that the m inute bubbles rise to the surface. T h e jolting o f the investm ent bowl helps to make them rise and also, w hen they come to the surface, it explodes them just as, in our childhood days, w hen someone stamped on the floor, the ja r w ould burst our soap bubbles. N ow these gas bubbles are all in there in a thick m ixture and cannot get out; and, in a thin m ixture, we must take the tim e necessary to get them to the surface, where they will explode, so that there w ill be no gas bubbles w hatever in the investment be fore the flask is filled. W h ile these gas bubbles are rising through the investment m aterial, they are very clannish, and, as soon as one touches the other, they coalesce, and, by the time they reach the surface o f the w ax pattern, they have form ed into quite a sizable gas bubble, which accounts fo r the nodules o f gold to be seen on the surface of the m ajority o f gold inlays. T h e d if ference in time required to get an in vestment m aterial into the proper w orking condition is tim e w ell spent, if our tim e is w orth $5, $10 or $20 an hour, because, i f we have these gold nodules on the inlay, it w ill take a quarter o f an hour or more to get rid o f the nodules on the surface of the casting; whereas, by m ixing our m aterial properly, and avoiding jarrin g our investm ent a fte r it is in the flask, these gold nodules w ill not form . These gold nodules also have a dis agreeable way o f form ing in some vital angle in the filling, and, under these circumstances, we have no guide as to
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how much to bur off in order to get rid o f them and allow the inlay to seat itself properly as the wax did. T h e lack o f fit o f an inlay at the gingival m argin, which occurs in so many fillings, is due not to the fact th at the gold has shrunk or that the wax has contracted as m uch as that the cavity has been poorly prepared and the w ax has never reached the gingival m argin. W hen it comes to the m elting o f the gold that is to be forced into the cavity mold, a fte r the wax has been burned out, the flask should be allow ed to come back to room tem perature, and then the gold should be melted as rapidly as possible. T o do this, I would advise you to get some kind o f a high heat blowpipe and not use a compressed air or blast blow pipe because this again brings the flask up to a high temperature and a high tem perature means expansion. But use these high tem perature flames in such a way that the flame w ill be a soft rather than the fast and furious one o f city gas, acetylene or oxygen. These unreasonably high tem peratures burn out the alloys in alloyed gold .which cost from 25 to 50 cents per penny w eight more than pure gold. H igh tem peratures burn out these alloys, and when the casting is finished, we have not the alloy fo r which we paid such a high price. In other words, dentists are spending hundreds of thou sands o f dollars fo r alloys that never get into the filling because they are volatilized into the air. I think the theory that the wax contracts or, as some say, expands is largely exploded. I w ill adm it that wax in a molten state, w hen poured into a m old, shrinks m a terially, but that does not signify that, w hen it comes back to the body tem pera ture, it w ill shrink, as the harder in gredients in a w ax act as a binder,
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holding the wax and keeping it from changing its form . W a x should be used at as low a tem perature as we can pos sibly use it and still get it into its place under firm and steady pressure, not let ting up on this pressure at any stage, putting the finger or thum b on a second or th ird time, as some do, in other words trying to knead it into place, because each tim e the pressure is taken oif and then renewed, the w ax is pushed into a new position, and w hen we finally get through w ith the pressure and have trim m ed and burnished the wax to the m argins as it should be, we have only a m arginal fit, as the in side o f the pattern has been shifted from one side to the other and can never make a true frictional fit; and yet the m arginal fit may look all right. W e should be a little bit shy o f men who have tw o or three columns o f statistics showing experiments under different temperatures. These statistics are meaningless, fo r the reason that the tem peratures given are beyond anything required in practice, and they have no bearing w hatever on the subject. In the early days o f casting, when I advo cated burning out the w ax at as low a tem perature as possible, which, at that time, I fe lt was not over from 250 to 280 degrees, W eston A. Price came out w ith a long and, apparently, exhaustive article, containing many statistics, and he based his conclusion as to my invest m ent m aterial on how it behaved a t a tem perature o f 1,000 degrees. A few years a fte r D r. Price had w ritten his article on investments, he came out trying to prove in a scientific way that wax w ould shrink in m aking patterns. T a k in g the statistics as they looked on paper and not as real facts, it looked as though he were m aking a point, and it did impress a great many dentists, and
numberless articles have been w ritten on the subject o f “ C ontraction in W a x .” T w o years a fte r these articles had been printed, D r. Price came to my office and told me that, a fte r fu rth er investi gation, he had found that the shrinkage o f w ax in inlay work was not w ell founded and was o f no importance. I w ant to make the point that these false scientific articles are very detrim ental to progress in art, especially when they come from prom inent men, fo r the rea son that the dentist, finding that his in lays do not fit, falls back on w hat he thinks is the broad theory that w ax changes its form so much, and thus ac counts fo r his poorly fitting inlays and, in this way, excuses him self, or at least salves his conscience into the belief that his fillings are as good as can be made anyhow by this process. W hereas, if he were encouraged by constructive science, there would be a better personal equation in his work. I f one man can do this w ork perfectly w ith any o f these invest ments on the m arket, any other man can do the same if he w ill do it the same way and take the same care in his technic. I have adm ired D r. Price fo r being m an enough to come out and tell me that he had made a mistake. These pseudoscientific papers are ridiculous. A m aterial should be tested under the conditions in which it is to be used and not in any outlandish way, with ex pectation that it w ill stand up under conditions which never exist in the real use o f the materials. W h a t I am trying to emphasize as thoroughly as I can is that one must first learn one’s business thoroughly. W h en these materials are used, they must be used absolutely ac cording to the directions that accom pany them and under no conditions m ust these be varied. O nce having learned this, one should strike out boldly
P a te r s o n — E s ta b li s h i n g th e P r o s th e tic C u r v e s o f O c c lu s io n fo r self-expression; that is, try to im prove on method and m aterial, because the more we know o f many things, the better we are able to appreciate every separate thing. T o know only a few things, even though we know these w ell, keeps one in a very small circle. O th er thoughts which I could in corporate in a paper may be brought out to better advantage in an open discus sion, and there has been enough m ate rial given here to bring out an exhaustive discussion. M y adm onition to you is not
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to w ander away from the subject m at ter. Discussions are quite apt to lead the discussers into other fields which have nothing to do with the papers that have been read. I rem em ber at one o f our early meetings in the Illinois State D en tal Society, a paper was read on the subject o f operative dentistry. I t was a good and instructive paper, but before the discussion was over, it wound up w ith a debate on the best kind o f an articulator to be used in making fu ll dentures.
ESTABLISHING THE PROSTHETIC CURVES OF OCCLUSION ON THE BITE BLOCKS IN FULL DENTURE CONSTRUCTION*
By ALEXANDER HORN PATERSON, D.D.S., F.A.C.D., Baltimore, Maryland T
O study any subject scientifically, it is necessary to analyze all o f the parts that constitute the whole, and, from the analysis, determ ine the basis upon which the subject is built. H aving determ ined the basis, each part in the reconstruction w ill assume a de finite place and the rearrangem ent o f the parts w ill be the method by which an intim ate knowledge o f the construc tion is obtained. In dentistry, as w ell as in other allied professions, the foregoing statem ent would apply only to a sm all group, classified as investigators, whose minds would dem and, in the study o f any mechanism or subject, a detailed know l edge. T h e great mass o f individuals do * R e ad b e fo re th e Section on F u l l D e n tu re P ro sth esis a t th e Seventh I n te rn a tio n a l D e n ta l C o n g ress, P h ila d e lp h ia , P a ., A u g . 24, 1926.
not possess minds curious enough to stim ulate the desire necessary to investigate but are satisfied to accept the fundam en tals established by the scientist in his investigation. T h is is as it should b e; all cannot be investigators because the talent or m ental vision necessary to make out a successful investigator is given to few ; nor is it necessary that the dentist be an investigator, provided the mechanism or subject presented fo r his consideration by the investigator is easily compre hended and simple to apply. T he den tist, ordinarily, is not concerned in the theory relative to the construction o f any o f the appliances used, but is vitally interested in the results obtained. In developing the fundam entals necessary fo r a complete understanding o f the method by which the curves o f occlusion are established and presented