Wax and Investing*

Wax and Investing*

W A X A N D INVESTING By L. B. MORRIS, D.D .S., Denver, Colorado O paraphrase Ruskin: “ T h e tru th must have revealed itself to T a g ­ g art in pr...

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W A X A N D INVESTING By L. B. MORRIS, D.D .S., Denver, Colorado

O paraphrase Ruskin: “ T h e tru th must have revealed itself to T a g ­ g art in proportion to his patience and knowledge, discovered itself kindly to his pleading and led him , as it was discovered, into deeper truths.” T h e promised im provem ent by fa ith ­ fu l research workers has, in every in ­ stance, failed o f realization in practice. In justice to our patients, we m ust fo r the present acclaim T a g g a rt as the m aster, and seek a better understanding o f his lif e ’s work. Precise knowledge o f the T a g g a rt technic is now im pera­ tive as has been revealed by the increas­ ing use o f radiogram s by careful technicians in detecting im perfections in recently inserted cast gold inlays. I t is my experience and profound be­ lie f that accuracy is in direct ratio to adherence to the original technic. T h e w ider the deviation from the original, the poorer the results. A ll heralded and o ften widely accepted improvements have been nothing more nor less than a nearer approach to the original. A

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*Read before the Section on Operative Dentistry at the Seventh International Dental Congress, Philadelphia, Pa., Aug. 26, 1926. *T he papers o f Drs. Morris and Gates, to­ gether with a paper by W. D . N. Moore, published in the November issue, were pre­ sented as a symposium on “Cast Gold Inlays.” Jour, A . D. A M a r c / i } 1928

different label does not alter the fact. T hese improvements were either re­ discoveries or a conscious or unconscious reverting to basic principles as given to us by the inventor. F o r each im perfection or failure, we are prone to place the blame on the accessories, while, in fact, the unhappy results so often obtained can always be traced to failure to follow the original technic. T h is is due either to careless­ ness or to failure to understand the physical facts involved in the process; and these basic physical facts, in my observation, have never been tran ­ scended. Regardless o f the claim , it has never, to my knowledge, been demonstrated that the fundam ental principles have ever been improved. T h e fact that hosts o f other m en, have, as I, fo r nineteen years earnestly endeavored to improve the process, should stand as an exceptional tribute to the scientific genius o f the inventor. Those who claim to have im proved the technic make their bow to the profession and, in w hat seems mock modesty, imply that the original process was a crude conception, but it has rem ained to their genius to refine the technic. T h e close follow ers o f the T a g g a rt technic never fail to detect the fallacy o f such claims.

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M orris— W a x and Investing Seldom, i f ever, in the history o f den­ tistry has a process so successfully w ith ­ stood attem pts tow ard im provement. T h e w ax should possess the property o f copying acute angles. I t should be sufficiently hard at open m outh tem ­ perature to prevent distortion during re­ moval fro m the cavity; capable o f being added to under moisture, and suffi­ ciently hard to allow o f heavy pressure in the cavity. I t should also show a m inim um am ount o f expansion and con­ traction under tem perature changes; carve w ithout flaking or dragging; re­ tain its fo rm during the interim be­ tween the taking o f the pattern and the time o f investing; burn out w ithout leaving a residue, and leave the m old unaffected. W ith in the scope o f my knowledge, only the T a g g a rt wax meets these re­ quirements. T A K IN G T H E W AX P A T T E R N

T h e portion o f wax which is to copy the acute angles should be o f u n ifo rm tem perature. A cone o f w ax is im ­ mersed in w ater at about 140 F . until it becomes light green. T h is is removed from the w ater and another cone o f sim ilar size is held in the flame and the end brought to the liquid state. T h is is pressed against the flat end o f the un ifo rm ally softened cone to which it adheres. W h en the softened cone returns to the original green, we have one portion o f w ax in the correct state o f softness fo r taking the pattern, w hile the added cone is sufficiently hard to act as a plunger. T h e original cone, before im m ersing in the w ater bath, is trim m ed to such

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Wedged-shaped proportions that there w ill be no resistance m et on the lateral walls. T hus, our first resistance is m et at the gingival w all, or base o f the cavity. I f there is an adjoining tooth, a suitable thin hand m atrix may be in­ serted between the teeth extending from just gingivally to the contact point to about 1 m m . beyond the gingival bevel. T h is prevents the w ax from flowing into the interproxim al space, holding the pressure tow ard the cavity and per­ m itting an accurate impression o f the contact point on the proxim ating tooth. T h e wax is forced into the cavity w ith all the force o f the thum b and index finger. T h e pressure should be heavy and constant till the w ax reaches open m outh tem perature. T h is long continued heavy pressure separates the teeth, and w ill, when the casting is completed, give correct tightness o f con­ tact; also, we shall have the correct location o f the contact point for these teeth. I f such pressure is not m ain­ tained until open mouth tem perature is reached, the teeth w ill not be sufficiently separated fo r tightness o f contact. T o add wax to the contact, or later, flow on solder, is to make only a crude guess at the correct location and tig h t­ ness o f contact. I t is an im m utable law o f physics that w ax expands w hen heated and contracts when cooled. T h e cavity is fu ll o f expanded wax, when wax is first inserted, and unless the needed pressure is m aintained, there w ill be a discrepancy at the gingival m argin, due to the contraction as the w ax cools, but w hen the correct pressure is m aintained fo r a sufficient time, more w ax w ill be

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forced into the cavity and w ill com ­ pensate fo r the shrinkage. T h e frequent shortage at the gin­ gival m argin is, in nearly every case, due to failure to get the w ax to correct position, or to shrinkage as it cools, or to both. W h en the entire technic is correctly employed, the frequent gin­ gival discrepancy is never present. O u r pattern should extend about 0.5 mm. beyond the gingival bevel. T h is gives a surplus, and when the pattern is re­ produced in gold, it may be disked to sym m etrical lines and w ill accurately register w ith the cavity margins. W h e n the cavity is correctly prepared and this technic employed, the shrink­ age o f the gold is o f no consequence in the inlay art. I realize that this is a dogm atic statem ent and w ill not be widely accepted; nevertheless, it is a demonstrable fact. A m ultitude of men may declare that a certain feat cannot be accomplished, but i f one m an suc­ ceeds in its accomplishment, then his premise is proved, the m ultitude to the contrary notw ithstanding. A seemingly accurate occlusion is now obtained and the approximate sur­ faces fairly w ell contoured. W h en ob­ taining occlusion, thè stress o f opposing cusps depresses the wax, which, w hen the pressure is released, returns in some degree to its original form . T h is may be obviated by allow ing the patient to chew on a sm all piece o f rubber dam o f m edium thickness. T h is w ill shave off a sufficient quantity to compensate fo r the am ount the wax returns to original form . By this method, the inlay, when inserted, should require no adjustm ent due to the usual plus con­ ditions.

T h e occlusion is now carved to cor­ rect anatomic form , and the surface smoothed w ith cotton carrying a very small am ount o f oil o f cajuput. T h is pattern, when correctly repro­ duced in gold, w ill require no polishing and no occlusal adjustm ent. W hen occlusal polishing is required, much of the detail o f the finer occlusal grooves is necessarily obliterated. T h e final approximal adjustm ent may be done more easily a fte r the wax has been reproduced in gold. A t this stage, the casting w ill have a saucer-shaped depression at the contact point. W ax, being fragile, is not easily carved in the hands. T h e gold is now disked, the operator rounding away from the contact point, but never touching the contact, to correct anatom ic form . T h e periphery is reduced to symmetrical lines and should register w ith the bevel o f our cavity m argins. T h is bevel, when correctly made, w ill be quite definitely marked in the casting. T h e question is o ften raised, “ W hy does the gold do all its shrinking at the gingival m argin ? ” I t does not do so. T h e trouble starts with a wax pattern which is already short at this surface, and no subsequent procedure can cor­ rect this discrepancy. T h e shrinkage of gold is interm olecular and absolutely uniform . T im e does not perm it of presenting physical laws in support o f this statem ent, which is quite generally refuted by the dental profession. Suffice it to say that physicists whom I con­ sulted are a unit in declaring this to be fact. W hen the accessories that I use are employed, any fa u lt whatsoever in the inlay is always due to faulty technic,

M orris— W a x and Investing and not to the m aterials used. W h en we do the same thing the same way every time, we shall always arrive at the same result. A n approximoclusal pattern m ay be removed w ith an exploring tine, in­ serted buccally or lingually to the con­ tact point. In case o f a mesioclusodistal inlay, tw o tines should be inserted in directly opposite positions to each other and the pattern removed w ithout distortion. T h e sprue fo rm er should be inserted at the same point as was the tine fo r rem oval. T h u s, neither the contact point nor the occlusal carving w ill be in terfered w ith. T h e size o f the sprue fo rm er should be somewhat in proportion to the size o f the w ax pattern, varying fro m 14 to 20 gage. A sprue fo rm er larger than 14 gage may allow a globule o f gold to drop into the sprue and block the opening, which prevents a success­ fu l casting. T h e length o f the sprue form er should in every case be threesixteenths inch from the w ax to the nearest point o f the crucible form er. T h e reasons fo r this length o f sprue w ill be om itted here; suffice it to say, they are im portant. T h e crucible fo rm er should be sym­ metrical. I f uneven, the hot spinning gold w ill dash about and may cause a globule o f gold to separate and block the opening leading to our mold. T h e sprue should be located at the lowest point in the crucible, this perm itting the gold to flow easily to its destination. A ttem pts have been made to raise the tem perature o f the w ax pattern fo r the purpose o f expansion, to counteract the contraction o f the gold. F o r inlay res­ torations, the contraction o f the gold, when the T a g g a rt technic is employed, is o f no consequence. E levating the

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tem perature releases the elasticity which is always present, and causes a marked distortion o f our pattern and a corre­ sponding misfit o f our inlay. I t w ould appear, fro m all available evidence at hand, that the best results are obtained by proceeding at practi­ cally u niform tem peratures when deal­ ing w ith the completed wax pattern. In the entire process o f inlay res­ torations, the prim ary and chief source o f error is due to faulty grinding. Even m inute indentations in the cavity re­ sult in corresponding protrusions on the pattern, which is distorted on removal. IN V E S T M E N T M A T E R IA L : ITS PH Y SIC A L P R O P E R T IE S

Investm ent m aterial should be com­ posed o f approximately three-fourths silica, and one-fourth plaster o f Paris, w ith the addition o f a small quantity o f graphite. T h e silica should be suffi­ ciently refractory to resist fusing by the hot gold used in casting, and should be as free as possible from iron con­ tam ination. G old, at the best heat fo r casting, reaches the fusing point o f iron (2800 F .) , which w ill incorporate with the gold, m aking ft sluggish. Iron also alloys with the gold, and robs it o f much o f its desirable qualities o f m al­ leability. T h e silica should be crushed, and o f exceedingly fine mesh. W hen the silica is not o f fine mesh, the in­ vesting m aterial w ill fail to copy the delicate angles o f the wax pattern, or if they are copies, they w ill be of plaster alone. T h e plaster o f Paris, when unpro­ tected by the refractory silica and graphite, w ill not successfully resist the process o f wax elim ination nor the effects o f the molten gold. T h e round or uncrushed globules o f silica used by

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most m anufacturers o f investm ent m a­ terial present a bland surface to the plaster o f Paris, which is the binder, and lessen its resistance to distortion in the process o f w ax elim ination, and also the internal pressure necessary to successful casting; w hile the acute angles o f the crushed variety present surfaces to which the plaster crystals readily and firm ly attach themselves, and furnish a bond which w ill success­ fu lly resist distortion during all stages o f the process. Plaster, which is at present the best available m aterial to use as the binder, is readily broken down and distorted by the high tem peratures necessary in the casting process; but w hen the m inim um quantity which w ill success­ fu lly bind the mass together is used, the refractory silica and graphite w ill afford ample protection to the plaster and prevent distortion o f our m old. T h e use o f coarse uncrushed silica is likely to be deceptive. I t w ill offer greater resistance to the passage o f a knife blade when attem pts are made to cut it, and may also offer greater re­ sistance to a crushing force; but it w ill not so successfully resist distortion as does the T a g g a rt investm ent. T h e latter possesses an ample m argin o f safety in the w ax elim ination and pres­ sure necessary in casting, and w ill not change its fo rm as do all other invest­ m ents w ith which I am fam iliar. Silica possessing the desirable qualities Is not uniform ally obtained in the open m arket. T h is should be obtained and selected fro m quarries which are know n to produce the most refractory variety, and should also possess the greatest freedom from iron contam ination. T h e various grades o f silica possess a wide range o f resistance to fusing. T h a t

which is purchased at random invari­ ably contains large quantities o f the low fusing variety, which becomes liquid under the influence o f the molten gold. T h is interferes w ith the accuracy so desired and necessary to the fit o f the inlay. T h e use o f a correctly balanced investment, and ability to fo l­ low the technic, are imperative. T E C H N IC OF IN V E S T IN G

T o attain the highest degree o f per­ fection, the follow ing equipment is necessary: 1. A large rubber bowl as free as possible from acute inside angles. T h is should be used only for m ixing investment. 2. A hard rubber spatula thick in the middle and thin on the edges, and curved to follow the contour o f the bowl. T o prevent too rapid setting o f our mass, the bowl should be thoroughly cleansed by friction. T h is is necessary to remove the last traces o f set plaster rem aining from a previous mix. Set plaster, in quantities macroscopically invisible, w ill unduly hasten the setting o f the mix, thus robbing us o f much o f the time necessary to carry the invest­ ing process to a successful conclusion. Should the w ater in a given locality contain undue quantities o f foreign m atter, fo r example, sodium chlorid, distilled w ater should be used. T h e tem perature o f the w ater is also an im portant factor in determ ining the rapidity o f setting. T h e higher the tem perature, the more rapid the setting; and, inversely, the low er the tem pera­ ture, the slower the setting. T h e setting time is in direct ratio to the tem perature o f the w ater and the freedom fro m set plaster. A clean bowl, and w ater at about 50 F ., w ill provide ample time to complete the technic described below. T h e materials

M orris— W a x and Investing furnished us should not be blamed when our results do not measure up to our ideals. T h e embarrassment can in­ variably be traced to our faulty technic. Each box o f investm ent contains a balance and fulcrum . W ith this, the correct proportion o f w ater to powder is easily obtained, insuring u n ifo rm re­ sults as fa r as the m ix is concerned. W hen the setting time is fixed by using a clean bowl, and w ater at about 50 F ., we can obtain better results by using a 60-grain w eight in the depression on the w ater end o f the balance. ' I use a steel ball bearing. Lessening o f the w ater content produces a denser m old and tends to prevent the liquid gold' from entering the surface o f the m old and producing a rough casting. A u­ thority fo r w eighting the balance is con­ tained in the instructions on the box. T h e correct proportion o f w ater to investm ent is now obtained and trans­ ferred to the m ixing bowl. T h e bowl is held in the palm o f the le f t hand and constantly rotated w ith the fingers, w hile the right hand uses the rubber spatula vigorously, sweeping it to and fro. T h is should be continued fo r a m inute and fifteen seconds. A t the ex­ piration o f this time, there should re­ main no unsaturated particles o f powder. B efore describing the method o f applying investment, we w ill discuss the condition o f the mold which is nec­ essary fo r the attainm ent o f the desired result. Surrounding the w ax pattern fo r the depth o f about 1 m m ., we should have a marked degree o f density in our in­ vestment. T h is is necessary in order to prevent the gold from entering, the sur­ face o f the m old; while, throughout the mass o f the investment, there should

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be a considerable degree o f porosity. T h e investment immediately surround­ ing the pattern we may call the filter coat. T h e thinness o f this coating allows ready passage o f air used in the casting and its density prevents the gold from entering the surface of the mold. Roughness o f the casting is always due to a degree o f porosity which per­ mits the gold to enter the surface of the mold, resulting in enlarging our casting, and causing a corresponding misfit. W hen the proper technic is em­ ployed, the filter coat w ill be impervious to gold, but w ill readily perm it the passage o f air; which results in a cast­ ing that is an accurate reproduction of the wax pattern. T h e technic employed to bring about the desired result is as follow s: W ith a small sable brush, the surface o f the investment is skimmed, and thus the finest particles are picked up. T h e coating o f invest­ m ent is carefully painted on the pat­ tern, being worked into the angles in such a m anner as to prevent the trap­ ping o f air. A fte r the first coating has been applied, a delay o f about one m in­ ute w ill cause the w ater th at is not nec­ essary to bring about crystallization to be dissipated into the surrounding at­ mosphere. T h is w ill produce a much denser surface in proximity to our pat­ tern than w ould otherwise be obtained. A nother coating o f investm ent is now applied to the first, being touched against the initial coating and not painted on it. Painting or friction upon the initial coat w ill disturb it. A fte r the second coating has been applied, a total of- about 1 mm. o f in­ vestment covers our pattern. Again, a delay o f about one m inute w ill pro­ duce a condition o f density as above

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described. W e now delay until the investm ent in the bowl has reached the stage o f crystallization at which it w ill not pour readily. T h e re should be a sufficient stiffness so that the pinching together o f the sides o f the bowl and tapping o f the upper surface w ith the fingers w ill become necessary to assist the flow o f the investment into the flask. T h e investm ent should flow down the side o f the flask, across the bottom and up around the pattern, this prevent­ ing trapping o f air. Should the invest­ m ent be poured before crystallization has advanced to the degree above de­ scribed, the free w ater w ill attack the filter coat, breaking it up, and w ill doubtless result in a roughness o f the casting. A corresponding result may be arrived at in another way, and I think perhaps in a m anner more certain fo r the m ajority o f dentists. T h is method is to apply the first coating as already described, and hold the pattern in the'path o f a rather forci­ ble stream o f compressed air. T h is explodes any bubbles which may be present and produces a m arked degree o f density. T h e second coating is now touched (not painted) on and held in a much softer stream o f air until the g lin t o f the w ater has disappeared, this resulting in a dull or etched ap­ pearance o f the investment. Im m ediately a fte r the g lin t o f the w ater has disappeared, the investment should be removed from the com­ pressed a ir; fo r i f it is le f t in too long, w ater, which is necessary, to the crystal­ lization o f plaster, w ill be extracted. Previously to discontinuing the stream o f air, the surface tension o f the w ater has caused the investm ent to pull away

from the thin m argins o f the wax pat­ tern. W e now do w hat we call rim m ing; that is, touching a portion o f the invest­ m ent to the thin edges o f our wax pattern, uniting the occlusal and cavity portions o f the investment. T h e excess w ater, having been removed fro m the occlusal and cavity surfaces, the rim ­ m ing portion w ill now readily remain in position. T h is should be held in a soft stream o f air fo r a period o f about twenty seconds,' or until the glint disappears. T h e pouring o f the investment should now be delayed until crystallization has advanced to the stage above described. T h e investment may now be poured just as described in the technic in which the compressed air is not used. W hen this has been correctly done, we shall have a m arked degree o f density o f about 1 mm. in thickness about our wax p attern; and the balance of the invest­ m ent, fro m which the w ater has not been extracted, w ill furnish the desired degree o f porosity to perm it the free passage o f air necessary to successful casting. T h e dense coating close about the pattern acts in a m anner corresponding to a w ater filter, which permits the w ater to pass through, but does not per­ m it the passage o f a foreign substance. T h e air necessary fo r casting w ill readily pass through this filter coat, for the reason that it is only about 1 mm. in thickness, but does not perm it the gold to follow , and thus we may expect and obtain an accurate reproduction qf our wax pattern, free fro m nodules and roughness. T h e use o f the compressed air is merely another method o f arriv­ ing at the result which the inventor in­ tended should be attained.