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Dentists' Tin Alloys for Moulds

used for taking dies from the mould-metal The gold plates on which artificial teeth are mentioned in No. 3435; but, as it melts at fastened, are fashioned to fit exactly to the nearly the same temperature, this requires mouth by being hammered between a mould care. It will be found of value in connection and die, cast from a plaster model of the with lead moulds made by dipping. (See No. mouth. The plaster model is obtained from 3435.) It is rather brittle for dies for partial a mould of wax, pressed while soft into the sets representing the teeth, as these are liable cavities of the mouth, and allowed to harden. to break on removing from the matrix; but it Duplicate moulds and dies are necessary, at is abundantly strong enough for swaging purabundantly strong enough for swaging purdifferent stages of the hammering, in order to poses. In combining these metals (which obtain a perfectly fitting plate. The neces- may be done in an ordinary charcoal furnace, sary characteristics of the metals used for the as it is by no means necessary to raise the moulds and dies are fusibility, hardness, or heat to the melting point of copper), place toughness, and, especially for the moulds, a the copper in a crucible and bring it to a red freedom from shrinkage in cooling. The heat, then pour in the tin and antimony, metal usually employed for the dies consists melted, and cover the whole with charcoal of 8 parts tin, 1 part lead, and 1 part bismuth. dust, to prevent oxidation. The copper will This compound is much harder than tin, melts soon liquefy, or dissolve, as it were, combining at a lower heat, shrinks little, or practically perfectly with the other metals, without furperfectly with the other metals, without furnone, in casting; is tough and strong. It ther elevation of temperature. To guard melts at about 330° Fahr. Although gener- better against volatilization of antimony, ally a harder and less fusible metal is used which takes place at a high red heat, it is for the first swaging, this alloy is particularly well enough to add to the copper but half the convenient for taking duplicate dies for finish- tin at first, and when these are combined, add ing. Its tenacity adapts it for cases of partial the antimony, and then the remaining tin. sets representing the teeth. The mould or This also cnables one to conduct the second This also enables one to conduct the second counter-die metal is made by adding to 1 part melting in a larger crucible, or, indeed, in an of this mixture 6 parts of lead. The result is iron ladle. It is best to let the melted mass harder than lead, and does not yield like it cool down some, before pouring it from the under the blow, presenting a resistance suffi- crucible, as, if poured out at too high a heat, cient to drive the plate up well against the the alloy oxidizes. A larger proportion of die. Its shrinkage is but slight; it melts at antimony and zinc increases the hardness of from 450° to 460°. It is designed for use the metal, but with a tendency to imperfect when the dipping process is resorted to. This castings. If tin be used in larger quantity, consists in pouring the melted metal into an the alloy is, of course, softer, and it shrinks appropriately shaped vessel or mould, and when cast. The relative proportion of zinc pressing the plaster model into the metal and antimony, in respect to each other, may before the moment of congelation. If used at be somewhat varied, without material modibe somewhat varied, without material modithe point of congelation, the plaster cast may be employed without previous baking; otherwise it should be baked to expel its water of crystallization. 3436. Hard Tin Alloys for Dentists' Moulds, The following formula affords a highly useful alloy, where toughness as well as hardness is essential: tin, 16 parts; antification of the qualities of the compound; but, for the best results, the sum of these two metals should hold to the quantity of tin employed the ratio of about 1 to 8. For fluidity, an excess of antimony over copper appears to be requisite. For non-shrinkage, the joint amount of antimony and copper should be to the quantity of tin as about 1 to 4; as, for mony, 1 part; zinc, 1 part. This alloy is example, 8 parts tin, 1 antimony, 1 copper; much harder than the preceding die metal, or, 10 tin, 1½ antimony, 1 copper; or, 12 tin, 10 tin, 14 antimony, 1 copper; or, 12 tin, and equals it in tenacity, being suited for any 2 antimony, 1 copper. For taking counterfor any 2 antimony, 1 copper. For taking counterkind of die; it requires a higher temperature dies cr moulds from dies of the last named to melt it, but it melts sooner than tin, cr alleys, a suitable metal, fusible at about 380° Fahr., is had by a mixture of 3 parts lead, 1 of tin have been added, the heat should be part bismuth, and not over Too part tin. It reduced to a dull red, to prevent oxidation; is wonderful how small a quantity of tin then add the remainder of the metal as above. serves to improve the alloys of lead and bis- In melting the composition, it is better to muth, giving them a white, clear lustre, keep a small quantity of powdered charcoal preventing oxidation, promoting fusibility-in on the surface of the metal. The above comshort, producing almost a new metal. 3438. Cadmium Alloys for Dentists' boxes, take 1 pound of this hardening and position is called hardening. For lining the Moulds. By the use of cadmium we may melt it with 2 pounds of Banca tin, which produce still harder alloys than any of the produces the lining metal for use. Thus, tho preceding, possessing in an equal degree every proportions for lining metal are 4 pounds other desirable quality. Thus, 10 parts of tin, copper, 8 pounds regulus of antimony, and 96 1 part of antimony, 1 of copper, and 1 of pounds Banca tin. cadmium, produce a compound which has about the hardness of zinc; it casts perfectly, method employed by the Chinese for working