3435. Dentists' Tin Alloys for Moulds

3435.  Dentists' Tin Alloys for Moulds. The gold plates on which artificial teeth are fastened, are fashioned to fit exactly to the mouth by being hammered between a mould and die, cast from a plaster model of the mouth. The plaster model is obtained from a mould of wax, pressed while soft into the cavities of the mouth, and allowed to harden. Duplicate moulds and dies are necessary, at different stages of the hammering, in order to obtain a perfectly fitting plate. The necessary characteristics of the metals used for the moulds and dies are fusibility, hardness, or toughness, and, especially for the moulds, a freedom from shrinkage in cooling. The metal usually employed for the dies consists of 8 parts tin, 1 part lead, and 1 part bismuth. This compound is much harder than tin, melts at a lower heat, shrinks little, or practically none, in casting; is tough and strong. It melts at about 330° Fahr. Although generally a harder and less fusible metal is used for the first swaging, this alloy is particularly convenient for taking duplicate dies for finishing. Its tenacity adapts it for cases of partial sets representing the teeth. The mould or counter-die metal is made by adding to 1 part of this mixture 6 parts of lead. The result is harder than lead, and does not yield like it under the blow presenting a resistance sufficient to drive the plate up well against the die. Its shrinkage is but slight; it melts at from 450° to 460°. It is designed for use when the dipping process is resorted to. This consists in pouring the melted metal into an appropriately shaped vessel or mould, and pressing the plaster model into the metal before the moment of congelation. If used at the 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

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; antimony, 1 part; zinc, 1 part. This alloy is much harder than the preceding die metal, and equals it in tenacity, being suited for any kind of die; it requires a higher temperature to melt it, but it melts sooner than tin, or than the mould-metal mentioned in the preceding receipt, from a matrix of which a die may be taken by it with safety. It affords, in sand, a perfect die, does not shrink, and, whether poured into a sand or metal mould, comes out with a smooth, bright face. It is the best combination of these three metals for the purpose. But when dies are made of it from sand moulds, and a more fusible metal is needed for taking counter-dies or moulds from them, it may be had by a combination of 5 parts lead, 2 bismuth, and 1 tin; or, 5 parts lead, 3 to 4 bismuth, and 1 tin afford a still more fusible compound, although harder. 3437. Copper Alloys for Dentists' Moulds. A very hard and most valuable alloy for general use may be had by a mixture of tin, 12 parts; antimony, 2 parts; copper,

1  part. It is not much inferior to zinc in hardness, casts without sensible shrinkage, and makes a perfect and very handsome die, bright and smooth. It is less fusible than the hard tin die metal in last receipt, but may be used for taking dies from the mould-metal mentioned in No. 3435; but, as it melts at nearly the same temperature, this requires care. It will be found of value in connection with lead moulds made by dipping. (See No. 3435 (Dentists' Tin Alloys for Moulds).) It is rather brittle for dies for partial sets representing the teeth, as these are liable to break on removing from the matrix; but it is abundantly strong enough for swaging purposes. In combining these metals (which may be done in an ordinary charcoal furnace, as it is by no means necessary to raise the heat to the melting point of copper), place the copper in a crucible and bring it to a red heat, then pour in the tin and. antimony, melted, and cover the whole with charcoal dust, to prevent oxidation. The copper will soon liquefy, or dissolve, as it were, combining perfectly with the other metals, without further elevation of temperature. To guard better against volatilization of antimony, which takes place at a high red heat, it is well enough to add to the copper but half the tin at first, and when these are combined, add the antimony, and then the remaining tin. This also enables one to conduct the second melting in a larger crucible, or, indeed, in an iron ladle. It is best to let the melted mass cool down some, before pouring it from the crucible, as, if poured out at too high a heat, the alloy oxidizes. A larger proportion of antimony and zinc increases the hardness of the metal, but with a tendency to imperfect castings. If tin be used in larger quantity, the alloy is, of course, softer, and it shrinks when cast. The relative proportion of zinc and antimony, in respect to each other, may be somewhat varied, without material modification 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 example, 8 parts tin, 1 antimony, 1 copper; or, 10 tin, l1/2 antimony, 1 copper; or, 12 tin,

2  antimony, 1 copper. For taking counter-dies or moulds from dies of the last named alloys, a suitable metal, fusible at about 380°