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types-first the fusible alloys such as the bismuth-lead-tin alloys
which are used in dies and counterdies, some of which melt at
temperatures as low as that of boiling water, and second the yellow
silvercopper technic alloys that look like gold. The workman
should try to exclude these base metals as far as possible from his
valuable wastes, as they not only consume time and chemicals, but
some of them, like lead and tungsten, complicate the refining even
when present in small quantities.
 Modern dental gold alloys always contain silver and copper, frequently
platinum, palladium, zinc and tin as well. Sometimes
iridium, rhodium, nickel, and indium1 are present in small amounts,
and the base metals aluminum, magnesium, manganese and iron
have been found.
 Much interesting research has been done on dental alloys during
the last few years, both by the firms that sell them and by the National
Bureau of Standards, with the cooperation of the American
Dental Association. If the reader is not already familiar with these
investigations, he should at once obtain some of the publications
described in Chapter E of the Appendix. A familiarity with these
recent researches will be of great usefulness both to the user and the
refiner of these alloys.

 The wrought gold alloys and casting alloys which are the main interest
of this chapter, are used for a variety of prosthetic purposes, and many
formulas have been proposed. Gold is the principal constituent of most
of them, especially the yellow ones. The silver content of the wrought
gold alloys ranges from about 5% to as much as 15%; this point will be
brought up again. later. Platinum and palladium together may run
from 10% to as much as 25%. Normally iridium is present in small
proportions-less than 1%-and probably got in by accident rather than
by design. In the white alloys there is considerable nickel-as much as
18%; copper runs from 9% to 15%, more or less, while zinc rarely runs
over 1%. The American Dental Association Specifications2 require

 1 Indium is a rare metal, of the aluminum group, sometimes added in small quantities.
See U. S. Patents 1,987,451 and 1,987,452, N. 0. Taylor, January 8, 1935.
Because of its rarity indium is not taken into consideration in this book. If you know
that your metal contains indium in sufficient quantity to justify its recovery, see
Raleigh Gilchrist; Research paper RP 1103, Journal of Research of the Bureau of
Standards, vol. 20, pp. 745-771, June, 1938.
 2 These specifications were prepared by the National Bureau of Standards, in
cooperation with the Dental Association and the manufacturers of dental gold materials.
See Chapter E of this volume.

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