(2)
loss of weight of acid treatment, (3) alkalinity of the surface after
ignition, and (4) development of cracks after continued heating. As
causes for loss on ignition the presence of iridium and base metals is
generally given. Loss on acid treatment after ignition is mainly due to
a content of iron and calcium. After a thorough consideration of the subject, the occasionally strong contamination by iron, iridium, and
other metals is given as the prooable cause of most of the defects.
Purchasers who desire ware of the highest quality should specify that
it must not show marked uneven discoloration on heating, and must give
no test for iron after prolonged ignition (two hours may be regarded as
sufficient), and that the rate of loss of weight per hour at 1,100°C.
over a period of not less than four hours and preferably longer, shall
not exceed, say, 0.2 milligrams. Iridium is rather easily volatilized
at high temperatures, especially above 1,100° C. Although the
volatility of platinum is much less marked, it is not, at this
temperature, a negligible factor.
USES OF PLATINUM.
Owing
to its high melting point and great resistance to acids, platinum is
extensively used for laboratory utensils. Platinum salts are employed
in chemical analysis. In the manufacture of sulphuric acid the metal
was used in making large concentration kettles, but of late gold has
been substituted for it. In photography, dentistry, and electric
installation much platinum is used. Of late the manufacture of jewelry
has consumed large quantities of it. It is extensively used for chains
and for the setting of diamonds, the claim being made that, besides
being more resistant than silver and harder than gold, the stones are
better offset by platinum and appear larger than in any other kind of
setting.
Substitutes for platinum.—The
high price which platinum has attained during the last two years has
necessitated the finding of substitutes for it. In the manufacture of
sulphuric acid as well as in ware for chemical use gold is now often
substituted for platinum. At one time much platinum was used in the
manufacture of incandescent lamps, but it is now almost entirely
replaced by tungsten; the osmium lamps had a similar fate when the
price of that metal rose to extremely high figures. Platinum triangles,
used extensively in laboratories, have recently been successfully
displaced by similar appliances made of an alloy of nickel and
chromium. Nevertheless, there remain so many industrial applications of
platinum for which no substitutes can be found that it is not likely
that the price will be much cheapened in the future.
The
present extensive use of platinum in the manufacture of jewelry is
unfortunate, as other metals could be substituted for it, and this fad
is undoubtedly one of the principal causes of the great increase in the
price of platinum.
OCCURRENCE OF PLATINUM.
UNITED STATES.
California and Oregon.—The
principal occurrences of platinum in the United States are in
California and Oregon, and the production of crude platinum is
practically limited to these States. In California a small amount of
platinum sand occurs in the principal placer mines
