Small quantities of platinum change the characteristics of gold in many respects. With a small percentage the color is noticeably lighter than that of pure gold, and the alloys are extremely elastic; alloys containing more than 20 per cent of platinum, however, almost entirely lose their elasticity. The melting point of the platinum-gold alloy is high, and alloys containing 70 per cent of platinum can be fused only in the flame of oxyhydrogen gas, like platinum itself. Alloys with a smaller percentage of platinum can be prepared in furnaces, but require the strongest white heat. In order to avoid the chance of an imperfect alloy from too low a temperature, it is always safer to fuse them with the oxyhydrogen flame. The alloys of platinum and gold have a somewhat limited application. Those which contain from 5 to 10 per cent of platinum are used for sheet and wire in the manufacture of artificial teeth.

Platinum-Gold Alloys For Dental Purposes

I II   III

Platinum........ 6 14    10

Gold............ 2 4      6

Silver........... 1 6     ..

Palladium...........       8

Platinum Silver

An addition of platinum to silver makes it harder, but also more brittle, and changes the white color to gray. An alloy which contains only a very small percentage of platinum is noticeably darker in color than pure silver. Such alloys are prepared under the name of platine au titre, containing between 17 and 35 per cent of platinum. They are almost exclusively used for dental purposes.

Imitation Platinum

I

Brass, 100 parts; zinc, 65 parts.

II

Brass, 120 parts; zinc, 75 parts.

III

Copper, 5 parts; nickel, 4 parts; zinc, 1.5 parts; antimony, 1 part; lead, 1 part; iron, 1 part; tin, 1 part.