Platinum as a Metallic Material
An alloying addition of gold to the platinum base leads to a significant increase in the contact angle of molten glass on the surface of the metal. In the range 1,100-1,400 the contact angle of a typical borosilicate glass on pure platinum is, depending on the temperature, between about 14° und 40°. This means that the surface of the platinum is very strongly wetted by the molten glass. An alloying addition of only 1 % gold increases the contact angle significantly: with 3 % gold the contact angle reaches 60° over a broad temperature range1). On the basis of this effect, the alloy PtAu95/5 has become the commercial standard material, as it combines good strength and forming characteristics with a low tendency to wetting by molten glass.
Over long periods of exposure at temperatures above approx. 1,000 , gold is depleted in the zone close to the surface as a result of the high vapor pressure of gold which is several orders of magnitude greater than that of platinum. This leads to a gradual increase in the tendency to wetting of the metal surface.