Though a rare metal, being at this writing more expensive than gold, platinum is much used for analytical apparatus. Between the years 1827-44 it was used by the Russian government for coinage.3 But it is now largely made into tubes, wire, crucibles, receptacles, etc., to be subjected to high temperature and to come in contact with various acids and alkalies necessary for rock analysis; also for leading-in wires for incandescent lamps and for sparking points for electrical apparatus.

1 "The Strength of Wrought Iron as Affected by its Composition and its Reduction in Rolling." Transactions American Institute Mining Engineers, Vol. VI, p. 101.

2 "Mechanical Engineer's Pocket Book."

3 Roscoe and Schorlemmer, "Treatise on Chemistry," Vol. II, p. 1350.

Platinum melts at 1760 deg. Cent., is seventh in the malleability scale and first in ductility (Prechtl). Heated in presence of oxygen, it begins to lose weight at 800 deg. Cent. It absorbs hydrogen when heated and gives it off again on cooling, and its surface becomes rough.1 Like iron, it is highly weldable; best at white heat, though also at a dull red. It does not oxidize in the air, hence needs no flux, though the surfaces should be polished.

The need for welding platinum sometimes arises, as in the case of analysis tubes made of sheet platinum, the joining of tubes, wires, etc., and the fabrication of apparatus and the insertion of patches in burnt-out crucibles.

Before a flame intense enough to melt the metal had been discovered, the platinum refiner took advantage of the welding property in making his ingot. Sponge platinum, resulting from the last stage of refining, was heated to redness. It was then pressed strongly together to form a cake. The cake was heated to white heat and hammered to a compact ingot.2

The oxy-hydrogen blast was applied to the heating process about 1847 by Dr. Hare, of Philadelphia; and later the hammering was dispensed with and the platinum was simply melted into an ingot.

Platinum was originally soldered with gold or with difficulty welded. The oxy-hydrogen flame makes the process much easier, as the metal readily melts under this heat. On account of its tendency to absorb hydrogen and consequent bubbling of the surface, it is necessary to keep a slight excess of oxygen in the flame.

The oxy-acetylene flame would also answer the purpose, though it would be necessary to keep a considerable excess of oxygen in the flame. Carbon from the acetylene would rapidly attack the platinum.

I am indebted to Mr. E. A. Colby, of Baker & Co., platinum refiners, for the subjoined special information, which covers points hitherto untouched in the literature of platinum and its allied metals.

1 Roscoe and Schorlemmer, "Treatise on Chemistry," Vol. II, p. 1350.

2 Encyclopaedia Britannica, Vol. XIX.

"The oxy-acetylene flame can undoubtedly be used for the welding of platinum, but is not by ourselves for the reason that the temperature available is far in excess of that necessary, and lack of experience leaves us in doubt as to the effect of the byproducts upon the metal from the acetylene flame. We consider the oxy-hydrogen flame far safer and, as the heat is not so concentrated, it is more useful where large surfaces are to be treated. Care, however, must be exercised to have the component gases (hydrogen and oxygen) present in approximately the necessary amounts for perfect combustion. Platinum takes up hydrogen at high temperatures and becomes more or less brittle, depending upon the amount of hydrogen retained.

"No flux is required in the welding of platinum or of other metals of the same group, osmium excepted.

"For the same section, the strength of the weld is undoubtedly weaker than that of the body of the metal. Just how much weaker we cannot state from observation, but, as a welded joint is not submitted to the same mechanical working as the body of the metal, its strength is not to be considered as equivalent. In practice, however, the welded portion is slightly increased in section over the body of the metal, and under these conditions there is no material difference in the strength.

"Iridium, osmium, and other metals of the platinum group, when present in small quantities, do not apparently increase the difficulty of effecting a joint, but the strength of the joint is considerably less, as alloys of platinum and members of that group become more and more brittle with increase of the foreign substances. No difficulty, however, is experienced in welding iridio-platinum containing as high as 30 per cent, iridium. Additions of even minute quantities of osmium to an alloy of this composition, however, make it extremely difficult to obtain satisfactory results.

"Platinum can be united to various other metals, such as copper, nickel, etc., but it is an open question as to whether the union can be considered as a true welding. Undoubtedly, an alloy is formed of the two metals at the surface of contact possessing sufficient mechanical strength for the purposes for which such welds are used. The only illustration of welds of this character are to be seen in the construction of the ordinary incandescent lamp, in which the copper wires attached to the filaments are joined to the platinum wires sealed in the glass by fusing a piece of copper onto the end of the platinum wire. This operation is conducted in automatic machines, and has proven very satisfactory for the purpose."