424 BELL SYSTEM TECHNICAL JOURNAL 



Various expedients were used, and one which utiUzed a unique principle 

 is worthy of note. 



It was observed about the middle of the century that platinum would 

 fuse at relatively low temperatures in the presence of arsenic^, and that, 

 on prolonged heating, the arsenic could be volatilized out of the fused lump 

 to leave behind a sponge of metallic platinum. This sponge could then 

 be heated and forged to solid form. Similar results were obtained using 

 mercury* or sulphur in place of arsenic; and the success of the forging 

 methods led other investigators to study the welding of grains of native 

 platinum or platinum scraps without the use of added elements to lower 

 the fusion point. 



Such was the situation in the early part of the nineteenth century when 

 Wollaston^ developed his method for the preparation of platinum ware. 

 Numerous other investigators^'^-^ had produced articles of platinum by 

 treatment of finely divided platinum or sponge, but by careful refinements 

 in the process with control of particle size, purity, compacting pressure 

 and sintering treatment, Wollaston obtained a superior product. Pre- 

 cautions were taken to use only the more finely divided platinum particles, 

 and to press the powder carefully in a mold while wet. This pressing of 

 wet powder is claimed to have been one of the main contributions made 

 by Wollaston since a much lower compacting pressure was allowable, and 

 the particles were not work hardened. The resulting cake was then slowly 

 dried to remove volatile matter and adsorbed gases before sintering at 

 800°-1000° C. The material was forged while still hot, and gave the first 

 really pure, blister-free platinum sheet. That the process developed by 

 Wollaston was sound is shown by the fact that the platinum produced 

 by powder metallurgy at present in England is made by essentially the 

 same procedure-^ The careful studies made by Wollaston in fabricating 

 platinum ware of high purity thus led to the basic principles utiUzed in 

 successfully producing massive metal parts from metal powder. 



During the nineteenth century, many metals were produced in powder 

 form, but there seems to have been no correlated effort to convert the powders 

 into coherent form. This may have been due to the development of better 

 melting furnace equipment that allowed ordinary melting and casting tech- 

 niques to be employed for most metals and alloys. On the other hand, 

 there remained some of the more refractory metals such as tungsten, tanta- 



* As an example of how new methods introduced can often be traced back to earlier 

 sources, the use of mercury to form an amalgam which could then be heated to leave a 

 powder sponge material, has been attributed to the monk Theophilus in the 11th century^. 

 In this case, the amalgam process was used with gold, and the end product sought was 

 gold powder which could be used as a pigment in inks for illuminating manuscripts. 

 There was no attempt, however, to carry the process further to make solid metal parts 

 as was the case with platinum as cited above. 



