CHAPTER XI

PLATINUM ALLOYS

§ 168. Platinum unites with most metals to form alloys, but in consequence of its high melting point, great difficulty is experienced in obtaining bodies of a definite composition containing volatile constituents. It should also be remembered that metals like silver and copper, which, under ordinary circumstances, do not vaporise, readily assume the gaseous state at the melting point of platinum. When a small quantity of platinum is heated with a large excess of a more fusible metal, the fusing point of the former is lowered sufficiently to enable it to melt and alloy with the latter at ordinary furnace temperatures. The alloys of platinum are, for the most part, much more fusible than platinum itself. When it is desired to alloy much platinum with another metal, a special arrangement, known as the oxy-hydrogen blowpipe furnace, is employed to effect the fusion.

Two well-fitting lumps of quicklime are hollowed out so as to form a cavity, in which the metal to be melted is placed. The cavity in the lower block is deeper than that of the upper one, as it has to contain the molten metal. The upper block is perforated through the centre with a hole, through which an oxy-hydrogen blowpipe passes, and a side opening between the two blocks permits the escape of the products of combustion, and serves as an outlet for the molten metal. The blowpipe is a double tube, the inner

one conveying oxygen, and the outer one conveying hydrogen or coal-gas. The intense heat, produced by the burning of hydrogen in oxygen, is sufficient to melt a considerable quantity of platinum or platinum-alloy. In preparing an alloy, the platinum is melted first, and the other metal or metals then added through an aperture in the top of the furnace, which is then closed with a lime-plate.

The flame can be modified by means of stop-cocks to suit the conditions required. If the oxygen be in excess during the process of alloying with a base metal, the latter will be largely oxidised, so that it is advisable to have a slight excess of hydrogen to prevent this loss. The molten metal or the prepared molten alloy is cast into bars or ingots in moulds of lime, in suitable sizes for wire-drawing or rolling.

§ 169. Platinum and Silver. These metals unite in several proportions, forming white or grayish-white alloys, which are harder and tougher than silver; and less fusible, malleable, and ductile, as the proportion of platinum is greater. There is a great tendency for the two metals to separate according to their specific gravities on cooling, the platinum settling to the bottom. Alloys with 17 to 35 per cent of platinum are used in dentistry, and known as platineau-titre. These alloys are less readily tarnished than silver or ordinary silver alloys. Lewis in the last century prepared small quantities of the following platinum alloys :

The mixtures in I, II, and III required a strong white-heat for fusion; the products were hard and brittle in proportion to the contained platinum. The constituents of No. IV melted readily, and the alloy was harder, grayer, and of a coarser grain than silver; it hammered tolerably well. He found a considerable separation of platinum at the bottom

of the moulds, unless the alloy solidified immediately on pouring.

1 Berthier states that an alloy with 7 per cent of platinum is brittle, but Mr. E. Matthey denies the truth of this statement. An alloy of 37.5 per cent silver and 62.5 per cent platinum is said to have a colour half-way between silver and platinum, to flatten under the hammer, but to crack in rolling. With alloys low in platinum, nitric acid dissolves a certain quantity of platinum along with the silver. An alloy containing only 5 per cent of platinum dissolves completely in nitric acid. With sulphuric acid the silver only dissolves. When the above nitric acid solution is heated with sulphuric acid, the platinum separates out.2

Messrs. Johnson and Matthey prepare an extremely ductile alloy of 2 parts silver and 1 part platinum, as an article of commerce. This alloy has been adopted as a standard of electrical resistance.

Platinum and Gold.-See Gold Alloys.

§ 170. Platinum and Copper.-Alloys of these metals may be obtained by fusion of the constituents in all proportions. A high temperature is required for their production, the oxy-hydrogen furnace being required when the platinum is in excess. They possess considerable ductility, malleability, and tenacity; are capable of forming a variety of shades of colour, and are less tarnished by the atmosphere than alloys of copper with base metals. As the proportion of platinum increases the alloys become harder, whiter, and more brittle. They are capable of a high polish and have been used for the reflectors of telescopes.

When zinc is added in addition to copper, alloys are obtained nearly equal to gold in colour and lustre, superior in durability, and used in the manufacture of jewellery and ornaments.

An alloy of 1 part platinum and 4 parts copper is

1 Traité des Essais (2), p. 800.

2 Handwörterbuch der Chemie (7), p. 958.

hard, ductile, of a yellow-pink colour, and susceptible of a high polish.

An alloy of equal parts by weight of copper and platinum, according to Clarke, is yellow, having the colour and specific gravity of gold, extensible, easily worked by the file, and tarnished by exposure to air.

An alloy of 4 parts platinum and 96 parts copper is malleable, rose-coloured, and exhibits a fine-grained fracture. An alloy of 3 parts platinum and 2 parts copper is nearly white, very hard, and brittle.

§ 171. The following alloys have a golden-yellow colour. No. IV, known as Cooper's gold, is malleable, ductile, and closely resembles 18-carat gold :

Cooper's Mirror Metal.-Copper 57.85, platinum 9.49, zine 3.51, tin 27·49, arsenic 1·66. The inventor claims for this alloy that it is indifferent to the weather, and takes a beautiful polish.

Cooper's Pen Metal.—The above alloy is said to be suitable for pens. Another alloy consists of copper 13 parts, platinum 50 parts, and silver 36 parts. The hardness and non-corrosive character of Cooper's alloys render them suitable for the manufacture of mathematical instruments and for chronometer wheels.

§ 172. Platinum and Iridium. These metals unite in different proportions, but the intense heat of the oxyhydrogen blowpipe is necessary to melt them and bring

them into union. The alloy consisting of 9 parts platinum and 1 part iridium is used as a standard metal bar for the metric system. It is extremely hard, as elastic as steel, more difficultly fusible than platinum, perfectly unalterable in air, and capable of taking an exceedingly beautiful polish.

In the year 1870 Messrs. Johnson, Matthey, and Co. prepared a standard bar of the above alloy for the Parisian Commission for the International Metrical System, and after it had been subjected to every possible test which could be suggested in competition with other materials, it was, after two years' trial, pronounced the best, and adopted as the material for the manufacture of all the standard weights and

measures.

The following alloys have been prepared by Deville and Debray, and their specific gravities determined :

:

Specific gravity of platinum 21.504; of iridium= 22-421. According to Deville and Debray, an alloy of 90 per cent platinum and 10 iridium has the same coefficient of expansion as the original metre preserved in the French Archives, which is known to have been made of impure platinum.

The 90 per cent platinum alloy is not attacked by aquaregia.

Alloys with 20 per cent iridium are malleable and capable of being worked. An alloy of equal parts of the two metals is brittle, but capable of welding to some extent. An alloy of 1 part iridium and 10 parts platinum, when laid on copper, serves for metallic mirrors.

A native alloy of platinum and iridium is found in the Ural Mountains and in Brazil.