This metal possesses properties which render it one of the most useful yet discovered, and the only bar to its greater employment has hitherto been its high price. There is an immediate prospect of this being enormously reduced by Webster's process, described below. While being very malleable and ductile, aluminium ranks second only to steel in tenacity; it is highly sonorous, 4 times lighter than silver, non-volatile at very high temperatures, conducts heat and electricity as well as silver, is inoxidizable in the air even at a red heat, is not acted upon by water, sulphuretted hydrogen, or ammonium sulphide, resists concentrated nitric acid and dilute sulphuric acid, and forms alloys (see p. 11) of con-siderable value.
The present method of manufacture consists in heating to redness a mixture of the double chloride of aluminium and sodium, or the double fluoride of aluminium and sodium (cryolite) with metallic sodium, by which sodium chloride is formed and metallic aluminium is separated. On the large scale, 10 parts of the double chloride, 5 of cryolite, and 2 of sodium are placed in a reverberatory furnace; immediately action has ensued, the fused metal and slag (consisting of common salt and aluminium fluoride) are run out, and a new quantity of the previous mixture is introduced. The current method of manufacturing aluminium was described at some length by J. L. Bell, at the British Association meeting of 1863 (see Soc Arts Jl. in. 769).
In making large quantities from the mineral bauxite (AlFe)2 O 4 H 4, contain-ing about 50 per cent. alumina and 25 iron oxide, the process is as follows: The mineral is pulverised, and the powder is mixed with soda and heated in a reverberatory furnace, when sodium alu-minate forms; the mass is withdrawn from the furnace, and treated with water, which dissolves the sodium aluposed by passing a current of carbonic acid gas through it, when sodium carbonate forme, and alumina is precipitated; when the reaction is complete, the whole is placed in linen filters and well washed, whereby the sodium carbonate is removed, and leaves the alumins behind in a polverulent rather than gelatinous condition; when dry, it forme a pure white powder. To convert this into aluminium-sodium chloride, it is mixed with salt and coal-dust, and formed into balls, which are rapidly dried, and transferred to upright fireclay retorts a, arranged in a furnace d, pipe to the chimney. The double chloride has next to be decomposed by metallic sodium; with this object, it is mixed with sodium and some cryolite, the latter serving as a flux; the quantities taken are 100 lb. of the double chloride, 35 lb. sodium, and 40 lb. cryolite; the operation is conducted on the sole of a reverberatory furnace by a gradually increased heat; the reduced metal collects on the bottom, and is run into iron moulds.
It always contains some iron and silica. The cost of manufacture for about 4000 lb. of aluminium in 1872 is thus stated:-
(a) Manufacture of the double chloride.
Hydrochloric acid *
Wear and tear
Cost of 1 lb .......................................
* To form the chlorine.
(6) Manufacture of metallic aluminium.
Sodium, 3.44 lb. at 4s. 7d. per lb.
Double chloride. 10.04 lb. at 10 2/4d. per lb.
Cryolite, 3.87 lb. at 24s. 7d. per cwt...
Goal, 29.17 lb. at 7d.per cwt. .
Wear and tear
Cost of 1 lb. .........................................
Webster's new process, before alluded to, is as follows:-A given quantity of alum and pitch, which are first finely ground, are mixed together and placed in a calcining furnace, by which means 38 per cent. of water is driven out, leaving the sulphur, potash, and alumina with oxide of iron. The calcined mixture is then put into vertical retorts, and steam and air are forced through, which leaves a residue of potash and alumina only. This residue is afterward placed in a vat filled with warm water, which is heated with steam. The potash is thus leached out, and the alumina left as a deposit. The potash liquor is then run off and boiled down, while the alumina precipitate is collected in sacks and dried. It is then ready for making chloride of aluminium. The alumina deposit thus obtained contains about 84 percent. of pure alumina, while that which is obtained by the old process of precipitation has only 65 per cent. Jones, the Wolverhampton borough analyst, certifies that the constituents of Webster's alumina deposit are as follows:-Alumina, 84.10; sulphate of zinc, 2.68; silica, 7.40; water, 4.20; alkaline salts, 1.62. In order to complete the process and convert it into aluminium, the aluminium chloride is treated with sodium, in order to withdraw the metal.
The operations are completed in a few days instead of 9 months, and the product costs only about 100/. per ton instead of 1000/. The sources of raw material are inexhaustible and everywhere distributed. Webster also claims to have found ways to solder and weld the metal.
To ascertain fully the mechanical properties of this metal, a bar of aluminium, 3 ft. long and 1/4 in. square, was obtained, and different parts of this bar were subjected to tests for tension, compression, transverse strain, modulus of elasticity, elastic range, and ductility. The experiments were carefully carried out under the direction of Prof. Kennedy, with his testing machine, at the London University, and the results are given in the table appended.
It will be seen that the weight of 1 cub. in. is .0972 lb., showing a specific gravity of 2.688, and that its ultimate tensile strength is about 12 tons per sq. in. The range of elasticity is large, the extension at the yielding point being 1/48 of its length. The modulus of elasticity is 10,000. The ductility of samples 2 in. long was only 2/5 per cent., but it is probable that the metal could be improved in this respect. Taking the tensile strength of the metal in relation to its weight, it shows a high mechanical value. Its characteristics in this respect, as compared with those of other well-known metals, are shown in the following summary :-
Weight of 1 cub.
Length of bar just
Cast-iron . .
Steel of 35 tonal per in. . .
Aluminium . .
It thus appears that, taking the strength of aluminium Id relation to its weight, it possesses a mechanical value about equal to steel of 35 tons per in. Its mechanical properties point to its suitability for those cases where strength, combined with lightness and a great range of elastic action, is required. The elastic range is about 3 times that of steel, and about 5 times that of wrought-iron. It has become somewhat common In Paris to make the frames of opera-glasses and telescopes of aluminium. But the most favourable use apparent for this curious metal, by reason of its low specific gravity, is the making of beams for balances : aluminium-bronze beams have been made for several years past, but, as far as lightness is concerned, they have scarcely any advantage over brass. Several reasons are given for the small amount of favour with which aluminium is regarded by mathematical instrument makers. Pint of all, there is the consideration of price; then the methods of working it are not everywhere understood ; and, further, no one knows how to cast it. Molten aluminium attacks the common earthen crucible, reduces silicon from it, and becomes grey and brittle.
This inconvenience, however, has been overcome by the use of lime crucibles, or by lining the earthy crucible with carbon or strongly-burnt cryolite. Were the catting of aluminium to become an industrial operation, the metal would be more freely used in the finer branches of practical mechanics.