ELECTRO-METALLURGY.—ALUMINIUM. 
185 
Cross’s electric crucibles are based on a different principle : instead of utilising 
the heat of the voltaic arc itself, a high temperature is produced by the incan¬ 
descence of large hollow carbons ; the sides of the crucible, also of carbon, 
form the positive electrode, while the crayons constitute the negative one. 
The above-mentioned furnaces are traversed by direct currents, but for some few 
years alternating currents have been in use. In Ferranti’s system the most 
remarkable feature is the absence of an electrode in the interior of the hearth; 
the crucibles are surrounded by circuits provided with iron plates to constitute a 
powerful magnetic field. Colby’s crucible forms the secondary circuit of a trans¬ 
former, the primary circuit being fed by an alternating current dynamo, and 
attempts are being made to manufacture steel in these. 
To sum up, in comparison with ordinary ones, electric furnaces have the advan¬ 
tage of reducing to a minimum the loss of heat by radiation, and of considerably 
restricting the mass to be heated. In many cases, if the motive power is cheap, 
the electric fusion of metals may be more economical than the ordinary methods. 
ELECTRO-METALLURGY. 
The art of depositing a thin metallic layer upon another metal has been long 
known and practised in the form of gilding, nickel plating, and electrotyping. 
Afterwards electrolysis was utilised for the production even of metals. One of 
its most important applications is the refining of copper. In the metallurgy of 
this metal, when it has been brought to the state known as black copper , there 
are about 5'% °f impurities, sulphur, iron, arsenic, antimony, lead, to be elimi¬ 
nated before it can be made malleable, and this elimination was only effected 
very incompletely by successive oxidations and a large expenditure of charcoal. 
The electrolytic process originated in 1872 in the Norddeutsche Affinerie of Ham¬ 
burg, and has been considerably developed since in Europe and America, being 
now in use in more than 80 manufactories, producing daily about 80 tons of 
electrolytic copper of most superior quality. When copper is chemically pure it 
is very ductile and of a high electrical conductivity, unattainable by the old 
methods. 
The general process of electrolytic refinement of copper consists in joining in 
series a hundred vats of wood or concrete lined with lead, and filled with an 
acidulated bath of copper sulphate dissolved in water. The kathode consists of 
a plate of pure copper *04" in thickness, and the anode of a plate of black or 
impure copper from ’I" to '6" in thickness. When the current passes through 
the bath the sulphate is decomposed and the copper is deposited on the kathode, 
while the oxygen and sulphuric acid attach themselves to the anode, attack it, and 
re-form a sulphate wherewith to regenerate the bath ; the current, so to speak, 
carries the metal of the positive to the negative electrode. The impurities of 
the black copper are dissolved and precipitated at the bottom of the vats like 
mud, from which substances of commercial value can be extracted, such as sul¬ 
phur and precious metals. At the Norddeutsche Affinerie 1'2 kg. (2*64 lbs.) 
of gold are annually extracted from the above residue. The strength of the 
current is generally from 25 to 35 amperes per square metre of the surface of 
the kathode. This gives about one gramme of copper deposited per ampere- 
hour. Important improvements in this manufacture are due to Mr. Thofern, 
viz., a better arrangement of the vats ; the use of a current of 1000 amperes at 
20 volts; advantageous arrangement of the conductors, whereby they absorb 
only 5 °/ 0 of the total energy; economical heating of the bath to 35° cent. (95° 
Eahr.) by the waste steam from the engines ; continuous circulation of the liquid 
by means of pumps and injectors analogous to those of sugar refineries; auto¬ 
matic cleansing of the vats. Erom the metallurgical point of view the chief 
feature of the above system is the preliminary oxidation of the anode \ the copper 
