Auci. 1, 1884.] 



♦ KNOW^LEDGE ♦ 



91 



required to be well done ; but the extra expense involved in 

 the sacrifice of the model, in taking two electrotyped 

 impressions, etc., forms an important factor in determining 

 «poa the course to be adopted and the method to be 

 ■employed. The method is, however, a safe one, and may 

 be relied upon to give a first-class representation of the 

 •original figuie. It was the one employed by Messrs. 

 Elkington in producing their colossal statue of the Earl of 

 Eglinton, l.'H ft. high, and weighing two tons. The 

 capacity of the bath employed was 6,680 gallons. Another 

 firm produced a work by this process weighing nearly three 

 and a half tons. 



Another process may be employed which does not involve 

 the sacrifice of the model, and that is to take sectional 

 plaster casts, and then, after saturating them with linseed 

 oil and well coating their surfaces with plumbago, to place 

 them in the bath, whence direct positive impressions may 

 be taken, and treated in the ordinary way. The various 

 sections are cleaned, fitted and soldered together, and the 

 joints bronzed over. The casts need not necessarily be 

 taken in plaster, wax or some other available substance 

 answering almost equally well. 



Bronzing is a branch of the subject to be dealt with 

 presently ; but before leaving the discussion of the typing 

 processes as applied to large work — large, that is to say, in 

 comparison with that with which we have next to deal — it 

 may l)e as well if a few remarks are made upon the probable 

 causes of failure and the steps that may be taken to avert 

 a failure. It must be distinctly borne in mind that the 

 perfect practice of every art is more or less the outcome of 

 experience and experiment, and that, therefore, one can 

 scarcely hope to turn out excellent work as the result of a 

 first eflbrt. 



In the great majority of cases failure may be ascribed 

 to an imperfect mould, or to an imperfect conducting 

 surface. A great deal, of course, depends upon the con- 

 dition of the bath and of the battery, but even when these 

 are all that could be wished, difficulties and disasters present 

 themselves which are attributable to one or other of the 

 causes referred to. In the first place, every care should be 

 taken to prevent " air-holes " forming between the mould 

 and the original. In using gutta-percha the injunction to 

 work it outwards from the centre and to apply a gradually- 

 increasing pressure must not be lost sight of, and generally 

 it may be said that to work from the centre is one of the 

 readiest meaus of securing the absence of these holes. 

 There are, however, times when even the greatest care is 

 insufiicient to insure immunity from this trouble, and when 

 one can only hope that things have gone well. This is 

 especially applicable when the object is so much under-cut 

 as to involve the production of an elastic mould. The chief 

 safeguard against air-holes in plaster moulds is to use a 

 thin material and to aid its distribution with a hard brush 

 or a small piece of wood. The brush should be transferred 

 to a vessel containing water immediately after it has been 

 used. Gutta-percha has the disadvantage of contracting 

 on solidifying, and hence the necessity for the increasing 

 pressura 



It follows from what has just been said that a careful 

 and thorough scrutiny of the mould, prior to its being 

 blackleaded, is imperative. Electrotyping is a faithful 

 process, and shows up the blemishes of a model quite as 

 prominently as the beauties. Being satisfied that the 

 mould is good — that is, that it reflects perfectly the object 

 we are desiring to copy — the next care is the blackleading. 

 Only the best plumbago should be used, and care is neces- 

 sary to reject any gritty particles that would be likely to 

 scratch the mould or to impart their impress to the face of 

 the electrotype. The plumbago must be rubbed on 



thoroughly and carefully, and nought should satisfy the 

 worker short of a bright, polished surface, free from spots 

 or specks. The appearance of holes in the copy is often 

 due to an imperfection in the conducting surface. The 

 plumbago should not be allowed to reach to the back of 

 the mould or to any other undesirable point. Should it 

 do so, the best plan is to cover up the excessive particles 

 with varnish, oil, wax, or some such substance. Plum- 

 bago, however, cannot always be used, and then the 

 best course open is to apply the phosphorous and silver 

 process. As we shall see hereafter, this process is not 

 always practicable, as the phosphorous is apt to render the 

 copper surface more or less brittle. Where the mould is in 

 considerable relief, the deeper parts require more careful 

 attention as regards blackleading, &c., than the more 

 prominent portions, because in the first place, the deepest 

 parts of the mould become the highest in the copy, and 

 therefore the most subjected to wear ; and, in the second 

 place, because the deeper portions are farthest from the 

 anode, and therefore the path between such part and the 

 anode is one of comparatively greater resistance. 



Concerning the manipulation of the cell, there is little 

 to say that has not been already said. The best materials 

 should be used — they are far the cheapest in the end, and 

 save one from interminable disappointment. Where 

 Daniell cells are used, the copper that is deposited upon 

 the negative plate forms the best anode for the bath that 

 can be procured, because of its extreme purity. The 

 bluestone, too, should be as pure as can be obtained. A 

 very frequent impurity is iron, which may be easily 

 detected by placing a small quantity of the solution in a 

 test-glass, and then adding "an excess" of ammonia, that 

 is, adding the solution of ammonia until the copper solu- 

 tion exhibits the alkaline property of rendering a jiiece of 

 red litmus paper blue. Should there be any iron pre.sent, 

 it will be deposited as a dark precipitate at the bottom 

 of the glass. The copper solution becomes of a beautiful 

 blue colour. Pin-holes are sometimes found in the copy, 

 due generally to too strong a current or too feeble a solution, 

 either of these circumstances causing the decomposition of 

 the water, with the result that bubbles of hydrogen gas 

 form on the negative plate (the mould). Sometimes, how- 

 ever, these holes may be due to bubbles of air which have 

 adhered to the mould after its immersion. That bubbles 

 do so adhere may be easily proved by placing a substance 

 under water. Be it never so smooth, the smallest grain of 

 foreign matter is prone to take with it an air-bubble, even 

 if it be only a small one. It is usual, therefore, to wet the 

 surface of the mould thoroughly before placing it in the 

 bath. With small models this is easily done, although with 

 large moulds of printing type it becomes a more difficult 

 task. Then, again, there is the damage that may be 

 efl[ected by removing the mould before the deposition is 

 completed. This, if avoidable, should rarely be done, and 

 under no circumstances should the surface of the copy be 

 touched with the hand, except under water. If the hand 

 or the finger is at all dirty or greasy, there is considerable 

 risk of its leaving a mark upon the mould, over which the 

 copper fails to be deposited. 



One of the greatest troubles attending the manipulation 

 of the cell arises from the impurities inherent to all market- 

 obtained copper, and precautions must be taken to prevent 

 them from reaching the mould. Should they succeed in so 

 doing, the probability is that they will seriously afTect the 

 deposition, giving it a dirty appearance, and often inter- 

 fering with the texture. The impurities of commercial 

 copper are very numerous, and equally varied in their 

 proportions. They generally separate from the anode as it 

 dissolves in the solution, and, falling to the bottom of the 



