48 



♦ KNOWLEDGE 



[July 18, 1884. 



the figure. The jar is filled up to an inch or two above 

 the figure. Thus filled, the jar is placed aside for a couple 

 of days or so, until, in fact, the mi.xture is thoroughly set. 

 The jar is then turned upside down, and a tap or two 

 releases the mixture from the sides of the jar, that is 

 unless the shape is an unusual one, preventing the mould 

 from slipping out. It is scarcely necessary to say that the 

 best form of vessel is one that tapers slightly towards the 

 bottam. The position of the back of the model being known 

 (by the mark placed on the outside of the jar), a clean, thin 

 sharp knife is inserted in the mould over the head of the 

 model, and passed down its back, keeping close to the 

 figure. The mixture which has cooled over the base of 

 the model is also carefully removed. The mould, being 

 highly elastic, may then be opened with the hands, and 

 another pair of hands being called into requisition, the 

 model may be removed. The mould, in virtue of its 

 elasticity, springs back on being released, and we thus 

 obtain a good negative representation of the figure. We 

 may, if we so please, take our copy from this mould, but 

 the task is rather a trouljlesome one, and the result 

 often disappointing. The admixture of treacle with the 

 glue prevents the shrinking which would otherwise 

 take place on cooling, but it does not overcome 

 the tendency to absorb water. Such an absorption 

 would matter little were it not that it produces a 

 considerable swelling, when the proportions of the 

 figure would be entiiely lost. Nor would our troubles end 

 here. The absorption may, however, be prevented by 

 adding to the hot mixture of glue and treacle a small 

 quantity of tannic acid to the extent of two per cent, of the 

 quantity of glue when cold. Another modeof excluding water 

 is to immerse the elastic mould in a weak solution of bichro- 

 mate of potash and allow it to dry in the sun. A thin im- 

 pervious film is thereby deposited. A good waterproof 

 coating is that previously referred to as guttapercha varnish, 

 made by dissolving the percha in bisulphide of carbon. 

 This, ob\'iously, must not be applied to the interior of the 

 mould, or we should get no deposit. The inner surface 

 may, however, be well pi-otected by a thoroughly good 

 coating of plumbago, which must be deposited, as facilities 

 do not present themselves for rubbing the blacklead in. 

 Of this, however, more anon. The mode of procuring the 

 deposit must also be deferred for a few minutes. 



Whether the elastic mould is or is not used to receive 

 the copper deposit, it should be bandaged up so as to 

 prevent it falling out of form, as its elasticity would 

 otherwise cause it to. 



When the deposit is not intended to be taken in the 

 elastic mould, it may be placed back in the vessel in which 

 it was moulded. A second mixture is then made by melting 

 together 2 lb. of beeswax, 1 J lb. of resin, and a j lb. of 

 tallow. During the heating the materials require to be 

 well stirred so as to ensure an intimate mixture. The 

 vessel containing the mixture should then be placed on one 

 side for a short time, until it has nearly set. It is then 

 poured gently into the cavity in the elastic mould until it 

 is filled up. The jar thus filled is put aside for some 

 hours until everything is quite cool, when it is inverted, 

 and the elastic mould with the beeswax mould inside 

 it slips out. The former may be again opened, and 

 the latter withdrawn. Had the beeswax been poured 

 in when warm it would most probably have melted, 

 and maybe have united with the treacle mixture, and so 

 have spoiled it. The beeswax mould thus obtained is 

 obviously a copy of the model, and can hardly be used to 

 ■obtain an electrotyped copy direct. It is, however, placed 

 in the jar in a position akin to that occupied by the original 

 (Sgure. A thin plaster of Paris paste is next prepared and 



poured steadily into the jar up to the level of the base of 

 the mould. When the plaster is thoroughly dry, the bees- 

 wax copy is melted out, and we have thus a plaster of 

 Paris negative. 



It is evident that such a mould cannot have plumbago 

 rubbed over its internal surface. To get a conducting sur- 

 face the cavity is washed out two or three times with a 

 solution composed of 32 grains of phosphorus to 480 grains 

 (l^oz.) of bisulphide of carbon. After this it is washed- 

 out with a solution of silver nitrate, one pennyweight of 

 the nitrate being dissolved in a pint of distilled water. 

 The plaster mould being prepared, it is placed in the bath, 

 or it may, for the matter of that, form its own bath. The 

 ca\-ity is filled with the bhie-stone solution, and, the 

 conducting surface being conuected with the zinc pole 

 of the battery, the copper pole is connected to the 

 anode, which should consist of a mass of copper 

 as near the general proportions of the figure as 

 possible. It has previously been pointed out that a great 

 deal depends upon the resistance in the bath, that is to say 

 upon the relative distance between the anode and the 

 mould. It is almost impossible to emphasise too strongly 

 the necessity that really exists for keeping the anode as 

 nearly parallel as possible with the various parts of the 

 mould. When the substance to be coated is cylindrical, 

 it is almost enveloped by two bent sheets of copper. When 

 the mould is full of irregularities, it is placed at a consider- 

 able distance from the anode so that the relative differences 

 of resistance are reduced to a minimum. The resistance 

 introduced by increasing the distance is compensated for 

 by the addition of another cell, that is to say, by increasing 

 the electro-motive force. Where there are a few cavities, 

 the deposit is sometimes started in them first by using a 

 small anode, and placing it in them. Another device is 

 that of employing leading wires, as indicated a fortnight 

 since. 



It is advisable, when coating a mould, such as the one in 

 hand, to cause a current of the liquid to flow through it, 

 otherwise the solution will degenerate. This is considerably 

 facilitated by boring a small hole in the bottom of the 

 mould, somewhere near or on the back of the head. 



Sometimes, more especially when the model is a large 

 one, the mould is made in two, or perhaps a number of 

 pieces. To make it in two, the model is embedded to half 

 its depth in fine sand, the surrounding surface being made 

 fairly level, two or more pegs are stuck in the sand, and 

 then, a little thin plaster having been brushed rapidly into 

 the crevices, a quantity of plaster is poured on. When it 

 has set it is removed with the model. The level surface of 

 the sand being spread over with oil, the reverse side of the 

 model is moulded in a manner similar to that adopted with 

 the first half. The plaster is then placed in a shallow tray 

 containing stearine until it is well saturated, when it is 

 taken out and thoroughly and carefully plumbagoed. The 

 electrotype is taken in halves, and when both halves have 

 attained a sufticient thickness they are trimmed, and the 

 edges soldered together, the joint being carefully bronzed 

 over. This process, however, cannot be adopted where 

 there is any considerable undercut, such as would be met 

 with, for example, in taking a copy of a figure recumbent 

 on a base. The number of pieces would then have to be 

 more numerous, unless the elastic mould process were 

 resorted to. 



Very large objects are generally sacrificed, but of this 

 more in my next. 



The Gas Company at Leipzig intends, says a contemporary, to 

 ask the Mnnicipal Council of that town for a concession for lighting 

 the streets and houses bv electric light. 



