The green patina on statues is usually given by some acid or salt, sal ammoniac being generally used. The surface is scratchbrushed and cleaned off as perfectly as possible, to get off every particle of grease, and then a strong or weak solution, according to the colour desired, is stippled on with brushes; if the status be warm, so much the better, as it makes the evaporation quicker. Some patinas are produced by heat. It is a very complicated subject, and one on which very little is known, and consequently the results are various There seems no scientific accuracy about it.' The Chinese and Japanese probably know more about it than we do.
In some cases, a small piece is cut out of the wax model for the introduction of the core, and is afterwards fitted on again. In larger works, openings must be left to get the core out, as it is not desirable to leave it in. That is one reason why large statues should not be cast in one piece. A colossal figure cast in wax in one piece at Pappi's in Florence, the height being 12 ft. 4 in., was a very large and difficult work to cast in one piece. In order to get the core irons out without injury, little boxes had to be made here and there, with lids, and these lids, instead of being put back into their places and closed up, were attached by little jets and vents of their own to the casting; then the core irons were got through the openings, and each little lid was afterwards fitted into its place, and all those joints had to be closed up. It seems just as well to have the statue in several pieces fitted with a good box-joint.
In a large statue the cores add very much to the weight; besides which the core might absorb moisture and freeze, and cause cracks in the statue. This has occurred in some bronzes at Venice where the core had been left in. In small articles there is no objection to leaving the core in, except the additional weight, which would not be great.
As to the proper thickness for the metal, in large work the thinner the better, in small work it would be difficult to cast if they were made too thin. What is very thin for a large work would be very thick for a small one; and small works are generally much thicker in proportion than large ones. It is very difficult to get a uniform thickness throughout if the wax is very thin. A casting no thicker than a penny would be very thin. The thickness is governed by the surface over which the metal has to run. It would not do to be too thin, or the metal would chill before the whole surface was covered. In the Wellington monument, the metal is about | in.; in the Sphinxes 1/2 in., because there is a greater surface for the metal to run over. In the Beaconsfield statue it is about 3/8 in.; but in some portions 5/8 in. Some portions of a statue may be almost solid, where there are folds in the drapery not large enough to get a core in properly; but it does not do to cast too many parts thick, or in cooling they will draw away from the thinner parts and break.
(6) The method of'making moulds for the Cera perduta process appears to me to be antiquated and by no means simple, or likely to give certain results. The material used for the cores, i.e., plaster of Paris and brick dust or loam, although firm when set, becomes friable and weak after firing, causing great liability to waste and risk of sandy dirty castings. A certain amount of plaster is necessary for constructive purposes, but some binding material, such as soda or other flux, should be mixed with the core to bind it together, when burnt; the venting of the cores may be greatly assisted by adding sawdust to the mixture, which burns out, leaving the core porous, and assisting its strength by the fusible salts contained in the wood.
For the outer mould a first layer of very fine loam, ground in petroleum oil, should be applied with a brush, and over this a few coats of mixture of fine sawdust and loam, to which has been added a proportion of short fibrous asbestos, the outer casing being made of the same mixture but coarser. Each coating must be put on in such a manner by the point of the brush as to leave a rough surface, which readily binds and adheres to the next coating, and it will, of course, be understood that the asbestos fibre must be mixed by hand; if ground in a mill the fibre is destroyed, and the asbestos becomes useless. A mould made in this manner pan be done rapidly, dries in a short time, and will never crack; in fact, it is strong enough to stand the pressure of the fused metal in small work without binding or any external assistance, and there is no risk in handling the moulds without any special care. I have lifted a block of this material 30 to 40 lb. weight when half dried, by one corner, and dropped it a distance of 3 ft. without a crack appearing. The system of pouring the metal from underneath is not so important with these moulds; they are as firm as an ordinary Bath-brick. At the same time it is always advisable for artistic work where fine surfaces are necessary.
The air outlets may be formed by thin sticks of wax, as they can be numerous and very small; but the " gate " or inlet for the metal is usually much better for large surface work if made wide than very narrow, and if of this shape it can usually be placed where it can be cut off, and no finishing is necessary.
There is one method of pouring which is only practised for some special purposes, but which would give perfect results in figure casting, as the metal flows calmly in without any rush or disturbance. In this method the gate or inlet is at one side near the bottom, the basin or "sow'" is immediately below it, and' the basin'and mould are fastened together and mounted on a swivel. As soon as the melted metal is placed in the basin, the whole is gradually tilted over, the metal flowing quietly from the basin into the mould, the vents and riser being of course on the same side as the gate or inlet, so that when the mould is turned completely over all the openings are on the upper side.