CHAPTER IV. (XI.)
How Models for large and small Bronze Figures are made, with the Moulds for casting them and their Armatures of iron; and how they are cast in metal and in three sorts of Bronze; and how after they are cast they are chased and refined; and how, if they lack pieces that did not come out in the cast, these are grafted and joined in the same bronze.
§ 55. The Full-sized Model for Bronze.
It is the custom of competent artists, when they wish to cast large figures in metal or bronze,[[169]] to make first a statue of clay as large as that intended to be cast in metal, and to perfect the clay statue as far as their art and their knowledge will allow.
§ 56. The Piece-Mould in Plaster.
When this, which they call the model, is finished and brought to this point of perfection, they then begin, with plaster that will set, to build over it piece by piece, making the pieces correspond to the relief of the model. On every piece they make a key, marking the pieces with numbers or letters of the alphabet or with other signs in order that the pieces can be taken off and register together. So they mould it part by part, oiling the pieces of the cast where the edges have to be connected; till from piece to piece the figure grows, the head, the arms, the body and the legs, to the last detail, in such a manner that the concave of the statue, that is the hollow mould, comes to be imprinted on the inner surface with all the parts and with the very minutest marking which is in the model.[[170]] This completed, the plaster casts are laid aside to harden.
§ 57. The Construction of the Core.
The workers then take a rod of iron longer than the whole figure that they wish to make, and that is to be cast, and over this they make a core of clay into which, while kneading it to make it soft, they mix horse dung and hair. The core has the same form as the model and is baked in successive layers so as to draw out the dampness of the clay; this is of use afterwards to the figure, for in casting the statue all this core, which is solid, leaves an empty space that is not filled with bronze, because if it were, the figure could not be moved on account of the weight. They make this core large enough and justly measured, so that when the layers are heated and baked, as has been said, the clay becomes well burned through and so entirely freed from damp, that in pouring the bronze upon it afterwards it does not spurt nor do injury, as has happened many times, involving the death of the masters and the ruin of the work. Thus they go on balancing the core and adjusting and examining the pieces, till they tally with it and represent it, so that there comes to be left exactly the thickness, or, (if we like to say so,) the thinness, of the metal, according as you wish the statue to be.
Frequently this core has an armature of rods of copper across it, and irons that can be taken out and put in to hold it with security and with greater strength.[[171]] The core, after it is finished, is yet again baked with a gentle heat, and the moisture, should any have remained, entirely removed; it is then again laid aside.
§ 58. The Piece-Mould lined with a Skin of Wax.
Returning now to the pieces of the hollow mould, these are lined severally with yellow wax that has been softened and incorporated with a little turpentine and tallow.[[172]] When the wax is melted at the fire, it is poured into the two halves of the mould made up of the hollow pieces in such a manner as causes the wax to come thin according to the worker’s idea for the cast, and the pieces, which have been shaped to correspond with the relief of the core already made of clay, are joined to it and fitted and grafted together.
§ 59. This Skin of Wax applied over the Core.
Plate VII
ILLUSTRATION SHOWING PROCESS OF PIECE-MOULDING IN PLASTER
With thin skewers of copper the pieces of wax pierced with the said skewers are now fixed to the baked core, and so, piece by piece, they are inserted and fitted to the figure and render it entirely finished. This completed they proceed to remove all the superfluous wax that has overflowed into the interstices of the pieces, and bring it as well as possible to that finished excellence and perfection which one desires in the bronze cast. Before going further, the craftsman sets up the figure and considers diligently if the wax have any deficiency, and he proceeds to repair it and to fill up again, putting on more or taking away where necessary.[[173]]
§ 60. The fire-resisting Envelope applied over the Wax.
After that, the wax being completed and the figure braced together, he puts it where fire can be applied to it[[174]] on two andirons of wood, stone, or iron like a roast, arranging so that it can be raised or lowered; and with moistened ash, specially fitted for that purpose, by means of a paint brush he covers the entire figure so that the wax is quite concealed, and over every hollow and chink he clothes it well with this material. Having applied the ash to it he replaces the transverse rods, which pass through the wax and the core, just as he has left them in the figure, because these have to support the core within and the mould without, which is the casing of the hollow space between the core and the mould, where the bronze is to be poured. When this armature has been fixed, the artificer begins to take some fine earth, beaten together with horse dung and hair, as I said, and carefully lays a very thin coating all over which he allows to dry, and so on time after time with other coatings, always allowing each to dry until the figure becomes covered with earth raised to the thickness of half a span at the most.
§ 61. The External Armature.
This done, he girds those irons that hold the core within with other irons which hold the mould outside, and fixes them together, so that chained and bound the one to the other they form a mutual support,[[175]] the core within sustaining the mould without and the mould without holding firm the core within.
§ 62. The Vents.
It is usual to make certain little pipes between the core and the outer mould called vents, that have issue upwards; they are put, for instance, from a knee to an arm that is raised, because these give passage to the metal[[176]] to make up for that which on account of some impediment may not flow properly, and these little tubes are made many or few, according as the casting is difficult or not.
§ 63. The Wax melted out.
This done, the worker proceeds to apply heat to the said mould equally all over, so that it may become united and little by little be warmed through, and he increases the heat till the mould is thoroughly hot throughout, so that the wax which is in the hollow space becomes melted and all flows out at the spot through which the metal is to be poured, without any particle of the wax remaining within.[[177]] To be sure of this, it is needful, before the pieces of wax are grafted in to their places on the figure, to weigh them piece by piece; in the same way after drawing out the wax, it must be weighed again, when by making the subtraction the artist sees if any wax be left between the core and the mould, and how much has come out. Notice that the skill and care of the artist is manifested in the process of taking out the wax; herein is seen the difficulty of producing the casts so that they come out sharp and beautiful, for if any of the wax be left, it would ruin the whole cast, especially that part where the wax remains.
§ 64. The Mould in the Casting-pit.
This finished, the craftsman puts the mould under ground near to the furnace where the bronze is melted, propping it so that the bronze may not strain it, and he makes the channels through which the bronze is to flow, and at the top he leaves a certain thickness, which allows for the surplus of the bronze to be sawn off afterwards, and this he does in order to secure sharpness.[[178]]
§ 65. The Composition of the Bronze.
The artist prepares the metal as he thinks fit, and for every pound of wax he puts ten pounds of metal.[[179]] Statuary metal is made of the combination of two thirds of copper and one third of brass according to the Italian rule. The Egyptians, from whom this art took its origin, put into the bronze two thirds of brass and one third of copper. In electron metal, which is the finest of all, two parts copper are put to one part silver. In bells, for every hundred parts of copper there are twenty of tin, in order that the sound of the bells may carry far and be more blended; and for artillery, in every hundred parts of copper, ten of tin.[[180]]
§ 66. Making up Imperfections.
There only remains to us now to teach the method of grafting a piece into the figure should it have a defect, either because the bronze coagulated, or ran too thin, or did not reach some part of the mould. In this case let the artificer entirely remove the defective part of the cast and make a square hole in its place, cutting it out under the carpenter’s square, then let him adjust a piece of metal prepared for that spot, that may project upward as much as he pleases, and when fitted exactly in the square hole let him strike it with the hammer to send it home, and with files and tools make it even and thoroughly finished.
Plate VIII
ENGRAVINGS ILLUSTRATING THE PROCESS OF CASTING IN BRONZE
From the French Encyclopédie
§ 67. A simpler Method of Casting small Figures and Reliefs.
Now should the artificer wish to cast small figures in metal, they are first made of wax, or if he happen to have them in clay or other material, he makes the shell of plaster over them in the same way as for the large figures, and fills it all with wax. But the shell must be moistened that the wax, when poured into it, may set (with a hard skin) by reason of the coldness of the wet cast. Then by shaking about and agitating the cast, the wax (which is not hardened) within the cavity is thrown out, so that the cast remains hollow in the interior: the craftsman afterwards fills up the vacant space with clay and puts in skewers of iron. This clay serves then for core, but it must be allowed to dry well. Thereafter he adjusts the mould as for the other large figures, giving it its armature and placing the tubes for the vents. Then he bakes it and gets rid of the (skin of) wax and thus the vacant space remains clear so that the bronze can easily be poured in. The same is done with the low and half reliefs and with every other work in metal.
§ 68. Chasing the Cast and Colouring the Bronze.
These casts being finished, the workman then, with suitable tools, that is, with burins, burnishers, chasing tools, punches, chisels and files, removes material where needed, and where needed presses inward the overflow of the metal and smoothes it down; and with other tools that scrape, he shaves and cleans the whole of it diligently, and finally with pumice stone gives the last polish. This bronze which is red when it is worked assumes through time by a natural change a colour that draws towards black. Some turn it black with oil, others with vinegar make it green, and others with varnish give it the colour of black, so that every one makes it come as he likes best.
§ 69. Modern Tours de Force in small Castings.
But that is a truly marvellous thing which is come to pass in our times, this mode of casting figures, large as well as small, so excellently that many masters make them come out in the cast quite clear so that they have not to be chased with tools, and as thin as the back of a knife. And what is more, some clays and ashes used for this purpose are actually so fine, that tufts of rue and any other slender herb or flower can be cast in silver and in gold, quite easily and with such success, that they are as beautiful as the natural; from which it is seen that this art is more excellent now than it was in the time of the ancients.