§ 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.