[171]. In the case of a heavy casting such an armature is necessary, and must be carefully constructed to give support at all points. The armature within the core of the horse of Louis XIV is shown in Plate VIII, D.
[172]. Vasari here describes a method of constructing the indispensable shell of wax which is to be replaced by the bronze. The hollow piece-mould is lined section by section with wax and a core is then formed to fill the rest of the interior and touch the inner surface of the wax at every point. The plaster mould is then removed and the wax linings of each of its sections are applied, each in its proper place, to the core, and fixed thereon by skewers. There is then a complete figure in wax, but, as this is made up of very many pieces, it has to be gone over carefully to smooth over the joins and secure unity of surface. Cellini’s plan seems a better one. He lines his hollow mould with a sort of paste or dough, and then fills up with the core. The dough is then removed and wax is poured in in its place, thus forming a continuous skin and securing a more perfect unity in the waxen shell.
[173]. On Plate VIII at B we see the core covered with the skin of wax and carefully gone over and finished in every part. The system of pipes with which it is covered are the ‘vents’ that Vasari notices in § 62, and also the channels through which the melted wax is to escape and the molten bronze to enter, as noticed in §§ 63, 64.
[174]. Vasari actually says that it must be put ‘al fuoco’ ‘to the fire,’ but it is clear that he does not mean that heat is at once to be applied to it. If this were done the wax would all be melted off the core too soon, before it was covered by the outer skin. It is only when the wax has been securely enclosed between the core and the outer skin that heat is needed to melt it away and leave its place free for the molten metal.
[175]. Plate VIII, C, shows this outer armature, with the ends of the transverse rods holding core and envelope together.
[176]. ‘Give passage to the metal.’ Their essential purpose is to allow for the escape of air which would be dangerous if driven by the metal into a confined space.
[177]. It should be understood that, in the process Vasari has in mind, the melted metal is introduced at the bottom of the mould so as to rise in it and expel before it the air. It is not poured in at the top. Hence the metal enters at the same orifice at which the wax flows out.
[178]. Plate VIII, D, gives a section through the model in the casting-pit, when all is ready for the actual operation of introducing the molten metal. The wax has all been run out, and the outline of the figure and of the horse is marked by a double line with a narrow space between. It is this space that will be filled by the bronze which will be introduced through numerous channels so that it may be distributed rapidly and evenly over the whole surface it is to cover. When in the pit the mould is packed all round with broken bricks or similar material, so that ‘the bronze may not strain it,’ nor cause it to shift.
[179]. The wax has already been carefully weighed, and in order to estimate how much bronze will be required for the cast a rough calculation is made based on the amount of wax.
[180]. The subject of the composition of bronze and of other alloys of copper is a complicated one, for the mixtures specified or established by analysis are very varied. Normally speaking, bronze is a mixture of copper with about ten per cent. of tin, brass of copper with twenty to forty per cent. of zinc. Vasari’s proportions for bells and for cannon are pretty much what are given now. In the Manuel de Fondeur (Manuels Roret) Paris, 1879, II, p. 94, eight to fifteen per cent. of tin are prescribed for cannon, fifteen to thirty per cent. for bell metal, the greater percentage of tin with the copper resulting in a less tough but harder and so sharper sounding metal. It will be noted however that for statuary metal Vasari specifies a mixture not of copper and tin but of copper and brass, that is, copper and zinc. Brass is composed of, say, twenty-five per cent. of zinc and seventy-five per cent. of copper, so that a mixture of two thirds, or sixty-six per cent., of copper with one third, or thirty-three per cent., of brass would work out to about ten parts of zinc to ninety of copper, and this agrees with classical proportions. The Greeks used tin for their bronzes, but various mysterious ingredients were supposed to be mingled in to produce special alloys. The Romans used zinc, or rather zinciferous ores such as calamine, with or in place of tin, and this is the tradition that Vasari follows.