[§§ 55–69, ante, p. [158] ff.]
Vasari’s account of the processes attendant on casting in bronze is intelligible and interesting, though he had himself little practical acquaintance with the craft. Benvenuto Cellini on the other hand was an expert bronze founder and the account he gives of the necessary operations in the first three chapters of his treatise on Sculpture is extremely graphic and detailed, and may be usefully employed to amplify and explain Vasari’s notice. An expert knowledge of the founder’s craft was not by any means universal among the Italian sculptors of the Renaissance. Donatello did not possess it, nor did Michelangelo. In the case of the former this is somewhat remarkable, for Donatello was such a vigorous craftsman that we should have expected to find him revelling in all the technical minutiae of the foundry. We are expressly told however by Pomponius Gauricus that Donatello lacked this knowledge, and never cast his own works but always relied on the help of bell founders (Hans Semper, Donatello, Wien, 1875, p. 317). Michelozzo on the other hand, who worked with Donatello, could cast, and so could Ghiberti, A. Pollaiuolo, and Verrocchio, while Alessandro Leopardi of Venice, who cast Verrocchio’s Colleoni statue, was famed for his practical skill in this department. It was customary, when expert help in casting was required, to enlist the services of bell founders and makers of cannon, but Cellini warns sculptors against trusting too much to these mere mechanicians who lacked ingenuity and resource.
The following general sketch of the processes of casting will render Vasari’s account more easy to follow. A successful cast in bronze consists in a thin shell of the metal, representing on the exterior the exact form of the model, or the complete design in the artist’s mind. The best way to procure this is to provide first a similar shell, perfect on its exterior surface, of wax, and then to melt away the wax and replace it by molten bronze. For this to be possible the shell of wax must be closely sealed between an outer envelope and an inner packing or core. It can then be got rid of by melting, but care must have been taken lest the core when it loses the support of the wax should shift its position in relation to the envelope. To prevent this, metal rods are run, skewer-fashion, through core and envelope to retain them firmly in their relative positions. Molten bronze may then be introduced into the space formerly occupied by the wax, and this, when it is cold, and the envelope and core are both removed, will be the cast required.
Attention has to be paid to secure that there shall be no moisture and no remnant of wax in the parts where the molten bronze is to come, otherwise steam may be generated and a dangerous explosion follow. Similarly, air holes or vents must be provided, so that the air may escape before the flowing metal. The cast when cold should, theoretically, give a perfect result, but as a matter of fact, unless very accomplished skill or great good fortune have presided over the operations, the metal will be blistered or seamed or flawed in parts, and these imperfections will have to be remedied by processes which come under the head of chasing, and are described by Vasari at the close of the chapter.
A direct and ingenious method of procuring the needful shell of wax is that described by Vasari in § 67 as suitable for small figures and reliefs. Over the model for such a small figure an envelope is formed, in the shape of a hollow mould of fire-resisting material, so constructed that it can be taken to pieces, the model extracted, and the mould closed up again. The mould must now be cooled with cold water and it is then filled with melted wax. Contact with the cold sides of the mould chills the wax, which hardens all over in a sort of crust or skin. The rest of the wax, still liquid, is then poured out and the skin or crust suffered to harden. The interior is then filled with clay of a kind that will stand heat. Rods or skewers are passed through this and the envelope, the wax is melted out and its place taken by the molten bronze.
This process, one of course only suitable for small objects, presupposes the existence of a completely finished model to be exactly reproduced in the metal. The simplest of all processes of bronze casting dispenses with this model. Vasari does not describe this simplest method but Cellini, who employed it both for his ‘Nymph of Fontainebleau’ and his ‘Perseus,’ gives an account of it which is worth summarizing because the process is probably the one adopted in most cases by the old Greek masters.
Cellini tells us that in making his large lunette-shaped relief of the ‘Nymph of Fontainebleau,’ now in the Louvre, he began by modelling up the piece in fire-resisting clay, of course over a proper armature or skeleton. He worked it out to full size and then let it dry till it had shrunk about a finger’s breadth. He then covered it with a coating of wax of rather less than this thickness, which he modelled with the utmost care, finishing it in every detail so that it expressed to the full his own idea for the finished work. This was then carefully covered in successive layers with an envelope of fire-resisting material, which would be properly tied by transverse rods to the core, and braced on the exterior by an armature. The wax was then melted out, and the core and envelope thoroughly dried, when the molten bronze was poured in so as to reproduce the wax in every detail.
It is obvious that this is not only the most direct but the most artistic method of work. The wax forms a complete unbroken surface and receives and retains the exact impression in every detail of the master’s hand. If the cast be thoroughly successful, the bronze will reproduce the surface of the wax so perfectly that no further work upon it will be needed, and an ‘untouched cast’ will be the result. This method would suit the genius of the Greeks, and was no doubt commonly employed by them, but it has the practical drawback that if anything go wrong, and the bronze do not flow properly, the whole work is spoilt, and will have to be built up again de novo from the small study. Cellini tells us that his ‘Perseus,’ which he was casting in this fashion, nearly came to grief from the cause just indicated, and he accordingly recommends what he calls the second method, a longer and less direct process, which has however the advantage that a full-sized completed model is all the time preserved.
This process is the one described by Vasari. A full-sized clay model is prepared and finished, and this is then covered with a plaster envelope made in numerous sections, so that it can be taken to pieces and put together again without the model, which may be preserved for further use. The next step is to line the inside of the empty envelope, or piece-mould, as it is called, with wax, and to fill up all the rest of the interior with a core. The piece-mould is then removed, and the surface of the wax is carefully gone over to secure that it shall be perfect in every part. Over it is then laid in successive coats a fire-resisting envelope between which and the core the wax is hermetically sealed. The wax can then be melted out and replaced with bronze. The piece-mould, which has been detached section by section from the wax, will serve again for other reproductions. The processes in which wax is employed are called cire perdue processes, because the wax is got rid of in order to be replaced by the metal. The usual process in vogue at the present day dispenses with any employment of wax. The figure to be cast is piece-moulded and reproduced in a suitable material, a certain thickness of which is in every part pared away according to the thickness required for the bronze. This core is then replaced with proper adjustments within a fireproof mould, and the bronze is poured into the space prepared for it.