This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
This is the last operation. The block having become sufficiently cool, it is surrounded with iron frames placed one above the other; the space between the block and the frames is filled by pressing into it ordinary moulding earth. This operation requires the greatest care; its object is to prevent the block from bursting when the liquid bronze is poured into it by the pressure of the gas and the expansion of the air while the fused metal is flowing through the mould, a comparatively small quantity of metal in fusion being capable of producing effects of incredible force which it is difficult to account for.
The block being perfectly iron-bound, a basin of iron covered with baked clay and pierced with a conical funnel is placed over the runner and closed with an iron stopper, from which projects a long stem. The hole of the basin communicates directly with that of the runner; the opening of the vent is left free, but in front of it a small basin is hollowed out of the block. Everything is now ready for the casting.
If the bust is calculated to weigh 50 lb., 80 lb. of bronze are put into the melting-pot in order to be certain of having enough metal, and it is necessary to allow for the runner, the vent, and the drain. The bronze which has hitherto given the best results is composed as follows : - 70 lb. red copper, 28 lb. zinc, 2 lb. tin.
The bronze being sufficiently melted, the crucibles are lifted out of the furnace and are emptied into the basin above referred to; a workman at the word of command takes out the iron stopper, the molten bronze flows into the runner, penetrates into the mould, fills up all the hollows, and returns to its level, the surplus metal flowing out at the vent into the basin that has been hollowed out of the block to receive it, preceded by the air and gas driven out by the entry of the metal.
If the operation has been made without producing noise, the casting may be considered to have been successful, but notwithstanding all the care taken to attain success, some fault may have occurred. The natural curiosity to learn the result may soon be satisfied, for in 1/4 hour the metal will have cooled sufficiently to allow the block to be broken up.
The workmen begin by lifting off the iron frames, and then, removing the earth that was pressed round it, commence to break up the block with iron picks, proceeding with precaution, and as soon as any portion of the bronze shows itself the picks are laid aside for smaller and lighter tools, with which the "potin" that surrounds and conceals the work is at length removed, the bust gradually appears, and it is possible to judge whether the casting has been successful; the bust itself, however, is covered with a white crust from the "potin" still adhering to it, and which only partially detaches itself. To get rid of this crust entirely is a work of some time.
The runner, the vent, and the drain, which have been transformed by the casting into solid bronze, are now sawn off, the core inside the bust is broken up, and the bust is emptied; it is then placed for several hours in a bath of water and sulphuric acid, and when taken out is vigorously scrubbed with hard brushes, rinsed in clean water, and allowed to dry. The bust is now handed over to the chasers, who efface the traces left by the runners and vents, remove any portions of metal that may fill up the cavities into which the "potin" has not penetrated, stop up with bronze the little holes left by the iron pins, and in fact place the work in a perfect state, leaving, however, untouched the epidermis of the bronze, for in this consists the merit and value of the "cire perdue" process, which renders so completely every touch of the artist that it seems as if he had kneaded and worked the bronze with his fingers.
The bust, now completed, is placed in the hands of the bronze decorators, who give it a "patina" in imitation of that produced by oxidation; the colour generally preferred for portrait busts is the brown tone of the Florentine bronzes. This artificial " patina" can be produced in a great variety of tones, light or dark, but in every case it is preferable that a well-modelled work should have a dead unpolished surface. The decoration of a bronze work is a question of taste or fashion for which there is no rule, though no doubt for many the success of a work depends very often on its decoration.