[250] Un morto: our workshop slang of the ‘dead horse’ would seem to meet Cellini’s meaning here.
[251] Una continova pratica.
[252] Nocciolo.
[253] Di poi sciolto il suo cavo.
[254] Tonaca.
CHAPTER IV. HOW TO CONSTRUCT FURNACES FOR CASTING BRONZE, WHETHER FOR STATUES, ORDNANCE, OR OTHER SUCH-LIKE THINGS.
Furnaces for casting bronze have to be made by each master according as the special needs of his piece of work require it. Inasmuch as at the beginning of my book I promised always to illustrate what I was describing by work of my own, I will do so with furnaces also. When working for good King Francis, I had to make a great bronze gate, for which a special furnace had to be constructed. This I did in my own castle, given to me in royal letters patent by his Majesty, whom I served most loyally for four years; and these letters patent I brought back with me to Florence, if only to show in Italy & my native land what great treasures may not be acquired, and how good it is after having been trained in Italy to leave home, and reap such useful & honourable fruits abroad.
Well, then, having to make a furnace, this was the way I did it. The hollow within was three Florentine cubits wide, which made it about nine cubits in circumference, the height of the vault of the furnace was equal in size and shape to the half circle of the bed.[255] To this bed, most gentle reader, I want you to give special attention; I do not intend making a drawing of it, because I have seen so many architectural drawings altered and spoiled, so I shall content myself with words only to convey what I mean, and such as I trust may suffice. [256] In a furnace of this nature, the bed, i.e. the place where the metal is put, must be constructed with a fall, just as I made the little one in question. The total fall to the bottom of the bed should be about one-sixth of a cubit, and see that it is shaped in the manner of the streets you go a-walking in that have in the middle of them what in Tuscany we call a rigaguolo or gulley; this gulley runs direct to the mouth of the outlet hole out of which the metal is to flow. The shoulders[257] of the gulley should slope up ever so gently, till they come to within about one-third of a cubit at the two gates[258] where you put the bronze. This third you can increase or not by one-sixteenth of a cubit according to whether you wish to increase or decrease the depth of your furnace. The third door, about which you need not be so particular, is the one through which the fire enters, &, as it is not brought into direct connection with the bronze, it need only be blocked with a little mound of earth about three fingers high. The bed of your furnace should be of bricks, specially constructed, they should be small, but bigger at one end than the other, & measuring one-sixth of a cubit in either direction; by far the best are those used for glass furnaces, which are made much in the same way as other bricks. Some have shaped their bricks with a cutting instrument[259] as they went along, but I find, after having tried one way & another, that the best results can be obtained by having them all the same size. Care should be taken to make the bricks of a clay that does not yield in the fire; in my native city of Florence we use a kind of white clay, said to come from Monte Carlo, and all our glass furnaces were constructed with it. In France I found another and ever so much better way. The bricks are one-fourth of a cubit long, & the same width as the above, they call them ciment, and they make them out of crucibles used in founderies, of which there are no end in those parts. But your master has got to accommodate himself to the conditions of workmanship in every place he works in. When your bricks are thus made, they must when quite dry, be carefully worked over with iron tools, somewhat in the nature of axes or large chisels made specially for the purpose, so as to make them cohere as well as possible. After this they are gradually cemented with quarry stones[260] to the thickness of about half a cubit, so that you get an absolutely firm floor for your furnace. These quarry stones should be at least one-third of a cubit in size, and ever so firmly united. The first or lower part of a furnace of this description should be, in diameter two-thirds of a cubit larger than the upper portion. Both must be walled with ordinary lime, provided it be good, and after the lower, you proceed to wall the upper, the portion in which the bronze has to be placed.
Having fashioned your bricks out of the fire-resisting earth just referred to, you take some of this earth, and make a paste of it as you would of lime, minding however, that it is well sifted and clean, and with it wall the whole base of your furnace. I insist again upon the need of your working over the bricks with your chisels carefully, & smoothing them well, so that they fit together absolutely, and in thus fitting them, you must make the jointings as thin as possible. Sometimes it happens, owing to some little negligence on the master’s part, if he mix the liquid earth too coarsely, that the tiniest little cracks form in the drying. These cracks, however small they be, are mighty dangerous, & may cause incalculable mischief; for, when the bronze grows liquid, such is its terrific force, that it penetrates into them, be they never so small, and I myself have seen the whole thing burst up owing to this.[261] But when due care is observed, & the walling made with the finest possible liquid earth, there is no need for cracks, your bronze may be safely melted, and all your work come scatheless from the furnace.
After the bed you build up the vault with similar bricks in the same way, in doing which you must remember, as I said before, to make your two openings for putting in the bronze; two-thirds of a cubit wide and three-quarters of a cubit high will suffice them, and they must be semi-circular atop. There must also be a third opening two-thirds of a cubit wide and one cubit high for the fire to enter in at, so that the flame, as is its wont, may curl powerfully to the top of the vault, and thence curl down again, [262] and with great heat play upon the metal, & melt it rapidly. Four air vents have to be distributed round the spring of the top of the furnace vault.[263]