In one of the vault bricks, at the lower end of the channel, a mouth must be made much like the air vents, & big enough for you to put in two fingers quite comfortably. The air vent too must be the same size. This mouth, out of which the bronze is to flow, must be made from one brick; & mind that it is a good sound one. The said brick, too, must, moreover, be built into its place just as the others were, and helping with them to gradually lock in the vault at the top. So that you don’t think me inaccurate, I would have you know that this mouth is called bocca della spina, the mouth of the plug; it must be half a finger wider inside than out, and before you pour out the metal you keep it stopped with an iron stopper luted with a little ash made into a kind of paste. Then you take a quarry stone about half a cubit square & make a hole through the middle of it. This hole is to be exactly the size of the mouth just made in the brick, that is to say on the side adjoining the brick, but on the other side, the side away from the furnace, it is to be six times as big, and it should be cleaned off[264] outside. Then you join it to the brickwork wall of the furnace with the earth, & in the manner mentioned above; but because the base and sides of the furnace have also to be considered, as I said before, you cement these with good ordinary lime. Similarly all the quarry stones must be the same size as the first piece, and be attached to the walling in the same manner, & they must be carried up to the height of the vault, but straight; so that in the event of any accident happening to the vault, for to that the craft is often liable, it may be mended or put in order. When you have walled your furnace round in this way you must be careful to join at the shoulders [265] of the principal orifice, by which the flame enters, [266] a hearth two-thirds of a cubit square & two cubits deep, measured from the bottom of this hole of the hearth. In this cavity you put six or seven iron bars, these are about two fingers thick, & of such length as to project beyond the sides of the hole about four fingers each way, and they rest upon pins set at intervals of about three fingers apart. This hearth made over the fire bars is constructed in just the same way, & with just the same bricks, and cemented with just the same mortar as was the furnace; it must stand from the ground to about the middle of the hole where the fire enters the fire bridge, and the part above this point must be narrowed to one-eighth of a cubit. Straight through this hole the wood is put. Under the grating a trench must be dug, & five or six cubits long, in the direction in which the draught is to pass through the grating into the hearth. Care must be taken that the draught only blows in one way, and that long-ways. We craftsmen call this trench bracciaiuola, the ash-pit, because all the ashes fall into it. How long the fire is to be kept up must be a matter of judgment; sometimes the master may have, owing to work he has to do on his mould, to keep it up for quite four or six hours. When the wood logs are burnt through, they fall into a great pile below the grating. And sometimes they heap up in such a way as to obstruct the force of the draught through the hearth, that it cannot do its work; heed must be taken then, that when the pile begins to grow big, the ashes must be raked asunder from time to time. To do this, you must have what we call a rastrello or rake, which you make as follows. You take a piece of iron half a cubit long and one-eighth of a cubit thick; on to the middle of this piece and at the upper and thicker side of it you weld an iron rod two fingers thick and two cubits long, at the other end of this is fashioned a ferule,[267] into which is fitted a wooden handle at least four cubits long.
Take heed, too, that when your whole furnace is duly made as above directed, you gird it round with two stout iron bands, the one round near the base, the other about one-third of a cubit higher up; the thicker and stouter these hoops are, the better, for I know by the experience of the casting of my Perseus how terrific the might of the fire is. The opening of the hearth through which the wood is put must be kept closed. The covering must be made in the form of an iron spade, of such a size as shall well cover the opening, and to this spade a handle of such convenient length that when, now and again, you have to manipulate it for putting on fresh wood or otherwise, you don’t burn yourself. It stands to reason that before all these things are accomplished, the metal has already been put in the furnace, & it must be stacked up in such a manner as to admit of flames playing easily through it, for this will make the working of your furnace much more effective.
Know, too, gentle reader, what up to now I have forgotten to tell you, that when with due care your furnace is made, you must, before putting the metal into it, heat it well through for a space of twenty-four hours; for if you do not do this, you will not get the metal to melt, nay rather will it stiffen, [268] and certain fumes will result from the damp earth that will so impede your work that it may be eight days before your metal begins to flow. That is what happened to me in Paris. I had made a little furnace and had put my trust in a very excellent old fellow, quite the best of his craft and about eighty years of age; but he hadn’t dried the furnace properly, and, sure enough, just as it was on the point of melting, & the fire at its fiercest, out came these earth fumes. When the old worthy saw that for all his heeding the metal was stiffening, he got into such a stew, the poor old chap, that what with his mighty exertions to overcome the difficulty, he fell flat down, and I took him for certain dead. Howbeit I had a great beaker of the choicest wine brought him, & since there was no such great risk in leaving the work as there was in the case of my Perseus; since, too, I served that most admirable of kings, and thus had not to bother so much about the peddling trivialities of making it pay, for however big it was it never mattered with him—I mixed a large bumper of wine for the old man, who was groaning away like anything, and I bade him most winning-wise to drink, & I stretched out my hand to him and said: ‘Drink, my father, for in yonder furnace has entered in a devil, who is making all the mischief, and, look you, we’ll just let him bide there a couple of days, till he gets jolly well bored, and then will we, you and I together, in the space of a three hours’ firing make this metal run like so much butter, & without any exertion at all.’ The old fellow drank, & then I brought him some little dainties to eat, meat pasties they were, nicely peppered, and I made him take down four full goblets of wine. He was a man quite out of the ordinary, this, and a most lovable old thing, and what with my caresses and the virtue of the wine, I found him soon moaning away as much with joy as he had moaned before with grief. When the appointed day came the fumes had duly evaporated, the furnace was quite ready and well heated, & in two hours we cast 1500 lbs. of metal, with which I finished certain portions that were left of my lunette of Fontainebleau. And that is why I insist upon your well heating the furnace, and also upon making two little quarry stone doors[269] at the furnace openings, and you make in each of them two holes one and a half fingers wide respectively, and four fingers apart from each other, and these holes serve for the insertion of an iron fork made specially to fit into them, with which now and again, as need occurs, you may open & shut the doors.
Remember, too, that each time new metal is to be put into the furnace it must be first put up against the doors[270] till it becomes red hot, for if you put it in too soon with the other metal already in, you run the risk of cooling the latter, & so caking it, [271] much as before referred to. Hence the very greatest care must be taken on that point.
In Paris have I seen craftsmen cast the most wonderful things imaginable, and also make equally wonderful blunders. And this is due to the fact that technical skill [272] serves you up to a certain point, but in some accident, for instance, you need the deeper knowledge of the principles of the art that leaves technical skill on one side, as I have evidenced to you above.
Indeed I may add that I have seen 100,000 lbs. of metal cast at one time with so much ease that I marvelled at it, so great was the technical skill with which it was done; at another time I saw a little error made that might easily have been remedied. I stood & watched whether they knew how to put it right, and I saw them throw it up, work and all, and so lose hundreds of scudi. Willingly would I have shown them what the remedy was, but their presumption was so huge that had they not known how to put my remedy into practice, they would have been quite capable of saying that I myself was the cause of all the ruin. So I stood mum and grew wise at their cost.
Gentle reader, let that suffice about furnaces and bronze casting, and let us now turn to other branches of the art.
[It is interesting to note that Biringoccio, a professed metallurgist, and a contemporary of Cellini, describing the reverberatory furnace (reverbero) in his celebrated metallurgical treatise ‘della Pirotechnica,’ 1540 (see above, [p. 106]), the first accurate treatise of its kind, gives fewer details than Cellini; he gives, however, diagrams which are very precious. Cellini’s decision not to give diagrams is much to be regretted. Biringoccio has, however, the following among other drawings of reverberatories. It serves to show what Cellini’s furnaces would have been in sectional plan. The letters are mine. It will be observed that no chimney is shown.]
