Chap. VIII. Secrets relative to the art of casting in moulds.

Valuable Secrets concerning Arts and Trades:
or Approved Directions, from the best Artists, for the Various Methods...
Printed by Thomas Hubbard,
Norwich, 1795
I. To cast a figure in bronze.

1. To cast a figure, or any other piece in bronze, you must, first, make a pattern with a proper clay. That clay ought to be mixed with sand, to prevent its cracking, when it comes to dry.

2. When the pattern is completed and the sculptor is pleased with his work, you mould it with plaister while it is still damp, because th drying, the parts of the pattern shrink, and lose their fullness. To that effect you begin by the bottom part of the figure, which you cover with several pieces, and by rows; as for example, let us suppose the first row from the feet to the knees; the second from the knees to the beginning of the belly; the third from the beginning of the belly up to the pit of the stomach, from thence to the shoulders, on which you lay the last row, which is to contain the head - Observe, how ever, that those divisions of rows admit of no particular rule, and ought to be intirely determined by, and adapted to, the size of the figure. For when the pieces are made too considerable, the plaister works too much, and fatigues itself, which is detrimental to its taking a true and precise impression of all the turns and shapes of the figure. So that at any rate, it is always preferable to make the pieces of the mould smaller than larger.

3. You must observe, that if the figure you are moulding have got any draperies, or other sorts of ornaments about it which require a good deal of trouble and nicety, you cannot help making a great many small parts and subdivisions in your mould, in order to enable you to strip them off the figure afterwards with more facility. In which circumstance, when all these small parts are made, and garnished with little rings to assist in pulling them off more easily, you cover them all over witg larger pieces, which containing several of the little ones, are called cases, and in French chapes.

4. When the mould is thus made and completed, you let it rest till it is perfectly dry. Then, before using it, they who are curious in their work, do not content themselves with imbibing it inwardly with oil, but they even make it drink as much wax as it can soak, by warming those separate pieces, and putting wax in them to melt. The motive, in doing this, is to render the wax-work, which is to be cast in it finer and more perfect. For if you imbibe the mould with oil only, the wax figure cast in such a mould always comes out a little rough and like flour, because the wax draws always the superficy of the plaister, and in reverse, the plaister draws also the superficy of the wax, which produces a great defect in the figure, and is a great obstacle to its coming out from the mould with that neatness it otherwise should.

5. The mould being therefore thus imbibed with wax, if you want it for a bronze figure, you assemble all the small parts of it each in their cases, and with a brush give them a coat of oil. Then, with another brush, give them another coat also of wax, prepared as follows. - Six pounds of wax, half-apound of hog's lard, and one pound of Burgundy pitch. - This preparation of the wax, however, must be regulated according to the country and the season. For in the heat of summer, or hot climes, such as Spain, Italy, and France, wax may be used alone, as it keeps naturally soft, and the other drugs above-mentioned, are added to it only to render it more traclable. Of this wax, therefore, whether prepared or natural, you lay another coat, as we said, in the hollow of the mould, to the thickness of a sixpenny piece. Then, with wax made in flat cakes, of the thickness of a quarter of an inch, more or less, according to that you are willing to give your metal, you fill all the hollow parts of the mould in pressing hard this sort of wax in them with your fingers. When thus fillet, you have an iron grate, larger by three or four inches every way than the plinth or basis of the figure. On the middle of that grate you erect one or more iron bars, contoured agreeable to the latitude and situation of the figure, and bored, from space to space, with holes to pass other iron rods of the size and length necessary to support the core (in French ame or noyau] of what you want to cast.

6. Formerly they used to make their cores with potter's clay mixed with hair and horse-durg well beaten together. With this compost, they formed a figure like the pattern; and, when they had well supported it with iron bars, length and crossways, according to its position and attitude, they scraped it, that is to say, they diminished, and took off from its bigness as much as they wanted to give to their metal. When that core was dry, they took the wax with which they had filled the hollow parts of their mould, and covered it with them. - This method is even practised row by some founders, especially for great bronze figures, because earth resists better the power of that red-hot melted metal, than plaister can; and this they reserve only for small figures, and those which are cast in gold or silver. However, when plaister is well beaten and mixed with brick dust also well beaten and sifted fine, it stands pretty well too. We shall therefore proceed on the method of casting on plaister cores.

7. You take then the first, or bottom rows, of the mould, filled by the last wax in cakes, as mentioned before, and assemble them on the iron grate round the principal iron bar, which is to support the core when made. When they are joined together, you give them a tye round very hard with cords, left they should vary from their position when you form the core.

8. To form this, as soon as the first set which completes the bottom row of the separate pieces of the mould is fixed, you pour phifter, diluted very clear, and mixed, as we said, with brick-dust, with which you fill up that bottom part of the hollow. Then, on this first bottom row of the mould, you place the second in the same manner as the first; then fill it likewise with your prepared plaister. Thus you continue to erect your mould from row to row, till you come to the last, and fill it as you go, with plaister, which is called forming the core. If the figure require it, you pass across the core some iron rods through the holes perforated forth at purpose in the perpendicular bars, in order to support the cort the better, and give it more strength and power to refill the effort of the metal when it comes in fusion upon it.

9. When all the pieces of the mould have been thus erected one upon another, and filled with plaister, yon limit stop a certain time to let it take a considence, then proceed to take off the cases and all the smaller parts of the mould contained in each of them, row by row, and one by one, in the same manner as you proceeded to erect them, with this difference, that in erecting them you begin at the bottom, and that in taking them off, you begun at the top; which, when done, leaves the figure to appear all in wax, covering the core, which is contained in the inside of it.

10. You are then to proceed to the repairing of the figure and finish it after the original. The sculptor, in that case, has even an opportunity of perfecting much some of the parts, in adding or taking off according as he thinks proper, to give more grace and expression to certain strokes, muscles, or features only; as for the disposition of the limbs, and their attitude, he can no longer mend or alter them.

11. The figure thus well prepared, you are to place what is called the pouring and the vent holes. The pouring holes are wax-pipes of the bigness of an inch diameter forsuch figures as are of a natural size; for they are to be proportioned not only to the size of the figures, but even to that of the parts of that figure whereon they are placed. The vent-holes are wax-pipes likewise, bat of much lesser size. Those pipes are cast in plaister moulds of what length you please. then cut to that of four or five inches, or thereabouts. They are cast hollow, to the intent of rendering them lighter, otherwise they might as well be cast solid. Those which serve for pouring, are placed in a straight perpendicular line, one above another, at six inches asunder, and sometimes nearer, when there are draperies, and much matter is used.

12. When the various pipes are placed and soldered against the figure, with wax, so that the end which is free should be upwards, and as much perpendicular to the figure as possible, you place another pipe of the same size quite perpendicular, which is to be fixed against every one of the ends of the others. All these pipes, both large and small serve for the pouring of the matter, and calling of the figure. You are to place three or four of them generally round the figure, which is determined by its size, bulk, and disposition.

13. But at the same time you are placing the pouring-holes, you must not neglect placing also those which are to serve for the vent. These iast are to be placed in the same line as and with the others, at the distance of four inches only from them, and fixed likewise by one end to the figure, and by the other to another long and perpendicular pipe, like those for pouring. Now, as it is necessary that all the wax, when you come to melt it, should, as we shall mention in its place, come out entirely from the mould, you must not fail to place those sorts of vent-pipes on all the rising and distant parts from the mean bulk of the figure, such as the arms, fingers, draperies, &c. &c. from which the wax must be got out with facility, either by means of particular vent-holes, so formed as to descend to the bottom of the figure, or by means of those large ones placed perpendicularly along-side of it. - Observe, always, to make the pouring-holes which come to the face and hands the smallest of any, that they may not affect too much the features and likeness, if any be intended, of those parts; and that you may the more easily repair those places with the chisel, when they are finished.

14. After these various pipes have been thus carefully fixed all about the figure, you must so place them that two of the main perdendicular ones should join together at five or six inches higher, and above the upper part of it, and be terminated by a wax cup of four inches deep, and as much diameter, under, and at the bottom part of which you solder them. This cup serves as a funnel to receive the metal, and introduces it into the pouring-holes, by means of its communcation with them, to convey it afterwards into all the parts of the figure at once, and form it. Therefore, if there be four perpendicular ascending pipes, you make two such cups, to communicate the metal to these pipes.

15. As for the vent-holes, you let them free above the top of the figure, and higher than the pouring ones, because they want no cups.

16. When the wax figure is thus completely repaired and garnished, with all its pouring and vent-holes, you prepare a composition of putty, and crucibles' powder, well grinded, and sifted very fine, which you dilute clear in a pan, like a colour for painting. With a brush take this composition, and cover all the figure, as well as the vent and pouring-pipes. This operation you repeat several times, ofeserving carefully to fill up all the cracks and crevices which may happe,in drying. When the wax is thus perfectly covered every where, you put with the same brush, another composition thicker than the first, and of a stronger sort.

17. This composition is made of the same materials as the other, buc with this addition, that you mix some free earth along with it, and horse-dung, quite clear from any draw. After having given six or seven coats of this, you give another coat again, much thicker still, of a stuff composed of nothing but free earth and horse-dung, and this being dry, you give half-a-dozen more of the same, allowing time between each to dry. At last, you put with your hand, and no more with the brush, two other coats of this same last composition, of free earth and horse-dung, mixed in form of mortar, obfering always that the one mould be perfectly dry, before laying on the other; and that there should be no part of the figure, whether naked or draperies, but what is equally covered with every one of the different coats we have mentioned.

18. Next to this, you must have flat iron bars turned and bent according to the disposition of the figure, which being fixed, by means of hooks at the sides of the grate on which it stands, rise up as high as the pipes, and joining close to the mould, unite at top by means of a circle of iron which runs through all the hooks, by which these bars are terminated. Then you surround again the figure with other iron bars, made in form of hcops, to prevent the others which go from top to bottom, and to which they are fixed by means of wires, from giving way; and, between every one of these bars, both perpendicular and horizontal, there must be no more than seven or eight inches distance allowed.

19. When all these bars are well fixed together, and enabled thereby to support and contain the mould, you take a compost of free earth, horse-dung and hair mixed together, in considence of mortar, and with this you cover the mould and the bars all over, without attending any more to the shape of the figure, so that there appears no more but a shapeless lump of clay, which ought to be of about four or five inches thick.

20. When the mould is thus completed, you are to dig a square pit sufficiently deep for the top of the mould to be somewhat lower than the superfice of the ground where the pit is dug, and sufficiently wide also to allow room of a soot and a half, free all round the mould, when descended into it. - At the bottom of that pit, you construct a furnace, on the top of which there is to be a strong iron grate supported by the arches and wall of the furnace, which is to be made of stone or bricks, as well as the four sides of the pit from top to bottom.

21. After the grate is placed on the furnace, you descend the mould on it by means of engines. Then, under the pipes which are to serve for pouring, as well as vent, you place pans to receive the wax which is to run off. This done, you light a middling fire to heat the figure, and all the place wherest stands, with so moderate a heat, that the wax may melt without boiling, and come entirely out from the mould, without there remaining any part of it; which would not be the case if the heat be so great as to make it boil, for then it would stick to the mould, and cause defects in the figure, when you come to run the metal. - When, therefore, you judge that all the wax is out, which you may know by weighing that you employed, and weighing it again after it is in the pans, you take these off, and stop the pipes, through which it came out, with clay. Then fill all the empty parts of the pit round the figure with bricks, which you throw in gently, but without order; and, when it is come up to the top, make a good brisk fire in the furnace. As the flame is interrupted by these bricks, it cannot ascend with violence, nor hurt the mould, and they only communicate their heat in going through all those bricks, which become so hot, that they and the mould are at last both red hot.

22. Twenty-four hours after the fire has been lighted, when you see that the bricks and the moud are equally red hot from top, bottom, you let the fire go out, and the mould cool, by taking all the bricks off. When there is no more any heat at all you throw some earth in the pit, to fill the place which had been occupied with the bricks, and, in proportion as you throw it in you tread it with your feet, and press it against the mould.

23. In order to melt the metal, you construct just by the pit where the mould is, a furnace, the lower part of which ought to be higher by two or thrre inches than the top of the said pit, in order to obtain a sufficient declivity from it to the pit for the running of the metal. Its construction must be after the form of an oven, with good bricks and free earth, and supported by good and strong iron hoops. There is a border raised all round, so as to make it capable to contain all the metal which is intended to be melted in it. On the side which looks towards the pit, there is an opening, which is stopped during the melting of the metal, and from that opening comes an earthen funnel practised, which goes to a bason of good free earth placed over the mould, and the middle of which corresponds and communicates to those cups we have mentioned before (No. 14). This bason is called by the workmen escheno. And in order to prevent the metal from running into these cups before the whole which is in the furnace is run into the escheno there are men on purpose who hold a long iron rud terminated by one end in the form of these cups, and stop them.

24. When the metal is melted, you unstop the opening of the furnace in which it is contained; this runs into the escheno, and as soon as it is arrived, the men take off the rod with which they stopped the cups, and the mould being instantly filled all over, the figure is formed in one moment.

25. After the mould is thus filled with the metal, you let it stayin that situation for three or four days, then, at leisure, you take off the earth which had been thrown all round it, which helps the mould to become entirely cold. As soon as you are sure there is no more heat, you break the mould, and the metal figure appears surrounded with rods of the same metal, starting out from it, occasioned by the vent and pouring-holes, or pipes, through which the metal was introduced, and which remained filled with it. These you must saw off, in order to unburden the figure of so much, and get it out of the pit more easily. Then you clean and scower with water and grinding-stone in powder, and pieces of deal or other sort of soft wood, and you search in all the hollow places of the draperies and other parts.

26. When the figures are small, they are generally washed with aquafortis; and, when it has operated, you may wash them again with common water. VWhen they are thus well cleansed, you repair, finish, and fault those which require to be treated more highly than others; for the large ones are seldom searched so minutely.

27. After they have been as much finished as they are intended to be, you may give them, if you like, a colour, as some do, with oil and blood-stone. Or, as some others practise it, you may make them turn green by means of vinegar. But without all that trouble, the bronze will in time take a natural varnish of itself, and becomes of a blackish hue.

II. How to gild such sorts of figures.

1. They may be gilt two different ways; either with gold in shells, or with gold in leaves. The first method is the handsomest, and at the same time the molt lasting, ii being always used for small sized works. To apply it, you make a mixture of one part of the hest gold, and seven, of mercury, which founders call silver in that sort of process. When these are incorporated together, you then heat the figure, and rub it with the composition, which whitens it, and heating it again over the fire the mercury exhales, and the figure, remains gilt.

2. As for the other method it is only for large sized works, and them on which one is not willing to a great expence; you scrape the figure with small files, and other proper tools, to make it quick and then you heat and lay on a gold leaf, repeating this four times.

III. Of the choice and composition of metals.

Any metal whatever may be used for the calling of figures, though the general composition runs as follows.

1. For the fine bronze figures, the alloy is half brass, half copper. The Egyptians who are said to be the inventors of that art used to employ two thirds of brass against one of copper

2. Brass is made with copper and calamine. One hundred weight calamine renders one hundred per cent. Calamine is a stone from which a yellow dye is drawn. It is to be found in France and at Liege.

3. Good copper ought to be beaten, not molten, when intended for statues. You must guard also against using putty, when in alloy with lead.

4. Copper may be forged either hot or cold. But brass breaks when cold, and suffers the hammer only when hot.

5. There is a sort of metalic stone called Zinc, which comes from Egypt: it renders the copper of a much finer yellow than the calamine, but, as it is both dearer and scarcer, they are not so ready to use it.

6. As for the composition for making of bells, it is twenty pounds weight pewter for each hundred of copper. And the artillery pieces take but ten pounds only of pewter to one hundred of the other. This last composition is pot good for the calling of figures, as it is both too hard and too brittle.

Ei kommentteja :