Abbildungen der Seite



which was watched until the boiling of the copper, arising probably from the escape of bubbles of air locked up at the lower part of the semi-fluid mass, ceased, and the copper assumed a bright red, but sluggish appearance; the zinc was then added.

Precaution is necessary in introducing the first quantity of zinc, not to set the copper, which is liable to occur if a large quantity of cold metal is thrown in, simply from the abstraction of heat; and it is also necessary to warm the zinc that it may be perfectly dry, as the least moisture would drive the metal out of the pot with dangerous violence. A small lump of the zinc, therefore, was taken in the tongs, held beside the pot for a few moments, and then put in with the tongs with an action between a stir and a plunge, regardless of the flare, and of the low crackling noise, just as if butter had been thrown in; the zinc was absorbed, and the surface of the pot was clear from its fumes almost immediately. The remainder of the zinc was then directly added, in about eight pieces, one at a time, much in the same manner, but the danger of setting the copper nearly ceases when a small quantity of the spelter is introduced. After each addition the pot was free from flame in a few moments, a handful of broken glass was then thrown in, the tile replaced, and the whole allowed to stand for about fifteen minutes to raise the metal to the proper heat for pouring, which is denoted by the commencement of the blue fumes of the zinc.

The pot was then taken from the fire, well stirred for one minute, and poured; the weight of the brass yielded was 34 lbs. 121 oz., showing a loss of llb. 34 oz., or one-tenth of the zinc, or the one-thirtieth part of the whole quantity. This experiment was repeated, and the loss was then 1 lb. 3 oz., the difference being only 1 an oz. By analysis, the mean of the two brasses was 311 per cent. zinc; or instead of being 8 oz. to the pound, it was only 74 oz.

Twelve pounds of each of these experimental mixtures were remelted six times, a bar of about 1ļlbs. being taken each time; the two series of trials were conducted in different foundries, by different men, and quite in the ordinary course of work; but the loss per cent. of zinc was in the six experiments exactly alike in each, that is, at the sixth melting each bar contained 224 per cent. or 4ž oz. to the pound of copper. The second fusion in each case sustained the greatest loss, (say nearly



two-fold); and in the others, taking all the accidental circumstances into account, the loss might be pronounced pretty much alike each fusion *.

In making the alloys with more zinc; the calculated weight of the first alloy was melted, and the amount of zinc was warmed and plunged in with the tongs, whilst the pot was in the fire, the whole was stirred and quickly poured: the losses in weight were rather large, but this is common when the zinc is in great quantity. To make the alloys containing less zinc than the alloy, the calculated weight of copper was first made red-hot, and the respective portion of the brass alloy was then put in the pot, by which means the two ran down nearly together: it being found that the copper, if entirely melted before the brass was added, incurred a risk of being set at the bottom of the pot; and remelting the mass, wasted the zinc. These alloys came out much nearer to their intended weights.

In making the tin and copper alloys, very little difficulty was experienced. The copper was put into the pot together with a little charcoal, which was added to assist the fusion and also to cause the alloy to run clean out; as in pouring gun-metal a small quantity is usually left on the lip of the crucible, which would have been an interference in these experiments. When the copper had ceased boiling, and was at a bright red heat, it was taken from the fire, and the tin, previously melted in a ladle, was thrown in, every mixture was well stirred and poured immediately.

In the fourteen alloys thus formed, each weighing about a pound and a half, namely, 1, 1, 1., &c., up to 8 oz. of tin to the pound of copper, (missing the 64 and 71,) no material loss was sustained in nine instances, and in the other five it never exceeded Joz. and that quantity was probably lost rather in fragments than by oxidation.

Alloys of 2, 4, 6 and 8 ounces of lead to the pound of copper were made exactly under the same circumstances as the last.

* Lord Oxmantown required the proportion of 2.75 copper and 1 zinc, to be very carefully preserved, as that alloy was found to expand equally with the speculum metal to which it had to be soldered. After many trials, Lord Oxmantown found that by employing a furnace deeper than usual, and by covering the metal with a layer of charcoal powder two inches thick, the loss each time was the smallest, and almost exactly the 180th each casting. To renew the charcoal dust it was folded up in paper and thrown in. See Trans. Royal Society, 1840, p. 507.



SECT. I. - GENERAL REMARKS. METALLIC MOULDS. We are indebted to the fusibility of the metals, for the power of giving them with great facility and perfection, any required form, by pouring them whilst in the fluid state into moulds of various kinds, of which the castings become in general the exact counterparts. This property is of immeasurable value.

Some few objects are cast in open moulds, so that the upper surface of the fluid metal assumes the horizontal position the same as other liquids, as in casting ingots, flat plates, and some few other objects ; but in general the metals are cast in close moulds, so that it becomes necessary to provide one or more apertures or ingates for pouring in the metal, and for allowing the air which previously filled the moulds to escape.

When these moulds are made of metal, they must be sufficiently hot not to chill or solidify the fluid metal before it has time to adapt itself thoroughly to every part of the mould; and when the moulds are made of earthy matters, although moisture is essential to their formation, little or none must remain at the time they are filled.

The earthen moulds must be also sufficiently pervious to air, that any vapour or gases which may be formed, either at the moment of pouring in the metal or until it has become entirely solid, may have free vent to escape; otherwise, if these gases are rapidly formed, there is great danger of the metal being driven out of the mould with a violent explosion, or when more slowly formed and locked up without sufficient freedom for escape, the casting will be said to be blown, as some of the bubbles of air will displace the fluid metal and render it spongy or porous. It not unfrequently happens that castings which appear externally good and sound, are full of hidden defects, because the surface being 318


first cooled, the bubbles of air will attempt to break their way through the central and still soft parts of the casting.

Fig. 139. a

_Ꭰ f The explanatory diagram, fig. 139, is intended to elucidate some of the circumstances concerning the construction of moulds, which in the greater number of cases are made only in two, but in other cases are divided into many parts. The figure to be moulded is supposed to be a rod of elliptical section, the mould for which might be divided into two parts through the line A, B, because no part of the figure projects beyond the lines a, b, drawn from the margin of the model at right angles to the line of division, and in which direction the half of the mould would be removed or lifted ; the model could be afterwards drawn out from the second half of the mould in a similar manner.

The mould could be also parted upon the line C, D, because in that direction likewise, no part of the model extends beyond the lines c, d, which show the direction in which the mould would be then lifted. .

The mould could be also parted either upon A B or upon CD, provided no part of the model outstepped the rectangle formed by the dotted lines b, c, or was undercut.

But the removal of the entire half of the mould upon the line E, F, would be impossible, because in raising the mould perpendicularly to E, F, that portion of the mould situated within the one perpendicular e, would catch against the overhanging part of the oval towards A. Were the mould of metal, and therefore rigid, it would be entirely locked fast, or it would not “deliver;" were the mould of sand, and therefore yielding, it would break and leave behind that part between A and E which caused the obstruction. Consequently, in such a case, the mould would be



made with a small loose part between A and E, so that when the principal portion, from A to F, had been lifted perpendicularly or in the direction of the line e, the small undercut piece, A to E, might be withdrawn sideways, on which account it would be designated by the iron founder a drawback, by the brass founder a false core.

All the patterns in the mould, fig. 140, could be extracted from each half of the mould, because none of them encroach beyond the perpendicular line, or that in which the mould is lifted; a and b, could be laid in exactly upon the diagonal, or upon one flat side, or partly embedded; and in like manner f, g, h, might be sunk more or less into the mould, their sides being perpendicular ; but the patterns in fig. 141 being undercut, the division of the mould into two parts only would be impracticable, and false cores or further subdivisions would be required in the manner represented, the construction of which will be hereafter detailed.

Fig. 140. a b c d


Extending these same views to a more complex object, such as a bust, it will be conceived that the mould must be divided into so many pieces, that none of them will be required to embrace any overhanging part of the figure. For instance, were it attempted to mould a human head, so that the parting passed through the central line of the face and down the back; the two halves could not be separated if they were made each in a single piece; as the inner angles of the eyes, the spaces behind the ears, the curls of the hair, &c. would obstruct it, and the head could be only thus moulded by making false cores or loose pieces at these particular places, in the manner illustrated by the former figures. These would require to be accurately adapted to the surrounding parts, by pins or contrivances to

« ZurückWeiter »