Читать книгу Henley's Formulas, Recipes and Processes (Applied Chemistry) - Various - Страница 184

—Many of the antique statues were made of genuine bronze, which has advantages for this purpose, but has been superseded in modern times by mixtures of metals containing, besides copper and tin—the constituents of real bronze—a quantity of zinc, the alloy thus formed being really an intermediate product between bronze and brass. The reason for the use of such mixtures lies partly in the comparative cheapness of their production as compared with genuine bronze, and partly in the purpose for which the metal is to be used. A thoroughly good statuary bronze must become thinly fluid in fusing, fill the molds out sharply, allow of being easily worked with the file, and must take on the beautiful green coating called patina, after being exposed to the air for a short time.

Genuine bronze, however strongly heated, does not become thin enough to fill out the molds well, and it is also difficult to obtain homogeneous castings from it. Brass alone is also too thickly fluid, and not hard enough for the required fine chiseling or chasing of the finished object. Alloys containing zinc and tin, in addition to copper, can be prepared in such a manner that they will become very thinly fluid, and will give fine castings which can easily be worked with the file and chisel. The best proportions seem to be from 10 to 18 per cent of zinc and from 2 to 4 per cent of tin. In point of hardness, statuary bronze holds an intermediate position between genuine bronze and brass, being harder and tougher than the latter, but not so much so as the former.

Since statuary bronze is used principally for artistic purposes, much depends upon the color. This can be varied from pale yellow to orange yellow by slightly varying the content of tin or zinc, which must, of course, still be kept between the limits given above. Too much tin makes the alloy brittle and difficult to chisel; with too much zinc, on the other hand, the warm tone of color is lost, and the bronze does not acquire a fine patina.

The best proportions for statuary bronze are very definitely known at the present day; yet it sometimes happens that large castings have not the right character. They are either defective in color, or they do not take on a fine patina, or they are difficult to chisel. These phenomena may be due to the use of impure metals—containing oxides, iron, lead, etc.—or to improper treatment of the alloy in melting. With the most careful work possible, there is a considerable loss in melting—3 per cent at the very least, and sometimes as much as 10. This is due to the large proportion of zinc, and it is evident that, in consequence of it, the nature of the alloy will be different from what might be expected from the quantities of metals used in its manufacture.

It has been remarked that slight variations in composition quickly change the color of the alloy. The following table gives a series of alloys of different colors, suitable for statuary bronze:

	Copper	Zinc	Tin	Color
I	84.42	11.28	4.30	Reddish yellow
II	84.00	11.00	5.00	Orange red
III	83.05	13.03	3.92	Orange red
IV	83.00	12.00	5.00	Orange red
V	81.05	15.32	3.63	Orange yellow
VI	81.00	15.00	4.00	Orange yellow
VII	78.09	18.47	3.44	Orange yellow
VIII	73.58	23.27	3.15	Orange yellow
IX	73.00	23.00	4.00	Pale orange
X	70.36	26.88	2.76	Pale yellow
XI	70.00	27.00	3.00	Pale yellow
XII	65.95	31.56	2.49	Pale yellow

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Perhaps the most satisfactory bronze metal is the alloy used in France for more than a century. It contains 91.60 per cent of copper, 5.33 per cent of zinc, 1.70 per cent of tin, and 1.37 per cent of lead. Somewhat more zinc is taken for articles to be gilded.