96 ON THE ALLOYS OF COPPER AND ZINC. 
are wrought. Singularly enough, at a point just beyond the limit at which the fibres 
cease to be apparent, viz. at 60 per cent of copper, an alloy of peculiar homogeneity 
occurs ; — its fracture, as seen when small bars are broken, being smooth and compact, 
and entirely unlike either the coarse, irregular, stringy fracture of alloys richer in 
copper, or that of alloys containing only a little more zinc, upon the fracture of 
which small bundles of fine crystalline fibres are often apparent. 
This alloy readily admits of being rolled, either hot or cold, and may be subjected 
to the operations of hammering or drawing without detriment, while alloys con- 
taining only a few per cent more copper can be rolled hot only when the sheets 
are raised to a very high temperature. Even then it is exceedingly difficult to 
obtain thin sheets without cracking their edges. 
On the other hand, alloys containing somewhat less than 60 per cent of copper 
cannot well be rolled hot, since, when the ingots are heated, the exterior — especially 
at the corners — is liable to become pasty before the centre is soft enough to admit 
of being rolled. 
In the preparation of the alloy of 60 per cent of copper, now so extensively used 
for sheathing vessels, under the name of Muntz's sheathing or yellow-metal, — also 
known as malleable brass,— it is the custom of founders to reserve a portion of 
the zinc which has been weighed out for a charge, until the alloy in their pots 
or furnace has become sufficiently hot;* the last portions of zinc are then added 
in small pieces, a sample of the alloy being tested after each such addition. This 
is done by dipping out a small portion of the melted mass and pouring it into 
a mould; a little ingot, five or six inches long by an inch or less in thickness, is 
thus obtained, which, after cooling, is broken on an anvil, and its fracture observed. 
If this does not exhibit a smooth and homogeneous surface, more zinc is added 
to the alloy. The accuracy with which an experienced workman can thus obtain 
the desired alloy is truly astonishing, the more especially since this homogeneous 
alloy is confined within very narrow limits. 
It is stated by founders that the alloy of 60 per cent of copper and 40 per cent of 
zinc will present almost precisely the same homogeneity of fracture, whether the test 
ingot prepared from it be cooled slowly, by exposure to the air, or rapidly, by plung- 
* The temperature to which the alloy is brought before pouring is considered to be a point of much 
importance by founders. In their opinion, a very inferior alloy would be formed if it were not heated much 
more strongly than would be necessary to maintain it in the liquid state. Compare Bolley, Zur Kentniss 
der Moleculareigenschaften des Zinks, Ann. Ch. u. Pharm., XCV. 302. This subject is evidently connected 
with the phenomena of tempering described in another part of this memoir. 
