COP
COP
( 3 '
{mall portion of filver. Neither of the abovemetals are imitable by the common procefles.
According to Dize, 1 when the iron formsonly T ' T of the mafs it is {till magnetical, butnot fo when it is only about Iron does
not whiten copper fo much, in equal quantities,as tin does, and {till lefs than arfenic.
Thefe metals unite to appearance very inti-mately by fufion, but, what is very remarkable,when a mafs of this alloy is expofed to a heatlefs than that at which the whole melts, thelead alone fweats out, leaving almoft all thecopper m a porous or honeycombed ftate. Whenthe copper holds a fmall portion of fdver, thelead carries the latter out with it, and this isthe principle of the old procefs of eliquution, for-merly much ufed in the extracting of silverfrom copper ores. Copper with about a fourthof its weight of lead forms pot-metal. m Theantient Roman pot-metal, according to Pliny ,was compofed of ioo of copper, 2 of lead, and2 of tin. The fame ingredients, but with more ofthe two latter, were the materials of many ofthe ancient Greek and Sicilian coins. “
Copper with Zinc.
Copper nearly faturated with zinc, that is,in which the latter makes about a fourth (moreor lefs) of the mixture, forms brafs, the molt im-portant of all the alloys of this metal, and whichhas been fully defcribed under that article.With a much lefs proportion of zinc the colourof the alloy approaches very nearly to that ofgold, and the malleability increafes. Mixtureschiefly of thefe two metals are ufed to form avariety of yellow or gold-coloured alloys, knownby the names of Tombac, Manheim or Dutch Gold ,Tinfel, Similar, Prince Rupert’s Metal , Pinchbeck,&c. but the precife compofition varies accord-ing to the fancy or the experience of differentmanufacturers. The Dutch gold may be beatenout into extremely fine leaves, which, whenfrelh, have nearly the brilliance of gold-leaf,and are ufed as a cheap imitation of it, but theytarnifh very foon. The mixture may be madeeither by directly melting copper and zinc, orby mixing brafs and copper. In either cafethe copper fhould be melted firfl, and the zincadded afterwards, the whole ftirred togetherwith wood, covering it with a little charcoal,and poured out immediately, to prevent thelofs by the burning off of the zinc.
Several direct experiments on the union ofcopper and zinc in different proportions were
1 Jour. Phvf, tom. 36. ” Watfon’s Effays. vol.
v Leoubardi.Wonerbuch,
17 )
made by Margraaf. 0 In all, the copper wasthe pureft Japanefe, and the mixture was madein the way above mentioned. With 8 dramsof copper and as much zinc, much of the latterunavoidably burnt off and the alloy only weighed12 in Head of 16 drams, the mixture was hard,brittle, yellow, and of a radiated textuie.With 16 drams of copper and 8 of zinc, thelofs by burning was only p of a dram. Thealloy was fofter than the laft, {fill radiated, yel-low, and began to be a little malleable. Fromthis, fucceffively diminifhing the proportions ofzinc, the alloy became fofter, more malleable,and of a colour more and more approaching togold : and at laft, with 11 or 12 of copper andx of zinc, the fineft golden tombac was pro-duced. According to Wiegleb p the Manheimgold is made by melting feparately 3 parts ofcopper and x of zinc, mixing them, coveringwith charcoal, ftirring with a flick and coolingimmediately. Thefe proportions fcarcely differfrom thole of fome kinds of brafs. Beaumegives for the fame metal 4 of copper and 1 ofzinc, whence it is obvious that the proportionsare quite arbitrary, but it appears that the alloyis not made, as brafs is, by cementation, but byAmple mixture of the metals. A very fmallquantity of tin is fometimes employed, but thismetal has the difadvantage of remarkably di-miniftiing the malleability of copper and itsalloys. A fine malleable tombac is made, how-ever, with 16 of copper, 1 of zinc, and 1 of tin.An alloy of 12 of brafs and r of tin is fcarcelymalleable.
A kind of tombac is the material of whicha large proportion of the Roman coins was com-pofed. Klaproth 13 on analyzing, feveral, {truckduring the firft century of the emperors, foundthem all to confift either of pure copper, or ofcopper and zinc, in which the latter metal madegenerally from a fifth to a fixth of the mafs. Alittle tin and lead were found in fome, but infuch fmall proportion as to appear only an ac-cidental impurity.
The alloys of copper and tin are extremelyimportant in the arts, and curious as chemicalmixtures. They form in different proportionsmixtures which have a diftindl and appropriateufe. Tin added to copper makes it more fufible,much lefs liable to ruft or corrolion by com-mon fubftances, harder, denfer, and more fono-rous. In thefe refpe£ts the alloy has a real ad-vantage over unmixed copper, but this is in
4. " Klaproth. • Mem Acad. Berlin, 1774.
’ Phil. Mag. vol. 17.
2X2