bra
C 169 >
a half broad, and eight inches thick. They areplaced one above the other, the upper one beingonly moveable, and furnilhed with a tackle andpullies for that purpofe, and before calling, thelurface is fmeared with cow-dung. To givethe plate the requifite thicknefs, hoops of ironof different dimenfions are adapted to the underHone, fo as to confine a determinate quantityof melted metal. The Hones are then gentlyinclined and the melted hrafs let in betweenthem. Thefe plates are afterwards laminated :fome of them are cut into flips by ftrong {hears,for the further purpofe of being drawn into wire,and otherwife manufactured in various ways.
A Angle procefs, where the fire is kept uplong' enough and the materials are good, is cer-tainly fufficient to make good malleable brafs,but it is probable that the excellence and beautyof the article are improved by making it undergoa fecond cementation with frefh calamine andcharcoal.
In the laboratory brafs may be made very wellin the fmall way in a much fhorter time. d Putinto a crucible a mixture of calamine and char-coal, bury it in the requifite proportion of copperfhot, cover the whole witn charcoal' powder,lute on a cover to the crucible, and heat flowlyin a wind-furnace for half an hour, till the zincbegins to burn off in a blue flame round the topof the crucible, then raife the fire and heatbrifldy for half an hour longer.
This procefs of cementation is alfo neatlyfliewn by the following management, as givenby Cramer. e Put the mixture of calamine andcharcoal into a crucible, cover it with a thinlayer of clay, over which when dry lay a thinplate of copper, cover the whole with fine char-coal powder, and lute on a cover to the crucible.Apply heat gradually, and the vapour of thereduced zinc will rife through the floor of clay,penetrate the red-hot copper plate above it, andgradually convert it into brafs, which at. theend of the operation will be found lying meltedon the ftratum of clay. The increaie of weightgained by the copper in this operation willafford a good practical teft of the goodnefs ofthe calamine, and its fitnefs for brafs-making inthe great way.
The molt important properties of brafscompared with copper are the following: thecolour of brafs is much brighter, and moreapproaching to that of gold; it is more fufiblethan copper; lefs fubjeft to ruft and to heafted on by the vaft. variety of fubvtances whichcorrode copper with fo much eafe; and it is* Sage. • Ars docinvdl.
equally malleable when cold, and more exten-fible than either copper or iron, and hence iswell fitted for fine wire. Brafs however is onlymalleable when cold. Hammering is found togive a magnetic property to brafs, perhaps how-ever only arifing from the minute particles ofiron beaten off the hammer during the procefsand forced into the furface of the brafs, butthis circumltance makes it neceffary to employunhammered brafs for compafs-boxes and fimilarapparatus.
The expanfion of brafs has been very accu-rately determined, as this metal is moll com-monly ufed for mathematical and aftronomicalinftruments, where the utmoft precifion is re-quired. Mr. Smeaton found that 12 inches inlength of call brafs, at 32°, expanded by 180degrees of heat (or the interval from freezingto boiling water) 225 ten thoufandth parts of aninch. Brafs wire under the fame circumftancesexpanded 232 parts; an alloy of 16 of brafswith 1 of tin expanded 229 parts. The expan-fion of hammered copper is only 204 fuch parts,but that of zinc is 253, fo that brafs holds a.middle place in this refpcct between its two-component metals.
Moll of the zinc readily burns off from brafswhen kept melted in a ilrong heat with freeaccefs of air. "When the heat is equal to thatof melted copper, the zinc takes fire and flowlyburns away. At laft little elfe but copperremains, but Hill united with a fmall portion ofzinc, which no further continuance of the firewill entirely feparate.
Some kinds of very fine brafs are faid not to-be made by cementation in the way alreadydefcribed, but by a more fpeedy and direfbunion of copper and zinc, care being taken to-prevent the accefs of air to the materials whilein fufion. Very fine brafs may alfo be madeby mixing together the oxyds of copper amizinc, and reducing them with a carbonaceousflux. This idea is ingenious, and from theintimate mixture of the two metals which itpromifes, it deferves to be further purfued.Sage gives the following experiment to this-purpofe. f Mix together 50 grains of theoxyd-of copper, remaining after the dillillation ofverdigris (which is very pure.) with 100 grainsof lapis calminaris, 400 grains of- black flux,and 30 grains of charcoal powder; melt the mix-ture in a.crucible till the blue flame is feen nolonger round the lid of the crucible, and whencold a fine button of brafs is found beneath the.fcoria, weighing a fixth more than the copper,-
' Journ. de Phyf, vol.-xwvlii.
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VOL. I.