OCR Text |
Show ENAMELS. NoTE XXI. The precipitates of gold, and the colcothar or other red preparations of iron, are called tender colours. The heat mufl be no greater than is jufl: fufficient to make the enamel run upon the piece, for if greater, the colours will be deCI:royed or changed to a 1 !ifferent kind. When the vitreous matter has jufl: become fluid it feems as if the coloured meta1lic calx remained barely intl!rmixed with it, like a coloured powder of exquifite tenuity fufpendcd in water: but by fl:ronger fire the calx is diflolved, and metallic colours are altered by Jolution in glafs as well as in acids or alcalies. The Saxon minf'S have till very lately almofl: exclufively fupplied the ren. of Europe with cobalt, or rather with its preparations, zafFre and fmalt, for the exportation of the ore itfdf is there a capital crime. Hungary, Spain, Sweden, and fome other parts of the continent, are now faicl to afford cobalts equal to the Saxon, and fpecimens have been difcovered in om own inand, buth in Cornwall and in Scotland; but hitherto in no great quantity. Calces of cobalt and of copper differ very materially from thofe above mentioned in their application for coloming enamels. In thofe the calx. has previouf1y acquired the intended colour, a colour which bears a red heat without injury, and all that remains is to fix it on the piece by a vitreous flux. · But the blue colour of cobalt, and the green or bluilh green of copper, are produced by vitrification, that is, by Jalution in the glafs, and a fl:rong fire is neceffary for their perfection. Thefe calces therefore, when mixed with the enamel flux, are melted in crucibles, once or oftener, ancl the deep ·coloured opakc glafs, thence refulting, is ground into impalpable powder, and ufed for enamel. One part of either ofthefe calces is put to ten, fixteen, or twenty parts of"the flux, according to the depth of colour required. The heat of the enamel kiln is only a full red, fuch as is marked on Mr. Wedgwood's thermometer 6 degrees. It is therefore neceffary that ..the flux be fo adjufled as to melt in that low heat. The ufual materials are flint, or flint-glafs, with a due proportion of red-lead, or borax, or both, and fometimes a little tin calx to give opacity. Ka-o-lin is the name given by the Chinefe to their porcelain clay, andpe-/un-t.fo to the other ingredient in their China ware. Specimens of both thefe have been brought into England, and found to agree in quality with fome of our own materials. Kaolin is the very fame with the clay called in Cornwall and the petuntfe is a granite fimilar to the Cornilh moorfl:one. There are differences, both in the Chinefe petuntfes, and the Englifh moorfl.ones ; all of them contain micaceous and quartzy particles, in greater or lefs quantity, along with feltfpat, which lafl: is the effential ingredient for the porcelain manufaB:ory. The only injurious material commonly found in them is iron, which difcolours the ware in proportion 10 its quantity, and which our moor{loFles are perhaps more frequently tainted with than the Chinefe. Very fine porcdain has been made from Englifh materials but the nature of the manufacture renders the procefs precarious and the profit hazardous; for the femivitrification, which confl:itutes porcelain, is neceffarily accompanied with a degree of foftnefs, or femifufion, fo that the veffels are liable to have their forms altered in the kiln, or to run together with any accidental augmentations of the fire. ,. |