[ 241 ]

porcelain than in any other. Long-necked matrasses forinstance could scarcely at all be formed by the potter, ofthat equal thickness, and internal smoothness, to whichthey are expeditiousty blown by the glass-maker.

The above account of the qualities of this porcelain inits different states, points out a caution to be observed inthe process, especially where mortars are to be changed,or in other cafes where great hardness is required ; viz.to discontinue the baking at the period of the greatesthardness ; for otherwise the matter soon becomes softagain, and even softer than the glass was at first. It seemsto have acquired its full hardness as soon as it has becomewhite and fibrous; and hence perhaps it may for mostpurposes be adviseable to discontinue the operation as soonas the glass appears covered with a moderately thick whitecoat. Fragments of the same kind of glass, put in alongwith the vessel to be changed, and occasionally taken outand broken, will serve to inform the operator how thechange goes on. Of wide-mouthed vessels, several maybe placed within one another, with sand between them.In all cases, care must be taken to apply the heat, asequally as may be, all over the crucible or pot containingthe vessels, that the change may be as uniform as possiblethroughout: no particular contrivances however are neces-sary for this use, the same caution and the same furnace,employed for baking the common fine wares, being suffi-cient for the baking of glass.

SECT. III.

Experiments of comparing the effeSls of different kinds of ma-terials on Green Glass by baking.

P IECES of green glass bottles were surrounded withpowdered charcoal, soot, and sundry kinds of earthybodies, in separate crucibles, which were all covered and

I i luted,