SECTION II.
Of glafs as a ground for painting with vitreous colours^or by burning.- -
T H E firft objedt to be regarded, is the choice of grounds} which fhould be jplates, or veflels, of glafs, that is of the firft degree of hardnefs, but at the fame timecolourlefs, and without fpecks or wavings. The glafs, which has thefe qualities in the greateft degree, is, the beft of the kinds of that ufed for windows, except fuch as is made for looking- glafles, which though molt colourless and clear, is fofter from the quantity of borax and other fluxes which enter into its compolkion. This fort, which is called crown glafs, being a glafe of falts, is hard and tranfparent; and, being ready formed into plates, may confequently be had in a Hate proper for ufe; but where paintings of any confequence are undertaken, a compofition, flail better fuited to this purpofe, ihould be employed ; and the glafs wrought in the fame manner as the looking-glals plates a- Jone are at prelent.
When larger objedls are to be depicted than the fize of Angle plates of glafs can contain, it is pradtifed to join feveral fquares together: which may be prepared for the painting in this manner. An even board, of the fizeof the whole of the plates laid together, Ihould be Iprinkled with a mixture of refin and pitch; which being melted by a flat iron held over it, the plates of glafs Ihould be placed on the board as dole to each other as poflible ; and will be firmly fixed in, the fituation they are laid, by the cement of refin and pitch as it cools. The glafs being lo fixt mull: be cleaned from any of the cement, which may have run through the joints, firft by leraping, and after by rubbing with fpirit of turpentine j and it will then be in a condition to be painted ewith the ground
colours: which being performed, the plates may be taken off the board, by palling the flat iron heated over them at a proper diltance ; •which melting the cement will let them loofe from the board: and they may then be burnt feparately without any inconvenience.