The colours thus prepared after having been rubbed up on a plate of ground glass with the spirits of turpentine or lavender, thickened in the air are applied with a hair pencil. Before using them, however, it is necessary to try them on small pieces of glass, and expose them to the fire, to ascertain if the desired tone of colour is produced. The artist must be guided by these proof pieces in using his colours. The proper glass for receiving these colours should be uniform, colourless, and difficult of fusion. For this reason crown glass made with a little alkali or kelp is preferred. A design must be drawn upon paper and placed beneath the plate of glass. The upper side of the glass being sponged over with gum-water affords, when dry, a surface proper for receiving the colours, without the risk of their running irregularly, as they would be apt to do on the slippery glass. The artist draws on the plate, with a fine pencil all the traces which mark the great outlines and shades of the figures. This is usually done in black, and afterwards, when it is dry, the vitrifying colours are laid on by means of larger hair pencils. The yellow formed with chloride of silver is generally laid on the back of the glass, for it is apt to run with the other colours while heating. The pigments used in painting on glass are principally matallic oxides and chlorides, and as, in most of these, the colour is not brought out until after the painting is submitted to heat, it is necessary to ascertain beforehand if the colours are properly mixed by painting on slips of glass, and exposing them to heat in a muffle. The painter is guided by these trial pieces in laying on his colours. To fire the paintings a furnace with a muffle is used. The muffles are made of refractory clay. 257. WHITE COATING FOR GOLD VARNISHES A quart of strong parchment size and half a pint of water are to be made quite hot, and to these are to be added, (in small portions from time to time,) two good handsful of common whiting, passed through a fine sieve; this mixture is to be left to infuse for half an hour, when it is to be stirred carefully so that the amalgamation may be perfect. This coating is preferable to any glue or cement for coating picture frames, &c., on which is to be laid the tin or silver leaf, to be varnished with gold varnishes or lackers. 258. LEAD COLOURING PAINT Whiting, 112 lbs...................... $1.12 Blue-black, 5 lbs..................... 0.25 White lead ground in oil, 28 lbs...... 2.24 Road-dirt, 56 lbs..................... 0.10 Lime-water, 5 galls................... 0.05 Residue of the oil, 2-1/4 galls....... 1.25 -------- Weights, 256 lbs....... $5.01