After colouring, the artist proceeds to produce the very light parts of his subject, by taking off the colour from them by a goose quill, cut like a pen without a slit. By working this upon the glass, he removes the colour from the parts where the lights should be the strongest ; such as the hair, eyes, the reflection of bright surfaces, and light parts of' draperies. The blank pen may be either employed to make the lights by lines, or hatches and clots, as will be most suitable to the subject, and the part where they are re quired, in the same manner as the black was used to pro duce the shades.
The glass is now ready for burning, to fix the colours, or rather stain the glass, by the preparations which have been laid upon it. The furnace most proper for the pur pose, is similar to that used by assayers or enanwIlers. It consists of a muffle, or arch of fire clay or pottery, so placed over a fire-place, and surrounded by Huss, as to re ceive a very considerable heat, hut in the most equable and regular manner throughout the whole; for if this is not at tended to, some parts of the glass will become too much stained, before the colour takes place others. The mouth of the muffle, and the entry to the fire through which the fuel is supplied, should be or opposite sides, to prevent as much as possible the envy of dust into the muffle; and the mouth of the muffle should be closed by iron folding-doors, with small openings or peep holes through them, to allow the artist to (userve the progress of the burning, by withdrawing small trial slips of glass, which arc stained with the pt incipal tints employed in the picture.
The muffle must be made of very good fire clay, flat at the bottom, and only five or six inches high, being so much arched at the top as is necessary to render the roof strong, and resist falling in by its weight when heated; the muffle must be so close on all sides as to admit no smoke or flame. Before the plate of glass is introduced into the furnace, a bed is prepared upon the bottom of the muffle. For this purpose, lime is used, It must be previously burned, so as to yield no more gas, and then reduced to a fine powder. In this state, the bottom of the muffle is strewed half an inch deep, and levelled with a feather The glass is then introduced, and sometimes the muffle is filled up with other glasses laid above the first, having beds of lime between each. The fire is lighted alter all
is shut up close, and the heat is raised very gradually at first, lest the glass should be broken by it ; but alter the lire has attained its full heat, it is continued for three or four hours, more or less, according to the appearances ob served upon the trial slips of glass, which are withdrawn for that purpose ; and the yellow colour is principally at tended to, as that is found the best test for the others. The operation of burning being thus finished, the fire is extin guished, or rather suffered to die away, and the heat to subside gradually, to prevent the glass becoming brittle, as it would do without this precaution; for the heat, when at the greatest, must be sufficient to make the glass very flexible, as is seen by the trial glasses.
\Ve shall conclude this Section with the preparations for a few principal colours.
Preparation for staining glass of a flesh colour.—Take one ounce of minium, and two ounces of red enamel, of that kind which is called Venetian glass enamel ; pound them to a fine powder, and grind it with brandy upon a hard stone. This mixture, when slightly baked, will pro duce a fine flesh colour.
Black colour.—Take 141 ounces of those scales of iron, which are found round the anvil of a smith's shop; mix with it two ounces of white glass, an ounce of antimony, and half an ounce of manganese ; pound and grind them together with strong vinegar. Brilliant black may also be obtained, by a mixture of blue with the oxides of manga nese and iron.
Another black colour.—Take equal parts of iron scales and of small beads or fragments of glass ; pound them exceedingly fine, and grind them to a consistence to work with a pencil.
Another black is made from three parts of glass of lead, two parts of the scales of copper, and one of antimony, treated as before.
For brown.—Take one ounce of white glass or enamel and half an ounce of good manganese ; grind them first very fine with vinegar, and afterwards with brandy.
Red, '-ose, and brown colours, are made from red oxi dated iroi prepared with nitric acid. Their flux is com posed of borax, sand, and minium, in small quantity.