Page:Cyclopedia of Painting-Armstrong, George D (1908).djvu/438

430 When solution is complete, mix the two, and keep well stoppered for use.

Waterproof Wash for Lime. This may be made by mixing the powder from 3 parts rock quartz, 3 parts broken marble and sandstone, also 2 parts burned porcelain clay with 3 parts freshly slaked lime, still warm. In this way a wash is made which forms a silicate if often wetted, and becomes finally like stone. It is applied thickly to the surface, allowed to dry for a day, and the next day frequently wetted, which makes it waterproof.

Whitewash that will not Rub off. Mix up half a pailful of lime and water, ready to put on the wall, then take $1/4$ pint of flour, mix it with water, then pour on it a sufficient quantity of boiling water to thicken it, and pour it while hot into the whitewash, stir all well together, and it is ready for use.

Writing on Glass. To write on glass, or, as it is properly termed, to etch, as the letters are eaten out by an acid, either liquid hydrofluoric acid or hydrofluoric acid gas is required, according to the effect desired to produce. The former eats away the glass but leaves it clear, the latter gives the part operated upon, a ground-glass appearance. For the first way, clean a piece of glass, warm it, rub over with white wax or beeswax, and trace the letters with a needle or penknife, going down to the surface of the glass, make a wall of wax all round the edge of the glass, and pour on hydrofluoric acid, and leave for two or three hours, then clean with turpentine. To produce letters with a ground-glass appearance, place in a leaden dish 2 parts of powdered fluoride of calcium, pour on 3 parts of sulphuric acid, and with a stick mix into a paste. Prepare the glass as before, except that there is no need for a wall of wax around the edges, cover the leaden vessel with this piece of glass, and by warming the vessel gas will be evolved which will attack every clear part of the glass. The workman must be very careful indeed in using the acid, of the gas, the