Take 1-1/2 oz. of red ochre in fine powder, the same quantity of calcined verdigris, 1/2 oz. of calcined borax, and 4 oz. of melted yellow wax; the verdigris must be calcined, or else, by the heat applied in melting the wax, the vinegar becomes so concentrated as to corrode the surface, and make it appear speckled.  These last three are colours for heightening compositions.

174.  Mosaic gold

Mosaic gold, or aurum mosaicum, is used for inferior articles.  It is prepared in the following manner:  1 lb. of tin is melted in a crucible, and 1/2 lb. of purified quicksilver added to it; when this mixture is cold, it is reduced to powder, and ground with 1/2 lb. of sal-ammoniac, and 7 ozs. of flower of sulphur, till the whole is thoroughly mixed; they are then calcined in a mattrass, and the sublimation of the other ingredients leaves the tin converted into the aurum mosaicum, which is found at the bottom of the glass, like a mass of bright flakey gold powder.  Should any black or discoloured particles appear, they must be removed.  The sal-ammoniac used here must be very white and clear, and the mercury quite pure and unadulterated.  When a shade of deeper red is required, it can easily be obtained by grinding a very small quantity of red lead along with the above materials.

175.  Dutch or german gold

A gilding powder is sometimes made from Dutch gold, which is sold in books at a very low price.  This is treated in the same way as the real gold leaf in making the true gold powder.  It is necessary, when this inferior powder is used, to cover the gilding with a coat of clear varnish, otherwise it soon loses its metallic appearance.  The same remark applies, though to a less degree, to Mosaic gilding.

176.  Copper powder

This is prepared by dissolving filings or slips of copper with nitrous acid in a receiver.  When the acid is saturated, the slips are to be removed; or, if filings be employed, the solution is to be poured off from what remains undissolved; small bars are then put in, which will precipitate the copper from the saturated acid, in a powder of the peculiar appearance and colour of copper, and the liquid being poured from the powder, this is to be washed clean of the crystals by repeated levigations.

177.  Common size

The size used by painters for most sorts of common work is prepared by boiling in water pieces of parchment, and of the skins of animals and fins of fish, and evaporating the solution to a proper consistency.  It only differs, however, from a solution of glue containing fewer foreign ingredients, and in not being so strong.

178.  Dr. John’s varnish for plaster of Paris casts

Take of white soap and white wax, each half an ounce, of water two pints; boil them together for a short time in a clean vessel.  This varnish is to be applied when cold, by means of a soft brush.  It does not sink in, it readily dries, and its effect may be heightened by lightly using a silk pocket handkerchief.