Yellow Colour.—Indigo, white, and lake; or fine Dutch bice and lake, shaded with Indigo; or litmus smalt and bice, the latter predominant.
Water.—Blue and white, shaded with blue, and heightened with white.
To prevent Colours from Cracking.—Boil 2 ozs. of the best and clearest glue, with 1 pint of clear water, and a 1/2 oz. of alum, till dissolved. With this temper those colours intended for the sky.
To make a Solution of Gum.—Dissolve 1 oz. of white gum arabic, and a 1/2 oz. of double refined sugar, in a quart of spring water; strain it through a piece of muslin, and bottle it to keep it free from dust.
To keep Flies from the Work.—Having prepared the gum water for the colours, add a little coloquintida.
To prepare Alum Water.—Take 4 ozs. of alum, and 1 pint of spring water; boil it till the alum is thoroughly dissolved, and then filter it through blotting-paper.
To use Alum Water.—Before laying on the colours, take some of this water, hot, and with a sponge wet the back of the paper, which, if not good, must be wet three or four times, letting the paper dry each time before wetting it again. This will prevent the sinking of the colours, and give them additional lustre.
To make Lime Water.—Put unslacked lime in a well-glazed pan; cover it with pure water, and let it remain for one day. Then strain off the water. This water will change sap-green into blue.
553. TO PREPARE WASH COLOURS FOR MAPS
Blue.—Dilute Saxon blue with water; or to the solution of litmus add distilled vinegar.