1438. To use Alum Water.—Before laying on the colors, 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 colors, and give them additional lustre.
1439. To make Lime Water.—Put unslaked 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.
TO PREPARE WASH COLORS FOR MAPS.
1440. Blue.—Dilute Saxon blue with water; or to the solution of litmus add distilled vinegar.
1441. Green.—Dissolve verdigris in distilled water and add gum arabic.