The composition for painting cardboard so as to produce a surface that can be written on with slate pencil is similar to that used for blackboards. Pour ounces of shellac should be dissolved in 1 qt. of methylated spirit, and then ground with 11/4 oz. of flour emery, 2oz. of ivory black, and 1 oz. of ultramarine blue. Other blackboard dressings are given on p. 230. Before using, the solution should be thoroughly shaken; a little is then poured out into a dish and evenly applied with a brush. Two or more coats will be required. If the cardboard is very porous, a coat of very thin size may first be applied.