To test the drying qualities of paint make some streaks of paint on glass slips and keep them at, say, 212° P. for about half an hour j the quickest drying will be the most tacky to the touch. In ordinary circumstances, the amount of white lead required for 1 lb. of driers varies from 10 lb. to 11 lb.; more will be necessary in summer than in winter. Too much driers will cause the surface of the paint to dry too rapidly, the result being the formation of a hard surface film and a tacky basis. For lead colour it would be advantageous to use more driers, as the black retards the drying considerably.