The system of resists or reserves possesses the inconveniences of not producing impressions of great firmness, and of requiring very strong vats. When the strength of the vat is partially exhausted, they may be thrown aside. These inconveniences are obviated by the system of discharges (_enlevages_). In this system the cloths are vat dyed of a uniform blue. The strength of the vat is of less importance, and it can be used until the indigo is quite exhausted. The means of destroying the indigo which has been fixed upon the fibre are founded on the use of active oxidizing agents, which transform the insoluble indigotine into soluble isatine. The agent generally used is chromic acid. As this acid cannot be incorporated with the thickening to be printed, as the thickening would produce oxide of chrome, the cloth is passed through a strong solution of chromate of potash, and dried in the shade. The required pattern is then printed on the cloth with a mixture whose principal elements are acids which are susceptible of setting free the chromic acid on the tissue, which then acts upon the indigo producing a white pattern. The acid generally employed for freeing the chromic acid is oxalic acid, thickened with British gum, dextrine, or starch, with the addition of pipe clay. To prevent running, nitric, sulphuric, or tartaric acid are sometimes used.[8] ------------------------------------------------------------- [8] Schutzenberger gives the following receipts:— PREPARATION FOR DISCHARGE. Water 2 litres. Yellow chromate 500 grammes. WHITE DISCHARGE ON BLUE BOTTOM. Tartaric acid 3 kilograms. Oxalic acid 250 grammes. Burnt starch 4 kilograms. Nitric acid 500 grammes. Water 4 litres. De Kæppelin gives the following:— WHITE DISCHARGE ON BLUE. Water 2 litres. Starch 1 kilogram, 800 grammes. Oxalic acid 500 grammes. Tartaric acid 250 grammes. Sulphuric acid 375 grammes.