Kodak's official E-6 process, which replaced Process E-4 for almost all applications, avoids the necessity of TBAB by adding a separate reversal bath containing the tin salt stannous chloride.
12.
While making a coloured liquid for a thermometer, Cornelis Drebbel dropped a flask of Aqua regia on a tin window sill, and discovered that stannous chloride makes the color of carmine much brighter and more durable.
13.
Therefore, if clear solutions of tin ( II ) chloride are to be used, it must be dissolved in hydrochloric acid ( typically of the same or greater molarity as the stannous chloride ) to maintain the oxidation by the air:
