Light insensitive physical developers.

Within the pH range 2.5-6.5 tungstic acid (an isopolyacid) prevents the reduction of silver ions by ascorbic or hydroquinone more effectively than either gum acacia or other protective colloids. The colloid state of tungstic acid can be stabilized with nonionic detergents, especially with Triton X-100. For buffering the system a mixture of acetic acid and sodium acetate is optimal. Physical developers constituted on the basis of these observations are, in contrast to those commonly used in histology, light insensitive, and remain clear for about 30 min at room temperature, 2-5 times as long as the time required for development.