This is 5 lb of Potassium Bromide from Photographers' Formulary. It is commonly used as a restrainer in developers, especially those containing hydroquinone, pyrocatechin, or pyrogallol, and helps to prevent fog during development. When used in paper developers, it also reduces contrast and provides a warming effect. Additionally, potassium bromide is used as an ingredient in intensifying and reducing toners, as well as a variety of other photographic solutions.