This is 1 lb of Rochelle Salt from Photographers' Formulary. It is commonly used as a component when mixing brown, sepia, blue, and maroon Kallitype developers. It is also used for silvering mirrors and is an ingredient in Fehling's solution.
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Sodium potassium tartrate, tetrahydrate Rochelle salt Rochelle salt tetrahydrate Seignette salt tetrahydrate Potassium sodium L-tartrate tetrahydrate Potassium Sodium Tartrate Tetrahydrate
Colorless to blue-white orthorhombic crystalline salt
It is an ingredient of Fehling's solution . It is used in silvering mirrors.
Also called Seignette salt after Pierre Seignette, an apothecary of La Rochelle, France, who was the first to make it (c.1675). Chemically, it is potassium sodium tartrate.