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19th century recipe
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Dissolve sulphate of iron (copperas, green vitriol) in water; boil the solution. Add nitric acid until red fumes cease to come off, and enough sulphuric acid to render the liquor clear. This is the persulphate of iron. To this add a solution of ferrocyanide of potassium (yellow prussiate of potash), as long as any precipitate is produced. Wash this precipitate thoroughly with water acidulated with sulphuric acid, and dry in a warm place.
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