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They started with an iron-containing electrolyte, ferrocyanide, that has been studied in the past.

He extensively used potassium ferrocyanide to bleach detail into dark areas.

The Harvard team realized that a possible bromine replacement was a charge-carrying molecule called ferrocyanide, which sounds dangerous but is actually used as a food additive.

He admonished them for sending their spies to a foreign country equipped with “soap made of potassium ferrocyanide or toilet water that contains lead acetate.”

The powdered ferrocyanide is then added, together with the colophonium, and the heating is continued until a slight smell of cyanogen is noticed.