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Dry-or solid-phase chemistry can trace its origins back to the ancient Greeks. Some 2000 years ago, copper sulfate was an important ingredient in tanning and preservation of leather. At that time, dishonest traders were adulterating valuable copper sulfate with iron salts. Pliny has described a method for soaking reeds of papyrus in a plant gall infusion or a solution of gallic acid that would turn black in the presence of iron. This is the first recorded use of the dry chemistry system. In 1830, filter paper, impregnated with silver carbonate, was used to detect uric acid qualitatively.