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An ancient transformation process
If natural red ochres are rare, the recipe for their artificial manufacture from yellow ocher is mentioned as early as Pliny the Elder, a first-century Roman writer and naturalist. He explains that red ocher is made by calcining yellow ocher for a long time in jars sealed with clay. His contemporary, the Roman architect Vitruvius, specifies that the yellow ocher is heated to incandescence before being suddenly extinguished in vinegar to give usta, burnt yellow ocher turned red.
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