The Ten Books on Architecture, 7.9.5

Vitruvius  Parallel editions

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Gwilt translation

5Vermilion is occasionally adulterated with lime. The following is a method by which its goodness may be proved. Let the vermilion be placed on an iron plate over the fire, and remain till the plate is red hot: when the heat has changed the colour, and it appears black, let the plate be removed from the fire. If, when cooled, it returns to its original colour, it may be considered pure. but if it remain of a black colour, it is quite clear that it has been adulterated.

Morgan translation

5Cinnabar is adulterated by mixing lime with it. Hence, one will have to proceed as follows, if one wishes to prove that it is unadulterated. Take an iron plate, put the cinnabar upon it, and lay it on the fire until the plate gets red hot. When the glowing heat makes the colour change and turn black, remove the plate from the fire, and if the cinnabar when cooled returns to its former colour, it will be proved to be unadulterated; but if it keeps the black colour, it will show that it has been adulterated.