MINIUM. This pigment is a peculiar oxyde of lead, consisting of two atoms of the protoxyde and one of the peroxyde ; but, as found in commerce, it always con tains a little extra protoxyde, oryellow massicot. It is prepared by calcining lead upon a reverberatory hearth with a slow fire, and frequent renewal of the surface with a rake, till itbecomes an ox yde, taking care not to fuse it. The cal cined mass is triturated into a fine pow der in a paint mill, where it is elutri ated with a stream of water, to carry off the finely levigated particles, and to de posit them afterwards in tanks. The powder thus obtained, being is called massicot. It is converted into minium, by being put in quantities of about 50 pounds into iron trays, 1 foot square, and 4 or 5 inches deep. These are piled up upon the reverberatory hearth, and exposed during the night, for economy of Mel, to the residuary heat of the furnace, whereby the massicot ab sorbs more oxygen, and becomes par tially red lead. This, after being stirred
about, and subjected to a similar low cal cining heat once and again, will be found to form a marketable red lead.
The best minium, however, called or ange mine, is made by the slow calcina tion of good white lead (carbonate) in iron trays. If the lead contains either iron or copper, it affords a minium which cannot be employed with advantage in the manufacture of flint-glass for pottery glazes, or for house-painting.
Dumas found several samples of red lead which he examined to consist of the chemical sesquioxyde, and the protoxyde, in proportions varying from 50 of the mer and 50 of the latter, to 95•3 of former and 4.7 of the latter. The more oxygen gas it gives out when heated, the better it is, generally speaking.