.Red Chalk or .Reddk.—An earthy red lacematite, found in all countries and most geological formations.
Red-lead.—This is prepared on the large scale by the oxidation of metallic lead in a reverberatory furnace with two fire hearths covered by an arched roof, situated at the extreme end, separated from the middle hearth, in which the leacl lies, by fire-bridges, and fed with coke. The lead, about 10 per cent. being hard, is worked about by an iron tool as soon as melted, the " massicot " or protoxide formed being constantly pushed to the side. The temperature must be kept at low redness, or the oxide will melt. The treatment is sustained for 24 hours ; the massicot is then removed, ground, and levigated, and again exposed in the furnace to the same heat for 48 hours, or till it exhibits a bright-red colour on cooling. The furnace is then closed, and allowed to cool as slowly as it will. The product is " minium " or " red-lead." Vermilion.—(a) Melt 1 part sulphur, and gradually add 5-6 parts mercury, continuing the heat till the mixture swells up ; then cover the vessel, remove it from the fire, and when the contents are cold, reduce to powder, and sublime in a closed vessel, so placed in a furnace that the flames reach about half the height. Gradually increase the beat till the lower part of the subliming-vessel
becomes red-hot ; break the cold sublimate, grind in water to fine powder, sift, and dry. It is a black sulphide of meicury. This, reduced to powder, and sublimed, gives a filamentous mass of violet hue, appearing scarlet on trituration. (b) Grind together 300 parts mercury and 114 parts flowers of sulphur for some hours, and gradually add 75 parts caustic potash dissolved in 450 parts water ; continue the grinding for some time longer, and gently heat the mixture in an iron vessel, first stirring constantly, afterwards at intervals, keeping the temperature as nearly as possible at 46° (115° F.), and renewing the water as evaporated. When reddening commences, increased care is needed, and when the colour is nearly fine, the heat rnust be maintained at a lower degree till a rich colour is produced. Eveiy precaution must be taken against inhaling the vapours. (c) To a mixture of 4 parts hyposulphite of soda and 4 parts sulphate of zinc in dilute solution, add drop by drop a solution containing 1 part corrosive sublimate. Ileat the whole gently for 60 hours at 45°-55° (112°430° F.).