(a) Melt 1 part sulphur, and gradually add 5 to 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 heat 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 mercury. This, reduced to powder and sublimed, gives a filamentous mass of violet hue, appearing scarlet on trituration. (6) Grind together 300 parts mercury and 114 flowers of sulphur for some hours, and gradually add 75 caustic potash dissolved in 450 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 115° F. (46° C), and renewing the water as evaporated. When reddening commences, increased care is needed, and when the colour is nearly fine, the heat must be maintained at a lower degree till a rich colour is produced.
Every precaution must be taken against inhaling the vapours, (c) To a mixture of 4 parts soda hyposulphite and 4 zinc sulphate in dilute solution, add drop by drop a solution containing 1 part corrosive sublimate. Heat the whole gently for 60 hours at 112° to 130° F. (45° to 55° C.)
Dry mix 2 lb. powdered alum, 1 lb. honey; powder, calcine to whiteness m a shallow dish, cool, wash, and dry
Spanish white and Paris white are practically the same article in different degrees of fineness, all being simply chalk, ground, elutriated, balled, and dried. Grinding-mills break up the chalk and mix it with water, which is constantly flowing in. On leaving the mills, the mixture passes along a series of wooden troughs, where the sand, which has a greater specific gravity than the chalk, is deposited, the chalk passing on into the settling-pits. On being taken from the pits, the whiting is partially, dried on a floor under which hot flues run; then cut up into large rough lumps, and placed in racks on cars which run round on tramways into an immense oven. The heat from the dues in this oven is greatly increased by an air-blast, which also carries off the moist exhalations from the drying whiting; 12 hours on the heated floor and 12 in the oven thoroughly dries the whiting, and it is ready for packing or the putty factory. Paris white of fine quality is used for finishing parlour walls, adulterating paints, making paper heavier and whiter, etc. For this purpose, what is called cliff stone, a better and harder quality of chalk, is used.
Paris white is made much on the same principle as whiting, only more carefully washed and more slowly dried.
After picking out the coarser impurities, the chalk is ground in a mill and formed into rolls, in which shape it is found in the trade. For painting purposes, it is still further purified by stirring in clear water, allowing it to settle, and decanting the first water, which is generally yellow and dirty. The washing is repeated, and the chalk is floated out into another vessel, after passing through a silken sieve. After settling, the water is decanted, and the pasty white residue is formed into cylindrical rolls, 3 to 4 in. long, and l 1/2 to 2 in. diameter. These are allowed to harden and dry in the air, and are then ready for painting, whitewashing ceilings, and for distemper painting with size.
Litharge is ground with sea-water till it ceases to whiten, and is then washed and dried.
(a) Add a filtered solution of lead nitrate or acetate to a filtered solution of neutral potash chromate so long as a precipitate falls; collect this, wash with soft water, and dry in security from sulphur-tainted air. (6) Dissolve lead acetate in warm water, and add sufficient sulphuric acid to convert it into sulphate; decant the clear liquid, wash the residue with soft water, and digest with agitation in a hot solution of yellow (neutral) potash chromate, containing 1 part of this salt for every 3 lead sulphate; decant the liquid, and drain, wash, and dry the precipitate, (c) 1/2 to 2 1/2 equivalents (according to colour required) sulphuric or phosphoric acid is added to a solution of potash bichromate in water. This mixture is added to a milk of white-lead or litharge (very finely divided and suspended in water), the addition being in the form of a thin stream, to prevent undue heating. The required colouring matter falls. (Werner.) (d) Lead chromate is prepared by mixing solutions of a lead salt and potash chromate or bichromate, whereby a precipitate of lead chromate is thrown down. The shade of the precipitate varies from sulphur-yellow to deep orange-red, according to the process of manufacture.
The lead salt used may be white sugar of lead, or chloride, prepared by mixing common salt and litharge with water to form a paste, occasionally stirring during the first 24 hours, and allowing to stand until all the oxide has been transformed into chloride. These methods do well on a small scale, but commercially the process is as follows: 4 wooden tubs, about 3 ft. diameter and 1 1/2 ft. high, are arranged one below the other. These are filled with granulated lead, and strong alcohol vinegar is poured into the first. After standing a few minutes, it is drawn off into the next, and so. on, until it is finally run into a large collecting tub placed at the bottom of the series. Although the first treatment with vinegar dissolves a very small quantity of the lead, it starts an oxidation on the surface, which the subsequent treatment with the vinegar (which is now allowed to remain an hour in each tub) easily dissolves off, and by this means a moderately strong solution of lead acetate is obtained. A copper pan, with an outside steam-casing, is used for dissolving the charge of potash bichromate, mixed with 10 to 12 times its weight of water. The proper proportion of the solution of acetate is now added, which exactly precipitates the whole of the chromate as lead chromate.