The drainings from the blocks of crystals in the drying house are transferred to a pan similar in description to the mixing pan. Hero a little acetic acid is added, the liquors are diluted with water to twice their bulk, the impurities that rise to the surface being skimmed off, and are boiled down to crystallizing point. After cooling and thoroughly settling, the clear liquid is siphoned off to shallow pans of copper, and " set " and dried in precisely similar manner to the first liquors. The blocks aro broke!' up when removed from the draining bench, and worked over again by careful introduction into the litharge solution. These second crystals are sometimes called "numbers," and the pan in which they are diluted and evaporated, the "number pan." All the residues—the settlings—from both the first and mother liquor processes are transferred to some suitable vessel and thoroughly washed with boiling water, the washings being added to tho liquors in the mixing pan. The final residue may be dried in a retort and smelted for the lead it contains.
When the mother liquors get too impure to be crystallized in the manner described—when tho blocks produced begin to come dirty—a pan of grey acetate is made. The liquors are trausferred to one of the " number" pans, and worked up in somewhat similar fashion to the " 'numbers" themselves. The crystals, however, which are so impure as to possess hardly any crystalline appearance, are not added to the " white " process at any stage, but are kept strictly by themselves, and when broken up are sold as impure sugar—or "grey "—from the dull grey appearance of tho fragments.
In the manufacture of white sugar of lead upon a large scale, it is found in practico that 1 ton of ordinarily pure litharge yields 31 cwt. of best acetate. Instead of being worked up in tho manner described, the spent liquors may be treated with carbonate of soda or lime, carbonate of lead and supernatant acetate of soda being obtained. Stoneware or glass crystallizing vessels may be used instead of copper, but the loss from breakage is considerable.
In some places in Germany a remarkably good white sugar is manufactured from spirits of wino, crystals of a very largo size and beautiful transparency being obtained. The process is, of course, too costly to be carried on to any great extent, and only where there are exceptional facilities for producing the spirits of wine.
The process for making brown sugar of lead differs but little from that employed to produce white, except in the matters of plant and choice of materials. Distilled pyroligneous acid is satu rated with litharge in a large tub, and the thick solution, after being thoroughly stirred up, is allowed to settle, and is then siphoned or run off into an iron pan 6 ft. long, 4 ft. wide, and 12 or 14 in. deep. Here it is boiled and diluted with a large amount of water, the impurities being skimmed off as they rise. The water must be added until the liquors have only a very slight colora tion. They are then evaporated to crystallizing point, and run off into wrought-iron pans about 4 ft. long, 3 ft. wide, and 6 in. deep. When "set," the pans are turned over, the masses of crystals
drained, dried, and broken up for market. A better article is made if the liquors are brought to the boiling point, and settled, before transference to the diluting and evaporating pan ;—which may conveniently he hemispherical, and of suffi dent capacity to hold 500 gallons.
Besides the processes described, acetate of lead, of exceedingly fine quality, may be obtained by exposing sheet lead to the action of air and acetic acid vapour in a closed chamber. A mixture of carbonate and acetate is formed upon the sheets, which is scraped off and dissolved in an excess of acetic acid. The solution is evaporated until a density of is attained, and allowed to cool. when acetate of lead crystallizes out in truncated and flattened prisms of the description shown in Fig. 30. If means be taken to secure rapid cooling, the crystals take the form of fine needles, separating out in clusters. Several patents have 'been taken out with a view to extend the principle of presenting the lead to the acetic acid in a state of vapour, but the process is expensive, and though producing a very fine article, cannot compete with the white sugar made in the usual way.
Sugar of lead is used by dyers and calieo-printers ; also in certain medicinal preparations, for its sedative and astringent qualities, and in the manufacture of the sulphide of lead used by makers of indiarubber goods. Its solution forms, with caustic potash or soda, a white precipitate, which is soluble in an excess of alkali. About 2000 tons per annum are produced in this country, the process being chiefly carried on in South Wales and London.
Besides the neutral acetate, lead forms certain sub-salts or basic acetates, which deserve atten tion. These are obtained for the most part by dissolving powdered litharge in hot solutions of sugar of lead, as much as six times the normal quantity of base being taken up. They are used to some considerable extent as n3ordants in dyeing and printing, and as a " resist " for china blues. Considerable difficulty, however, is experienced in thickening them. The diacetate, crystallizing in long needles from a solution of 6 parts of sugar of lead and 7 of litharge in 30 parts of boiling water, is used in the manufacture of white-lead. The Goulard water of pharmacy, " Acetum Saturni," is an aqueous solution of various haic acetates, ehiefly the tribasic salt--a white powder obtained by dissolving 3 parts of the diaeetate and 1 of litharge in 9 parts of hot water. The manufacture of these basic aeetates is carried on only to a liraited extent.
Ordinarily good cornmercial acetates of lead should give the following results upon analysis:— White. Brown.
Acetic acid 27.6 21.8 Oxide of lead .. 58.4 59.9 15.5 Carbonate of lead and insoluble matter .. — 2.8 100.0 100.0- They may be te3ted by precipitating the lead as sulphide by a current of sulphuretted hydrogen, exactly neutralizing the acetic acid, which is liberated, by a standard alkaline solution, aud calculating the result after the method described in treating of aeetic aeid.