LEAD, ACETATES OF.—The most important of this series of salts is the neutral acetate, tech nically termed "sugar of lead" from its sweet, but at the same time astringent, taste. It is also called salt of Saturn (Fa. Sucre, or Sel, de Saturne ; GEE. BieizUCker), When carefully prepared, sugar of lead forms colourless, transparent, prismatic needles belonging to the monoclinic system, extremely light, though with lead as the base. The commercial article, however, is usually a con fused mass of irregular crystals somewhat resembling loaf sugar, but of a yellowish colour if the acetic acid used has not been pure. The crystals are soluble in rather more than their own weight of cold, and •75 parts of hot water ; also in 8 parts of alcohol. At ordinary tempera tures a very alight efflorescence takes place. Melted at a gentle heat, the water of crystallization is driven off, and an anhydrous acetate obtained. Beyond 100° (212° F.) the anhydrous salt is decomposed, losing acetic acid and being converted into a aub-salt. Distilled at a high tempera ture, acetic acid, carbonic acid, and acetone are given off, and finely divided metallic lead left in the retort. Great care should be taken in dealing with all preparations of this salt, as it is extremely poisonous. A good acetate should be completely soluble in water, and when the lead is exactly precipitated by dilute sulphuric acid, the clear supernatant liquid should be wholly volatilizable by heat. The aqueous solution is partly decomposed by the carbonic acid of the air, carbonate of lead being formed and a portion of acetic acid being given off.
There are three qualities of sugar of lead known in commerce, white, grey, and brown. The white, or purest, salt is the most esteemed, inasmuch as the chief use of the compound is to prepare other acetates, and it is therefore especially valuable for the acetic acid which it contains ; when used as a mordant, however—for the sake of the base—the brown salt is the most economical, as it contains a larger amount of lead than an equal weight of white.
The process for the manufacture of the white salt upon a large scale is as follows :—A charge of beat litharge is added to acetic acid—usually good "second " acid—in a copper pan about 6 ft. long, 4 ft. wide, and 1 ft. deep, with, preferably, a strip of lead soldered upon the bottom to prevent the acid acting upon the copper. The best proportions are 325 parts of Illharge to 575 parts of acid, and the mixture should not be more than 5 inches deep. The pan is heated by a small coal fire placed underneath iu the ordinary way, and—a precaution to be observed in all the processes—the mouth of the fireplace, and the fire-hole, should be situated outside the building, the pan being set against the wall. The plan is
shown in Fig. 29.
By this means a more per fc,:t cleanliness is secured. The litharge is added gradually to the acid in the pan, and the mixture stirred up from the bottom during the addition and until the whole is thoroughly dissolved ; an operation usually occupying about half an hour.
The mixture must be tested from time to time to see that it is still acid, as the formation of any basic salt interferes with the after crystallization. As soon as a thorough solution is effected, the pan is filled up with water to within a couple of inches of the top—that is to say, the quantity of water added is equal in bulk to the solution itself. By this dilution the impurities are separated out, rise to the surface, and are skimmed off. The liquor is then thoroughly boiled and a judicious amount of the crystals from the mother liquor of previous crystallizations is dissolved in it. After cooling and settling, the clear liquor is siphoned off to a similar pan placed alongside, and set in the same fashion. The first " mixing" pan is carefully cleare.1 ont, and a fresh batch of litharge dissolved. The liquor in the second pan is evaporated to about density, samples being taken from timo to time and set aside to eool in small shallow dishes, that it may be judged from the way in which they sot if a proper degree of concentration has been reached. As soon as one of these samples upon cooling forms a compact mass of crystals, the liquor is siphoned off into shallow copper crystallizing pans about 3 ft. long, 2 ft. wide, and 4 inches deep, with bevelled sites. In these compact masses of crystals are formed. They are then taken into the drying room and emptied, the blocks of crystals being set up on end along a sloping bench (conveniently formed of two balks of timber set longitudinally) with a spout below to carry off the drainings, or " mothers," to a cistern sunk in the floor at the end of the bench. Only a gentle heat, about 32' (90° F.) from a fireplace outside the house, should be maintained in the drying room, as sugar of lead is somewhat dangerous; dry wood, linen, matting, and other fibrous materials impregnated with the salt boing liable to take fire very readily. When drained and dry the crystals aro removed into the packing room, which should be maintained at about the same temperature ns the drying room. Here they are carefully scraped, broken up into fragments, and caskeil ready for the market. When of good quality the fragments consist of masses of beautifully white, small crystals, exceedingly light. If large crystals are required they have to be specially "grown." Tho scrapings from those first crystals in the packing room aro added to the litharge solution in the mixing pan and worked over again.