CERUSE, or WHITE LEAD, is commonly made by coiling up very thin cast sheets of lead into rolls, so as to leave a small space between each fold. The rolls thus formed are lightly jammed into the mouths of a number of earthen ware pots, which are about half filled with vinegar. These pots are placed in a situation where a very gentle heat will cause a slow evaporation of the vinegar, in order that it may gradually operate upon the lead. A layer of several hundreds of them are usually deposited either in a bed of tanner's spent bark, or dung, contained in a wooden frame. Boards are then laid over the tops of these pots, and on this temporary floor, which is supported by boards placed edgeways, is laid another layer of pots similarly furnished. Seven such layers of pots are placed in succession over each other, to form a stack. When the vinegar is found to be completely evaporated in these stacks, which generally occupies three months, they are taken down ; the corroded lead is taken out of the mouths of the pots, but as the adhesion to the latter is very strong, many of them are broken in getting out the lead, and the poisonous dust which the workmen inspire by this operation, lamentably impairs their health, and they are often in consequence afflicted with the disease called the Devonshire, or painter's colic. To avoid this serious evil, and economize the process, a different and more mechanical arrangement has been made. The base of the stack is made of a layer of pots filled with vinegar only ; over these is supported, by planks on edge, a floor of boards, pierced with numerous holes to allow the passage of the acid vapour; on these perforated boards are then laid the rolls of lead, then another perforated floor, and another layer of rolls, until the stack is completed. By this arrangement it is said that the manufac turer obtains one-third more white lead, saves his pots from breakage, and avoids the dust so pernicious to the workmen by first sprinkling the rolls with water. Upon uncoiling the plates, the white substance that falls off is the purest white lead, and is disposed of for the finest work under the name of flake white; a portion of the latter is ground up with water, formed into small lumps, and sold under the name of ceruse. But all the white lead that is subsequently taken of the plates is ground up either with water, and sold in large lumps, or it is ground with oil, and is usually more or less adulterated with whiting, to suit the various prices at which it is required in the market. As the processes employed in France and Germany for the manufacture of ceruse appear to be well worthy of attention, we shall here annex some account of them. In
France the first part of the process consists in dissolving 174 lbs. of finely ground litharge in 65 lbs. of pyroligneous acid, of such strength that 22i grains of this acid will saturate 25 grains of sub-carbonate of soda ; fifteen to twenty times as much water is usually added. The whole is left for a short time, and the clear portion being decanted off, some fresh acid and water is poured on the sediment, to take up any oxide that might have escaped the action of the first parcel The decanted solution is run into large shallow covered cisterns, into which carbonic acid gas is passed through numerous pipes. When the settling appears to be completed, the whole is passed into a deep cistern, and left there for some hours, when the liquid part is to be poured off, in order to be combined with more litharge, some fresh acid being also added. After the desired tint has been given to the lead, it is well drained, put into glazed pots of the proper shape to imitate the Dutch white lead leaves, then dried in stoves, and lastly, packed in blue paper to heighten the effect of its beautiful colour.
In Germany the first part of the process consists in casting the pure lead of Corinthia into very thin sheets, which is effected by pouring the liquid metal upon inclined sheets of iron. The sheets of lead are trimmed to a proper size, and suspended over an acid liquor contained in boxes, which are usually about 5 feet long, 1 foot broad, and 10 inches deep ; and they are pitched internally on the bottom, and rising therefrom about 2 inches at the sides. Sticks are placed across the boxes, and the sheets of lead are doubled so as to be suspended in the middle by them, but so as not to touch each other, nor the acid liquor deposited underneath them. The liquor in some manufactories is made of equal parts of vinegar (obtained from crab apples) and wine lees, and about two gallons of this mixture is apportioned to each box. Some manufacturers use 20 pinta of vinegar, 8 pints of wine lees, and 1 lb. of pearl ash. The usual mode is to dispose the boxes in a large room heated up to 87• Fahrenheit ; a greater heat would evaporate the acid too fast. In about a fortnight the corrosion is finished, rendering the sheets about a quarter of an inch thick, and partially covered with crystals of sugar of lead. Those crystals that are easily detached are carefully washed ; during this operation a white scum appears, which is taken off, and a little pearl ash being added to it, it is changed into ceruse of a beautiful whiteness, and is sold for the choicest purposes; the remainder is mixed in different proportions with the pure sulphate of barites, brought from the Tyrol.