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MANUFACTURES.
Manufac to more than forty feet. This required ten additionaltuies. fire-places, as a fire was necessary to every five feet oflength when the vessel is ten feet broad, which is a sur-face of fifty square feet for each fire to act upon. Trans-actions of the Society of Arts , vol. xxviii.
Litharge,
(606.) This is an oxide of lead, made by the simpleaction of heat and air on the metal; it is produced inthe process of extracting silver from lead, and has beenalready explained under the latter head.
Minium or Red Lead Works.
(607.) The process of making red lead in Derbyshireis described by Dr. Watson in his Chemical Essays, vol. iii.as follows. The furnace is much like a baker’s ovenwith a low vaulted roof, and on each side of the furnaceare two party walls rising from the floor of the furnace,but not reaching to the roof. In the interval betweenthese walls and the sides of the furnace the coal is burned,and the flame, drawn over the top of the party wall andstriking against the roof, is thence reflected down uponthe surface of a quantity of lead which is laid on thefloor of the furnace. The metal soon melts,, and in-stantly becomes covered with a pellicle which is succes-sively raked off till the whole is changed into a greenish-yellow powder. This is taken out, ground in a mill,and washed, to separate the portion of lead that still re-mains in the metallic state, by which it becomes a uni-form yellow colour, and is then thrown hack into thefurnace and constantly stirred, so that every part maybe equally exposed to the action of the flame, and afterabout forty-eight hours of calcination it is convertedinto red lead.
Some practical nicety is required in the managementof the fire, which, if too slack, gives only a yellow ororange-coloured powder, and, if too fierce, makes theminium dusky, and destroys that brilliant gloss for whichit is so much admired. There are besides other minutercircumstances of management, and probably kept secretas much as possible. Jar mentions in particular that ofcooling the minium very gradually, and closing all theopenings of the furnace, otherwise, he says, the beauty ofthe colour is much impaired.
Vt lute lead. (608.) This is a carbonate of lead prepared in the firstpart of the process of making the acetate of lead. First,lead is melted and cast in a case or mould, so as to forma sheet of about 2 feet 5 inches broad, and from one-six-teenth to one fourth of an inch thick. The lead in thisinstance is cast at once of the proper form, and not me-chanically flattened like sheet lead, that its texture may bemore open, and more easily penetrated by the acid vapour.These plates are then rolled up into a loose coil, andeach is placed perpendicularly in an earthen pot, like acommon garden pot, holding from two to six pints each,but with a ledge on the inside about half way down, onwhich the coil of lead rests, so that it may not touch thebottom. Each pot is filled with vinegar of any kind,just so high as not to wet the bottom of the lead, andthe whole is also covered with a plate of lead fitted veryclose. The pots are then ranged under a building, so asto shelter them from the weather, and buried pretty deepin fresh stable litter or tanner’s bark, layer upon layer,
according to the number of pots. The he&v of the dung Mechanic*-soon fills the upper part of the pots with the vapour of P rcic esses ,the vinegar below, and the lead kept constantly in con-tact with the acid vapour, but not immersed in theliquor, presently begins to corrode and oxidate at thesurface. The pots remain under the litter for abouttwo months, at the end of which time they are taken outand the coils of lead are found deeply corroded, and thesurface converted into a whitish, scaly, brittle oxide.
This is separated by passing the plates between rollers,which causes it to pee) off, leaving the lead beneath inthe metallic state. This oxide is then mixed with alittle water, and passed between a pair of mill-stones;then the finer parts are separated from the coarser bysuccessive washings, the former being longer suspendedin water than the latter, till the whole of the finer oxideis obtained. This is then dried either in the air or inlarge airy rooms, warmed by a small stove, and is thenperfect ceruse or white lead fit for use. Of late yearsthe scales of oxide, instead of being separated fromthe coils by dry laminating, which raised a dust of leadhighly injurious to the health of the people about them,are detached by spreading the coils upon a perforatedwooden floor covered with water, and drawing them toand fro by rakes, which detaches the oxide, and causesit to sink through the water and the holes of the floor tothe bottom of the vessel below. Aikin’s Dictionary.
Acetate or Sugar of Lead Works.
(609.) Sugar of lead is used very largely in various ma- Sug* rnufactories of this Kingdom, particularly in calico print- eing, and the works for preparing it, although very sim-ple, are confined to very few places. Most of the sugar oflead used in this Country is imported from Holland. Thissalt is made very nearly in the manner of ceruse ; that is,lead sheets are put into pots with vinegar, and digested asufficient time, but here the vinegar is distilled, and theplates, instead of being entirely out of the liquor, are halfimmersed in it. This being done, the upper half is sooncovered with an efflorescence of ceruse, after which itis immersed in vinegar, and the part which was beforeimmersed is now brought up to be converted into ceruseas before, when the plate is again turned, and the newlyoxidated surface in its turn immersed in the liquor. r l heplates are thus turned about two or three times a day,and the vinegar in saturating itself with the cerusebecomes milky, and soon sufficiently impregnated to beboiled down to the crystallizing point, which is done intinned vessels to about a third of the original quantity.
This is then strained, and, on cooling, deposits the acetatein small long needles in irregular whitish crystals. Themother-liquor is again evaporated for a fresh crop ocrystals, but these are browner and somewhat deli-quescent. Acetate of lead may also be made directly bydissolving ceruse or litharge in vinegar, and probablythe natural carbonate will answer the same purpose.
Some technical nicety appears to be required in makingthe salt crystallize in the large way.
(610.) This is an article of great application in various 1branches of the Arts and Manufactures of this Country,and is principally obtained from the combustion °^' e ^tables, although it exists also in a mineral state. If 1woody or annual stems of vegetables, which have g ro ' v1 'in soils unimpregnated with common salt, after being