1134 WHITE LEAD 



neutral acetate, is formed as we have already supposed. Now there are two sub- 

 acetates : one composed of six molecules of oxide of lead to one of acetic acid ; and 

 the other consisting of three molecules cf oxide of lead to one of acetic acid. We select, 

 in preference the former, as it is the one which forms naturally when acetic acid acts, 

 at common temperatures, on an excess of oxide of lead. The composition of this 

 salt is such, that, if we can conceive slow combustion to take place, or that its acetic 

 acid combining with the oxygen of the air is resolved into water and carbonic acid, 

 then the carbonic acid produced would be exactly sufficient to saturate four atoms of 

 the oxide of lead, and leave a compound of the precise composition of white lead. 

 On this view, the first action in a white lead stack would be the production of sex- 

 basic acetate of lead ; and the next would be the destruction of this by eremacausis, 

 and the formation of white lead. 



The apparatus employed in the manufacture of white lead is extremely simple, and 

 consists merely of certain large enclosures or spaces, called ' beds,' in which the stacks 

 are built up, together with the earthenware-pots needed for holding the vinegar, and 

 the machinery used in casting the lead and grinding the white lead, so as to n't it for 

 the market. The metallic lead was formerly used in the shape of sheets or coils, which 

 were placed perpendicularly over the vinegar pots ; but this practice has been almost 

 everywhere abandoned, and at present the lead is generally cast into what are culled 

 'crates' or 'grates,' and having the appearance of lattice-work; the object being to 

 expose as large a surface as possible of metallic lead to the action of the vapour of the 

 vinegar. The beds are of considerable size ; and, in this respect, some diversity of 

 opinion prevails amongst practical men ; but it seems pretty certain that no advantage 

 is gained when the area of a bed comes to exceed 300 square feet ; and there are many 

 reasons for believing that, with beds of twice this area, the gain, in point of diminished 

 labour, is much more than compensated for by the reduced produce in white lead. 

 Nevertheless, each manufacturer seems to entertain an opinion of his own in respect 

 to this matter ; and there are even some pretensions to secresy concerning it. In fact, 

 everything depends upon the construction of the bed, for it is this which regulates 

 the production of white lead ; and, as a proof of the great importance connected with 

 this circumstance, we may here mention, that, whilst one manufacturer has produced 

 as much as 65 per cent, of corrosion during a long course of years, another in his im- 

 mediate neighbourhood has never been able to exceed 52 per cent. The beds of the 

 former are 16 feet square, whilst those of the latter are 19 feet square; and, in 

 dwelling upou the details of this operation, we shall find that theoretically, a bed may 

 be too large, as the above practical fact indicates. 



In forming a stack, it is necessary to begin by laying, in the first instance, a bed of 

 spent tanners' bark, 3 feet in thickness, over the surface of the bed ; and upon this 

 are placed the earthenware-pots containing the vinegar. These are arranged side by 

 side, and filled to about one-third of their contents with vinegar, of a strength equal 

 to 6 per cent, of anhydrous acetic acid. Upon these pots are placed the crates of 

 lead, and over all a series of boards are arranged, which form a floor for the next 

 layer of spent tan. Such an arrangement as we have described is denominated ' a 

 bed,' but there is this difference between the beds, viz. that the lowest or bottom bed 

 has a bed of tan 3 feet in thickness, whereas but one foot only is needed in the others. 

 Having finished the lowest bed, 12 inches of spent tan are now placed upon the 

 boards, and a similar arrangement of pots, crates, and boards takes place, which con- 

 stitutes the second bed ; this is followed by a third, a fourth, and so on, until at last 

 the uppermost bed is finished; when a layer of spent tan, 30 inches in thickness, is 

 placed over the whole, and the operation may be said to commence. In six or eight 

 days the tan begins to ferment and evolve heat ; and this goes on increasing for some 

 weeks, when it gradually diminishes, and at the end of about three months the whole 

 has become cool, and the stack is fit to be taken down. When examined, the pots, 

 which formerly contained vinegar, will now be found to be quite empty, or to hold a 

 little water merely, but no acetic acid ; the leaden crates will be discovered to have 

 increased sensibly in bulk, to have become coated with a thick and dense incrustation 

 of white lead, and in somo places even to have become altogether converted into this 

 substance ; whilst the tan, having lost its fermentative quality, is now useless, except 

 as fuel. 



The successive beds constituting the entire stack are next carefully removed, so as 

 to obtain the white lead with the least possible admixture of the tan ; and as a portion 

 of this substance always adheres to the crates, these are washed in a kind of wear or 

 trough, by which the whole of the tan is thoroughly separated. When this is seen 

 to be complete, the corroded part of the plate or ' white lead ' is detached from the 

 uncorroded or ' blue lead,' either by means of rollers or with a mallet. The blue lead 

 is weighed, and, for the most part, remelted and again cast into crates ; whilst the 

 white lead is first crushed, and afterwards ground in water into a fine powder, when 



