Thénard was the first to point out a process for the manufacture of white lead, which was applied later by Mr. Roard in a large establishment near Paris, the products of which are known under the name of Clichy White Lead. The chemical reactions on which this process is based are as follows: -,,If a solution of basic acetate of lead, sometimes called Extract of Saturn, be treated with carbonic acid, part of the oxide of the salt is converted into carbonate of lead, and the remainder becomes neutral acetate. By adding a new proportion of litharge or oxide of lead to the solution of neutral acetate, this becomes basic again by the solution of the oxide. We see therefore, that these reactions permit of the manufacture of white lead by a continuous and economical production of basic acetate.,Without thoroughly considering all the manipulations of this process we shall indicate the mode of operation.,A solution of basic acetate of lead, marking from16 o to 18 o Bé., is made by boiling a solution of neutral acetate (sugar of lead) with very finely powdered oxide of lead (litharge). There is no difficulty in this operation.,When the litharge has become dissolved, and the ,[79] basic solution is well saturated, the liquor is decanted from the impurities in the litharge or the acetate, into a closed vessel. Then the carbonic acid is introduced, which gas may be produced by several methods, such as the calcination of chalk or the combustion of carbon. At all events the gas should be previously well washed, so as not to add impurities to the white lead.,As soon as it is ascertained that all the basic excess of oxide of lead is transformed into carbonate, the liquors are allowed to settle. The carbonate falls to the bottom, and the supernatent solution of neutral acetate is decanted to be boiled again with oxide of lead, and become, as we have said, basic acetate.,There is however, at each operation a certain loss of neutral acetate, which must be replaced and rendered as small as possible by careful manipulation.,The settled carbonate of lead is first washed with a small proportion of water, which is added to the decanted solution of acetate. The washing is then continued with larger quantities of water, which are thrown away, since they are too poor in acetate. The paste of white lead is put into pots, and dried in the stove room.,This Clichy white lead is in impalpable powder and as white as snow; but with those of Krems and Holland, it has less density and body, that is, covers less.,The manufacture of white lead, by the Thénard process, has been established at Portillon, near Tours, by M. M. Pallu and Delaunay, with a perfect under standing of its theory. Thanks to a report made in 1836 to the 'Société d'Encouragemnet' by MM. A. ,[80] Chevalier, F. Barral, and Gaultier de Claubry, we are enabled to explain this manufacture.,,Preparation of the oxide of lead.-- The works of Portillon, as stated by the above delegates, contain five furnaces with double fire-places, four of which are in constant operation, and use bituminous coal as fuel. The furnaces are built directly in the rock, and calcine 1800 kilogrammes of lead at each operation. The leads employed bear the best brands of Andalusia and of England, and are analyzed at the works so that the best only may be received. The bed of each furnace is built with fire-brick holding as little silica as possible. The shape is nearly circular, about 3,40 metres in diameter, and with two lateral fire-places. It is hollow, so as to retain the molten metal. The vault is surbased, and 60 centimetres (o,60 metre) is the greatest distance between the bed and the ceiling of the vault.,During the heating, the gases of the combustion escape through an opening or hood, placed in front of the aperture used for charging the metal or extracting the oxide. This hood connects with an upper furnace where the transformation of the oxide of lead into red lead takes place.,Twelve hours are required for oxidizing 1500 kilogrammes of lead; but the oxide still contains a large proportion of metal or blue lead, which is separated and returns to the calcining furnaces. Half of the oxide produced is for the manufacture of white lead, and the other half for that of red lead.,,Manufacture of white lead.- The oxide of lead intended for the preparation of white lead is moistened with water, and spread over a wooden floor above two saturating pans lined with copper. These pans are,[81] supplied with stirrers composed of a wooden frame with bronze projections, which reach to abot 1 or 2 centimetres from the bottom. One of the pans is raised above the other, so that the excess of liquid in the upper one may run by a spout into the lower one. The latter pan, at the middle of its height, is connected with a duplex bronze pump.,The two pans are filled with water rendered acid by about one-fortieth of pure pyroligneous acid marking 30 acetimetric degrees. While the stirrers are in motion, a certain proportion of damp oxide of lead is poured in, and becomes dissolved in part. The pump is then set to work, and forces the solution into three large tanks, lined with copper, placed in an upper story, and which connect with each other. These tanks have stirrers like those of the saturating pans, and which are kept in motion during the whole operation.,Besides the pipes for conducting the liquors, these three apparatus are provided with pipes and inverted gutters, perforated with numerous small holes, through which a continuous stream of cabonic acid escapes. The average specific gravity of the solution is 5 o Bé. ,During this operation the pump takes from the saturating pans the solution of basic acetate, and carries it into the precipitating tanks where it is brought into contact with the carbonic acid. The white lead is immediately formed, and the liquid, which must still retain a certain proportion of basic acetate, passes into the settling tanks where the white lead becomes deposited. The liquor then goes back in the saturating tanks, and the operation begins anew. It is, as we see, a system of circulation in which machinery performs most of the work, and hand labor is reduced to a minimum.,[82] After a certain lenght of time, the settling tank is sufficiently filled with white lead, that is, when this material reaches the level of the overflow. The solution is then made to pass into other vessels and the white lead is washed in washing tanks, which are providedwith wooden horizontal stirrers having a rotary motion.,The settled white lead is covered with twice its volume of pure water and stirred. Three washings take place, and at each, the material is allowed to deposit, and the water above is decanted.,The white lead is then conducted into large basins built of porous stones, which absorb part of its dampness. After a few days, the material is divided into blocks which are still wuite wet, and which are pounded by wooden vertical stamps falling into a wooden trough inclined from the front backwards.,This stamping reders fluid the white lead which appeared half dry before. The stuff is then put into small movable boxes, holding about 400 kilogrammes, and which are carried to the drying room. It is sufficient, for filling the pots, to open and close the trap-doors at the bottom of these boxes.,When the white lead is to be sold in powder, the stamped paste is run into wooden frames, which are set upon a brick platform heated underneath. When dry it is put upon a distributor similar to that used for red lead, but larger, and which projects it upon a sheet-iron ventilator having four wings. The ventilator is inclosed within cast-iron plates, and is followed by a rectangulat trough of the same metal, about 1 metre long. At the top end of the trough there is a sheet-iron pipe 35 centimetres in diameter, 8 metres long, and nearly vertical, which communicates at its,[83] upper end with a large sheet-iron chamber, to the bottom of which are fixed two funnels or hoppers closed by lateral sliding plates. Below the opening of the vertical pipe, and in the cast-iron trough, there is a cast-iron hexagonal prism which rotates and pulverizes the coarse portions of white lead which have not the ventilator. The white lead deposited in the iron chamber above is in impalpable powder.,The distributor, like that for red lead, has an aspirator, so there is no danger of dust being inhaled by the men.,The above operations apply to the preparation of white lead in lumps and in powder, as is generally required by the trade. However, for several years past, part of the white lead has been ground in oil, which is a hygienic progress, since numerous cases of lead colic have been observed among those who grind white lead in the shops of color dealers.,The grinding of white lead in oil is done at the works of Portillon as follows: the stamped and still damp material is introduced into a kneading machine with the given proportion of oil, and soon transformed into dough, which is removed through a side opening. The paste is then ground between metallic rollers heated by steam, and the water expelled. After another passage through an ordinary grinding apparatus, the paste is put into zinc cans soldered or closed tight.,The carbonic acid used for the manufacture of white lead, and which passes through the solution of subacetate, is produced by the combustion of cheap charcoal dust, cemented into bricks by means of a small quantity of clay. the gas is aspirated from the ,[84] combustion furnace by means of a series of inverted drums plunging into water, and which act as pumps.,Here are also a few data on a modification of this process, practised in England, and the description of which is due to Mr. Preisser.,The lead is smelted in a cast-iron kettle with a spout, which delivers it upon the bed of a large reverbaratory furnace, in which air is constantly injected by a ventilator. The lead becomes divided, offers a large surface to the air, and runs into a channel the lateral sides of which are perforated with small holes. The lead is oxidized, and the litharge escapes through small apertures which may be opened at the same time. the silver, if any, remains at the bottom of the channel. This mode of preparing litharge is very easy and rapid.,The litharge is then finely divided, and, after being moistened with 1 per cent. of acetate of lead dissolved in water, is put into horizontal troughs, closed on top and communicating one with the other. A stream of impure carbonic acid, produced by the combustion of coke in a reverbaratory furnace, with air projected by two powerful centrifugal ventilators, passes all the while through the layers of oxide. The pressure exerted by the ventilators is sufficient to overcome the resistance of the layers of litharge. The gases are cooled in pipes immersed in water.,In order to bring all the particles of oxide into contact with the carbonic acid, and aid the combination , a system of rakes, moved by machinery, keeps the mass constantly stirred.,The white lead obtained by this process is good for painting, and is perfectly white. It covers well, and is ,[85] preferred in England to that prepared in the wet way, which contains crystalline particles.,,