adulterants any further; some notes bearing on this point will be found in the descriptions of each individual pigment, while reference should be made to works on quantitative chemical analysis, such as that of Prof. Sexton, for fuller details. By multiplying the weight of the carbonic acid by 5.05 the amount of lead oxide with which it is combined can be calculated; the two amounts added together give the quantity of lead carbonate in the white lead. Deducting the amount of lead oxide combined with the carbonic oxide from the total present, and multiplying this difference by 0.077, gives the amount of water combined with it to form lead hydroxide, and the two amounts added together gives the amount of the latter body present. Paste White Lead can be quantitatively examined as follows:-Two grammes are treated with strong nitric acid at a gentle heat; this converts all the oil into an insoluble greasy matter, of which no account need be taken; the process then becomes identical with that for dry white lead. Should it be desired to ascertain the amount of the oil, then 10 grammes must be weighed into a filter paper and placed in a Soxhlet or other form of fat-extractor and the oil extracted by means of petroleum ether; the ethereal solution is then run into a weighed glass, the ether evaporated off and the oil weighed. If a fatextractor is not available it will suffice to agitate the white lead with some petroleum ether in a beaker, allowing the pigment to settle, pouring off the liquid into a weighed glass, again pouring on more ether, and again allowing the white lead to settle; the ether is poured off into the glass, or the mass may be filtered. Finally, the ether is evaporated off as before. It is sometimes recommended to burn off the oil from the white lead in a crucible, but this course is not so satisfactory as treating it with petroleum ether, as not only is the oil burnt off, but the white lead is decomposed; whereas in the ether method the lead is left in its original form for further examination if necessary. SULPHATE OF LEAD PIGMENTS. Sulphate of lead, Pb S O,, forms the basis of a number of white pigments which are made on a large scale and sold under a variety of names, such as Patent White Lead, Non-poisonous White Lead, Sublimed White Lead, &c. These do not consist entirely of lead sulphate but contain other bodies, such as zinc oxide, barytes, magnesia, &c., in varying quantities; they are made by different methods, and most of those now sold are produced by patented processes. Lead sulphate can be made by dissolving lead in strong sulphuric acid, the action is, however, but slight and does not form a commercial method of manufacture. It is mostly made by adding sulphuric acid to solutions of either lead acetate or lead nitrate; perhaps the best method is that where lead acetate is used. Metallic lead is granulated by melting and pouring the molten lead into cold water; the object of granulating is to obtain the metal in such a form as to expose a large surface to the action of acids and air. The granulated lead is placed in a large tub fitted with a closed steam coil so that the action of the acid may be facilitated by heat, if necessary. Acetic acid diluted with its own volume of water is poured on to the lead; the action of the acid is at first rather sluggish, but by allowing the action to go on for about 12 hours, then running off the acid and leaving the lead in the tub without any liquor, a certain amount of oxidation goes on, resulting in the formation of a deposit of oxide on the lead, so that when the acid is again admitted to the lead, the acid acts more rapidly, and a strong solution of lead acetate is soon obtained; this action is facilitated by a gentle heating of the contents of the tub. From the tub the solution is run into a large wooden vessel and to it is added strong sulphuric acid in small quantities at a time with constant stirring; lead sulphate is thereby precipitated. Care is taken that the amount of sulphuric acid used is not sufficient to throw down all the lead, but that some of the latter is left in solution. The lead sulphate is allowed to settle, and then the clear supernatant liquor is pumped back again into the lead tub, for it contains all the acetic acid; for, as will be seen from the equation— acetic acid is reproduced as the result of the reaction which goes on between the lead acetate and the sulphuric acid, and this acetic acid can be used over again for preparing fresh solution of lead for precipitation; thus a comparatively small quantity of acetic acid may be used to prepare a large quantity of lead sulphate; theoretically speaking, beyond the first charge no more acetic acid is required, but practically, there is a small loss which requires to be restored by new additions of acid from time to time. The sulphate of lead which is precipitated is washed with water, drained on a filter, and dried; after which it is ready for use as a pigment, or can be combined with other white pigments if desired. = The acetic acid used should be as pure as possible; the usual commercial variety of a strength of about 1.050 10° to 12° Twaddell is sufficiently good for the purpose; this contains about 35 per cent. of actual acetic acid, although some makes contain more. 100 lbs. of it will dissolve about 60 lbs. of lead supposing all the acetic acid exerts its solvent power, but in practice this never or but rarely happens, nor indeed is it necessary that it should. From 20 to 28 lbs. of sulphuric acid will be required to precipitate the lead dissolved by this quantity of acetic acid. These figures are only approximate and are given simply as guides to actual practice. Lead sulphate has the formula Pb S O, and contains Lead oxide, Pb O, and of metallic lead 68.31 per cent. 73.55 per cent. It is a white, somewhat crystalline, and very heavy powder, its specific gravity being about 6.3. It is only slightly soluble in water, insoluble in dilute acids and in alcohol, but soluble in solutions of ammoniacal salts, and in strong sulphuric acid; from the latter solution it is precipitated on the addition of water. Boiling concentrated hydrochloric acid dissolves it, and crystals of lead chloride fall down as the solution cools. As a pigment it is not satisfactory, its crystalline character reduces its body and covering power, causing it often to work streaky or livery under the brush; this defect can be remedied to some extent by grinding it. It is not readily acted upon by sulphuretted hydrogen, and is, therefore, more permanent than white lead under exposure to air. Owing to its solubility being less it is free from the poisonous character of white lead, and, therefore, white pigments containing it are often sold as poisonous white leads." Its colour or hue is a good white, but slightly yellower in tone than white lead and about equal to barytes. It is used as a diluent in the manufacture of pale chromes. non Many have been the attempts to make lead sulphate the base of commercial white leads, the records of these are to be found in the publications of the Patent Office, where they lie buried in an almost unknown condition, and it would really be most instructive for colour-makers and would-be inventors if they would peruse these records and see what has been done in the past. A few of these inventions may be briefly noticed here, and a fuller description given of such as are at present used on a large scale. Richardson, in 1839, patented the use of the sulphate only as a pigment; in 1853, Carter & Marriott prepared a chlorosulphate of lead made by treating 100 lbs. of litharge with 25 lbs. of salt, and the product so obtained with 5 lbs. of sulphuric acid. Woods, in 1866, took out a patent for the preparation of a white pigment from lead fume, which is a mixture of lead, lead oxide, and lead sulphate; this he treated with hydrochloric acid thus forming a chloro-sulphate; or he calcined the fume in a furnace, whereby it was converted into a white mixture of lead oxide and lead sulphate, which was then treated with hydrochloric acid as before. Groves, in 1826, treated galena with potassium nitrate and sulphuric acid, whereby the lead sulphide was converted into lead sulphate, which was dried and sold as a pigment. In 1866, Messrs. Bell & Fell patented the use and preparation of what they called a sub-sulphate of lead, prepared by precipitating a solution of lead nitrate with sulphuric acid and then boiling with an alkali. SUBLIMED WHITE LEAD. This product is the invention of G. T. Lewis, and was first patented in 1879. About 1870 a vein of ore containing both lead and zinc, being a mixture of galena and blende, was discovered in America; this was smelted for lead, which, owing to it being different in properties from ordinary lead, was distinguished as "Bartlett lead;" the presence of zinc in lead ores is very detrimental, as the zinc cannot wholly be removed from the lead, while it imparts to it properties which, by causing it to become hard and brittle, prevent its application to those uses for which lead is of great service. The process of white-lead making now to be described was to some extent devised to utilise this lead-zinc ore. The manufacture of sublimed lead depends upon two facts, which also are the principles that underlie some of the ordinary processes of lead smelting-first, when lead ore (galena, lead sulphide) is heated in a furnace with access of air it undergoes oxidation, partly to lead oxide, Pb O, partly to lead sulphate, Pb SO4, the amount of the oxidation depending upon the amount of air which comes in contact with the ore; if this is small then the sulphate is mostly formed. The temperature also has some influence on the result; in fact, it is probably the chief factor in the process. If low, then the sulphate is chiefly formed; if high, this is likely to be decomposed into oxide and sulphur dioxide. Secondly, during the operation of lead smelting a large proportion of the lead is carried off in the form of "fume," which collects in large chambers or flues built for the purpose. This lead-fume is a mixture of metallic lead, lead oxide, lead sulphate; if there be any zinc in the ore it will be mostly found in this fume. It has frequently been noticed that the composition of the fume varies from time to time, according to the conditions under which the furnaces are being worked. Lewis proposes to utilise the two principles laid down and, by carrying out the process in such a manner that the great bulk of the fume. produced consists of lead sulphate, to prepare a white pigment from any lead ore. This is effected in the Lewis and other processes based on the same principle by causing the operation to be carried out in a blast furnace. The preparation of sublimed white lead takes place in two stages; the first may be called the fume stage, in which the lead ore is transformed into fume of more or less complex composition; the second stage may be called the colouring stage, in which the pigment is made of a proper degree of white ness. In the first stage a furnace is used resembling the Wetherill zinc furnace. Four of these are placed back to back and side by side, and is hence not unlike four American lead-smelting hearths. The furnace is worked from two sides, known as the fronts; a water back, containing tuyeres, is also placed between the bench of furnaces; above the water back is an air chamber divided into two compartments by a partition, so arranged that the air which enters into one of these compartments passes to one front of the bench of furnaces, while the air in the other passes to the other front. This air is heated by passing through the hot walls of the furnaces, and by suitable means is sent as a powerful blast through the tuyeres into the hearth of the furnace, and through any ore which may be on the hearth. The ore is placed on the hearth of the furnace with the fuel necessary to effect its melting, and air is sent through it; part of the lead is reduced to the metallic state, and is collected by being allowed to flow out of the furnace into a suitable receptacle. Some of the lead is blown by the blast of air into fume, and is more or less oxidised to oxide and sulphate during that operation; this fume |
