2 molecule of lead hydroxide, Pb H, O; this is the composition of the best make of Dutch white lead, which has all the good properties of white lead in the highest degree of perfection. It is scarcely necessary to point out that as white lead is made by many processes it must necessarily vary in composition; indeed the white leads yielded by the same process do not always have the same composition, as is evinced by the analyses given here and on p. 19; these have been collected from a variety of sources. 99.74 99.83 99-84 99.83 100 23 100.0 99.53 from which the composition of the white leads can be calculated to be Lead carbonate, Pb C 03, 63.35 72.15 91.21 71.93 87:42 68:36 70-87 Lead hydroxide, Pb H2 O2, 36·14 27.68 8.21 27.88 12:33 31.64 28.66 Moisture, No. 1. English make. No. 2. English make. No. 3. Krems white. this sample is deficient in No. 4. German make. medium quality. No. 5. German make. but deficient in body. No. 6. German make. No. 7. German make. quality fair. 0.25 0.42 0.02 0.48 Made by the Dutch process; of very good quality. Precipitated by sodium carbonate; it is only of Precipitated by carbonic acid gas; of good colour, Made by the Dutch process; a good white. Made by precipitation with carbonic acid gas; The second form in which lead is sold is that of a paste with linseed oil. To make this, the dry white lead, above described, is first mixed in a mixing mill, with about 8 to 9 per cent. of its weight of raw linseed oil; then it is run through a grinding mill several times, to ensure a thorough mixture of the oil and white lead. This form is much favoured by painters, as it is more readily miscible with oil and turps to make into paint. The following are two analyses of ground white lead: In making this ground white lead only the best raw linseed oil should be used; boiled oil is not admissible, as there would be too much tendency for the lead to become a hard dry mass before it could be used. It is customary to keep ground white lead under water to prevent it drying up too rapidly. Besides its use by painters, this form of white lead is also largely used for other purposes, as a cement for gas-piping, &c. White lead is soluble in dilute nitric acid, and in acetic acid with effervescence, due to the evolution of carbonic acid gas. It is also soluble in boiling dilute hydrochloric acid with effervescence; on cooling the solution fine transparent needle-shaped crystals of lead chloride separate out. Boiling with sulphuric acid decomposes the white lead, insoluble lead sulphate being formed. Solutions of white lead in acids give white precipitates of lead sulphate with sulphuric acid; of lead chloride with hydrochloric acid, soluble on boiling in water; and of lead carbonate with sodium carbonate. Neutral solutions of white lead give a yellow precipitate of lead chromate with potassium bichromate, and a black precipitate of lead sulphide with sulphuretted hydrogen and solutions of sulphides. As a pigment white lead possesses all the good qualities desired by a painter-viz., good colour, body or covering power, and permanency. It is distinguished from all other pigments by the ease with which it mixes with oil and by forming a paint which readily flows from the brush, whereas most pigments, as for instance, barytes, tend to work what the painter calls slimy or livery, and streaky; white lead does not exhibit this property, but flows freely and evenly from the brush. This feature is due to the lead hydroxide in the white lead combining with some of the oil and forming a lead soap which, dissolving in the rest of the oil used in the preparation of the paint, forms a kind of varnish; this varnish takes up the lead carbonate to which is due the body or covering power of the pigment. Sometimes this chemical combination between the lead hydroxide and the oil extends to the lead carbonate and then the white lead loses its opacity and becomes more or less transparent or horny; the conditions most favourable to the production of this change, which is of rare occurrence, are not properly known. This fact of the white lead forming a chemical combination with the oil is well known to colour makers, who have endeavoured, by the addition of basic bodies, to bring about a similar action in the case of other white pigments, such as zinc white and barytes, but so far without any great success. When exposed to light and air white lead is fairly permanent and will resist exposure to normal conditions for a great length of time; on the other hand, when exposed to the fumes of sulphuretted hydrogen and other sulphureous gases, white lead turns brown or black through the formation of the black sulphide or lead. The production of this body is more likely to occur in large towns, such as London and Manchester, where large quantities of gas are used for lighting and other purposes, which usually contains some sulphuretted hydrogen or other sulphur compounds. By oxidation this black sulphide can be transformed into the white sulphate of lead; the only agent which can be safely used for this purpose in restoring paintings which have become discoloured is peroxide of hydrogen, but the action of this body is very slow and is much interfered with by the oil which is present. White lead can be mixed with all pigments except those which, like cadmium yellow, ultramarine or king's yellow, contain sulphur; such pigments sooner or later cause the formation of the black sulphide and thus bring about the discolouration of the pigment or paint. White lead is frequently adulterated, the pigment most used for this purpose being barytes, because it more nearly approaches white lead in specific gravity, and is, on that account, not so readily detected; whereas the use of whiting or gypsum would soon be detected on account of the great difference in weight between genuine white lead and white lead adulterated with them. This adulteration of white lead is exceedingly common and is well understood by makers and dealers; in fact it is the custom for makers to send out several qualities of commercial white lead distinguished as "genuine," "No. 1," "No. 2," and so on; the degree of adulteration being regulated by the price which is paid for the product. The question whether this is adulteration is a matter of opinion; if by adulteration one means the admixture of cheap products with dear products with a view of deceiving purchasers of the latter, then the admixture of barytes with white lead under the conditions named is not adulteration, for the purchaser knows what he is buying, and only pays a fair price for such mixed white leads. The author is, however, of opinion that this custom of mixing barytes with white lead would be much better honoured by the breach than the observance of it. ASSAY AND ANALYSIS OF WHITE LEAD. White lead may be assayed for colour and covering power by the usual methods (see Chapter X.). Dry White Lead.-The purity of this pigment is ascertained by dissolving some of the lead in pure dilute nitric acid (1 acid, 2 water); strong nitric acid does not dissolve white lead owing to the insolubility of the lead nitrate which is formed in the acid; the ordinary commercial nitric acid contains sulphuric acid, which would lead to the formation of the insoluble sulphate of lead, the production of which might lead to the condemnation of a pure sample. On adding dilute sulphuric acid to the solution, after diluting it with water and filtering off the precipitate of lead sulphate thus obtained, no further precipitate should be obtained on successively adding ammonia, ammonium sulphide, and ammonium oxalate to the filtrate. A white precipitate with ammonium sulphide would indicate the presence of zinc white, which is a rare thing to find with white lead; a white precipitate with ammonium oxalate would indicate the presence of whiting. The insoluble residue, if any, will consist most probably of barytes, as other adulterants (for reasons already pointed out) are rarely used; still any lead sulphate, china clay, gypsum, or strontium sulphate which may be used would also be left as an insoluble residue on treating white lead with dilute nitric acid. To distinguish these bodies, boil the residue in hydrochloric acid and place the solution on one side to cool; if crystals of lead chloride separate out and the solution gives a white precipitate with barium chloride, then lead sulphate is present. The hydrochloric acid solution should be diluted with water and sulphuretted hydrogen passed through it; the black precipitate of lead sulphide which may be obtained can be disregarded; this is filtered off and the filtrate boiled for some time to concentrate it and to drive off the sulphuretted hygrogen it contains. Then ammonia is added, when a white precipitate of alumina may be obtained indicating the presence of china clay; this is filtered off, and to the filtrate is added ammonium carbonate, which will precipitate any calcium that may have been added, in the form of gypsum or whiting. A little of the insoluble residue from the hydrochloric acid should be held on a piece of platinum wire in the lower part of a Bunsen flame when, if it contains barytes, the flame will be coloured green; if strontium sulphate is present a crimson flame will be obtained. This test is not always easy to carry out, but with a little care the coloured flames can be obtained, and they are good proof of the presence of the pigments named. A quantitative analysis of white lead may be made as follows: Weigh out 2 grammes and dissolve them in a beaker with the smallest possible quantity of pure dilute nitric acid, remove the insoluble matter by filtering, wash the residue well with warm water, adding the first wash waters to the filtrate, then dry the residue, place the filter paper and its contents in a weighed crucible and burn the paper; when completely burnt allow the crucible to cool in a desiccator and then weigh it. From the weight so obtained deduct the weight of the crucible and of the filter-paper ash, the difference is the weight of the insoluble residue. To the filtrate add dilute sulphuric acid and a little alcohol, filter off the precipitate of lead sulphate which is obtained, wash it dry and burn it in a crucible as before. By multiplying the weight of the lead sulphate so obtained by 0·73554 the weight of lead oxide in the white lead can be found. The carbonic acid can be ascertained by treating 2 grammes of the white lead with nitric acid in a Schrotter's or other form of apparatus for the estimation of carbonic acid. The water may be determined by taking the difference between the amounts of lead oxide and carbonic acid thus found and 100. Hygroscopic water can be ascertained by heating 2 grammes in an oven at 110° to 120° C. until no further loss of weight occurs. If the white lead be adulterated with barytes, lead sulphate, china clay or some of the other insoluble white pigments, these will be left behind as an insoluble residue on treatment with nitric acid; their amount is ascertained by filtering off, washing, drying, and burning the residue in a weighed crucible in the usual way. Soluble adulterants like whiting, strontium carbonate, barium carbonate, and magnesite will be dissolved; if the presence of these is suspected, to the filtrate should be added more dilute sulphuric acid, which will precipitate the lead and barium; this precipitate can be filtered off, the two can be separated by boiling with hydrochloric acid, which dissolves the lead sulphate but not the barium sulphate. To the filtrate ammonia and ammonium oxalate are added; this precipitates the calcium and the strontium, while the magnesite (if present) will remain in solution, and can be precipitated by sodium phosphate. It is not necessary to describe in detail the methods of separating these |