The pure product dissolves completely in dilute nitric acid, as well as in potash and in soda lye. When exposed to the sulphuretted hydrogen or moistened with ammonium hydrosulphide it turns brown or black, whereby it is distinguished from zinc-white. When heated with the access of air it yields its carbonic acid, and at 572° Fahrenheit passes into lead oxide and finally into minium. When digested under pressure with carbonated water for some time, the water may contain 0.22 drachm of lead per quart. The difference in covering power is due to the form, size, density and composition of the smallest particles. The white lead obtained by the French or precipitation process is looser, of a coarser grain, and possesses less covering power than the product obtained by the Dutch or German process, which is denser, of a finer grain, and never crystalline, and, though of a greater specific gravity, requires less oil.

When exposed to the 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 sulphurous gases, white lead turns brown or black through the formation of the black sulphide of lead. The production of this body is more likely to occur in large towns, where great quantities of gas are used for lighting and other purposes, which usually contains some sulphuretted hydrogen or other sulphur compounds.

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 causing the formation of the black sulphide, and thus bringing about the discoloration of the pigment or paint.

In commerce white lead is found in two forms, as a heavy white powder, having a specific gravity of about 6.47 and weighing about 180 pounds to the cubic foot, or as a paste containing about 8 per cent of the linseed oil. To make the latter, the dry white lead is first mixed in a mixing mill with about 8 to 9 per cent of its weight of raw linseed oil; it is then run through a grinding mill several times to ensure a thorough mixture of the oil and white lead. This form is much liked by painters, it being more readily miscible with oil and turpentine to make it into paint.

In order to obtain a cheaper product, white lead is frequently mixed with barytes which is distinguished by its white color and great specific gravity. The mixture is often effected in fixed proportions and for certain varieties of white lead, which are known by special names or numbers, remains unchanged. Thus Venetian white consists of equal parts white lead and barytes, Hamburg white of 2 parts barytes and 1 part white lead, Dutch white with up to 75 per cent barytes. The so-called Kremnitz white is a pure white lead. It is produced by placing trays containing a paste made of litharge and either acetic acid or lead acetate upon shelves in a chamber built of brick or wood. When the chamber is filled carbonic acid gas is sent into it and is absorbed by the lead oxide present in the paste.

White Lead

The nitric acid solution obtained as before is boiled nearly to dryness, and if a precipitate forms a little water is added, then dilute sulphuric acid in small quantity, and the boiling continued for some minutes to expel the last trace of nitric acid. An excess of sulphuric acid is then added to precipitate the whole of the lead and lime as sulphates, which appear as a white heavy powder. The beaker and its contents are then cooled by immersion in cold water. When cool, double the bulk of alcohol is added, and the whole allowed to stand for some time - over night if possible. It is then filtered and washed with alcohol until the washings are no longer acid, dried, ignited in a porcelain capsule, and weighed as above. From the weight obtained deduct .05 grain, the amount of ash contained in an ordinary Swedish filter paper, the remainder multiplied by five for the percentage of sulphate of lead, and (if chalk is present) this again by .852 for the percentage of white lead. If the sample contains chalk, the percentage of white lead must not be estimated until the percentage of chalk is determined. The percentage of chalk is converted into its equivalent of sulphate of lime.

This latter deducted from the percentage of sulphates of lead and lime, and the remainder multiplied by .825 for the percentage of white lead.

White Lead

The presence of lead should be ascertained. This is indicated by the black precipitate given with sulphate of ammonia. It may be best ascertained by boiling the nitric acid solution to expel the free nitric acid, adding dilute sulphuric acid to the clear solution, dissolving the white precipitate of sulphate of lead thus formed in ammoniac acetate, and adding potassic ehromate to this solution. A heavy yellow precipitate (chromate of lead) forms when lead is present.