Manufacturer and builder 8, 1869
The value of white lead depends upon its color, body, and purity. The first property is not always a guarantee of purity. White lead made after the French process is generally of a brighter shade than that obtained by any other method, though all the different kinds may be quite pure. Kremser white, on the other hand, is whiter than the pigonent prepared by the Dutch method, while, from the purity of the metal employed, the English white lead, which is manufactuired after the latter method, at least equals the Kremser white. The rose tint, sometimes observed in white lead, is attributed to a trace of silver, and not to copper, es was proved by Mr. Baker. With respect to the body of the pigment, it is generally admitted that the best Corinthian or Kremser white is superior to all others.
Pure white lead will go farther, or cover a larger surface, than that which is impure; but white lead mixed with about ten per cent of sulphate of baryta, is generally whiter than the pure pigment. It is generally second qualities, that is, white lead spoiled by the tan-bark of the beds, that are mixed with baryta. In order to ascertain the body of two or several sorts, it is best to pursue the following course: Weigh fifteen grains of each sort, which should not be ground in oil, but dry. Mix each kind iatimately with one grain and a half of oil, add the necessary amount of turpentine, then weigh equal portions of the paints and spread them, each separately, on planed boards of equal surfaces. This trial affords a fair test, not only in regard to the power of covering, but also as to the purity. Instead of being spread on the board by itself, the dry paint may also be mixed with definite, proportions of lamp-black or other pigments.
The most common admixture to white lead is sad phate of baryta, and it is safe to say that thousands of Comas are used yearly in this country for this purpose. Some time ago, the daily press of New-York, in articles relating to the adulteration of drugs, dwelt. largely on the "fraud" practiced by the admixture of baryta to white lead. Now, it may be well to say that there is perhaps no painter in the Union who, in purchasing white lead, does not know that the cheaper kinds are mixed with baryta, and no dealer in paints, who mikes it a secret that it is the custom to mix time paint in question with heavy spar. Among European, immufacturers, it is even customary to stamp upon the. packages their different qualities, and time percentage ot baryta, and it would be well if the manufacturers in this country would adopt the same plan. Though baryta lessens the adhesiveness of the pigment, it cheapens it on the one hand, and on the other it makes it brighter. Moreover, a mixture of white lead with baryta absorbs less oil than the pure color.
In many cases, it may be well to know the percentage of baryta in white lead. Weigh, for this purpose, one hundred grains of the dry color, put it in a tumbler; add, first, some water, and then so much nitrie acid, until all effervescence ceases. The latter should be added gradually. If no residue remains, the white lead inay be considered free of baryta, as well as of plaster of Paris and clay, which latter adulterations, however, are of but rare occurrence, they depriving the pigment of its brightness. The weight of the residue, when lixiviated with water, and dried, indicates the percentage of the admixture.
With respect to the discovery of other adulterations, we have arranged the following table, which needs no further explanation:
- - - - - - - - - | Muriatic Acid. | Soda Lye. | On being heat'd.
Zinc white. | Effects solution without effervescence. | Effects solution. | Appears yellow, but white upon cooling.
Bone Ashes. | Effects solution, but only when heated, and with effervescence in the beginning. | Leaves them unchanged. |When laid on moist turmeric paper, when cold, produces a brown color.
Chalk. | Dissolves it with considerable evolution of gas. | Leaves it unchanged. | Produces a brown color on turmeric paper.