A New Process For Determining the Comparative Value of White Lead

Scientific American 19, 8.5.1869

By Prof. C. F. Chandler

To determine the comparative value of different samples of white lead, it has generally been considered necessary to submit them to chemical analysis; an operation which can only be performed by a skillful chemist, and which, if carefully executed, requires much time, and involves, consequently, considerable expense.

It is therefore extremely important that a simple test should be devised, one so easy of execution that it may be applied by all persons interested in paints. Such a test was recently suggested to me by a person who has had many years of experience, in the white lead industry, and who desired me to submit his plan to a careful examination to test its reliability. This I have done, and the results are so satisfactory that I desire to place them before your readers, that those interested in the purchase and use of white lead may avail themselves of the test.


Properties of white lead.

The great value of white lead as a pigment depends upon its opacity, or as painters express it, its body," or "covering power." Pure white lead differs in opacity to a limited extent, according to the process by which it is made; that prepared by the Dutch method leaving the greatest covering power. The commercial varieties of white lead differ, however, to a far greater extent, owing to the extensive adulteration which is now practiced; sulphate of baryta, or barytes, a very heavy mineral, much cheaper than white lead, being the chief adulterant employed. The objection to the barytes is its transparency or want of body; it is not opaque, and consequently it does not cover well. A much larger quantity of the adulterated paint is required to produce the desired effect.

There is another objection to barytes, it has no affinity for the oil, and, consequently, when the adulterated paint is applied to surfaces which are exposed to the weather, as on the outside of houses, the oil quickly disappears, leaving the pigment loosely attached, and ready to be washed off by the rain.

Such paint rubs off readily upon our clothes when we come in contact with it.

White lead is a compound of carbonate and hydrated oxide of lead, which unites with the oil to some extent, producing a hard surface, which resists for a much longer tune the action of the elements. Oxide of zinc has a similar property.

From these statements the importance of a simple test of the quality of white lead will be readily seen.


The test.

The value of white lead depends upon its opacity; the more opaque it is, the more completely will it conceal a dark color. The test consists, therefore, in mixing a definite quantity of a dark pigment with a definite quantity of the white lead, spreading the mixture on a suitable surface, and noting the tint produced. In my experiments 100 grains of the pigment white lead, ground in oil, as it comes from the mill, were mixed with onehalf grain of Eddy's best lampblack, and four drops of boiled linseed oil. These substances were thoroughly incorporated, and then spread upon sheets of window glass, 6 by 12 inches, with a steel spatula. A few of my experiments will best illustrate the test. Pure carbonate of lead and pure sulphate of baryta, both ground in oil, were employed; onehalf grain of Eddy's dry lampblack, and four drops of boiled linseed oil, were mixed with—

	White lead.	Barytes.	Color produced.
1st	100 grains	0 grains	light drab.
2d.	95 "	5 "	slightly darker drab.
3d.	90 "	10 "	"
4th.	66 2/3 "	33 1/3 "	"
5th.	50 "	50 "	"
6th.	33 1/3 "	66 2/3	"
7th.	0 "	100 "	black.

The specimens were submitted to six different persons successively, and all agreed in pronouncing them as above recorded. Five per cent may therefore be considered the limit of the accuracy of this test.


Oxide of zinc

As oxide of zinc is often mixed with the white lead, experiments were made to determine the effect of this pigment upon the tints. The best American zinc white was employed. One-half grain of lampblack, and four drops of boiled linseed oil were mixed with—

	White lead.	Oxide of zinc.	Barytes.	Gave color.
1st....	33.33	33.33	33.33	lightblexish drab.
2d.....	50	25	25	darker bluish drab.
3d.....	50	12.50	37.50	still darker bluish drab.
4th...	50	6.25	43.75	"

The tint of the mixtures containing oxide of zinc was quite different from that obtained without the addition of this substance; while with white lead alone, or white lead and barytes, the color was a pure drab; the presence of six and a, quarter per cent of oxide of zinc was sufficient to communicate a very decided bluish tint. I think as little as two percent of oxide of zinc would make itself apparent in this way. This difference in tint makes it a little difficult to decide between two samples of adulterated white lead when one does, and the other does not contain oxide of zinc, as between tints so different in character, it is not easy to decide which is the darkest. In doubtful cases, however, this difficulty may be overcome by adding to both samples the same weighed quantity of oxide of zinc, say ten grains to each.

The colors communicated by the lampblack will then be of the same bluish tint, differing merely in intensity.

The only practical objection to this method of testing will arise from the difficulty of weighing half a grain of lampblack with sufficient exactness.

In my experiments, a chemical balance, which shows the thousandth part of a grain, was employed. The practical painter, however, will have no difficulty in applying this test with sufficient accuracy, if he will weigh out in ordinary scales, say 100 ounces (6 1/4 lbs.) of each sample to be compared, adding to each half an ounce of dry lampblack and an equal quantity to each sample of boiled linseed oil. After mixing the lead, black, and oil together, very thoroughly, spread each sample on glass, wood, or other smooth surface, as nearly alike as possible, when the difference in depth of color, produced by the black, will determine the comparative value or body of each sample.

The sample most discolored will have the least body, aired that least discolored the most body.

Another very simple test for determining the comparative value of any white paint ground in oil, was suggested by the same person — the correctness of which I have fully demonstrated — namely, weigh out, say 100 grains of each paint to be compared, add three drops of linseed oil to each, and spread them with a steel spatula on sheets of glass, 6 by 12 inches, as nearly as possible in the same manner. Place the samples thus prepared between yourself and the light, and you will have no difficulty in deciding which is most opaque. The sample which has obsured the light the most, or appears the darkest when held between yourself and the light, must have the greatest body or covering capacity.


Conclusion.

After having made a great number of experiments with these tests, I am satisfied that when they are applied with ordinary intelligence, they will not fail to determine the comparative value of the different grades of white pigments. would advise ,every person who makes use of these tests to begin by preparing a series of standard plates for comparison selecting samples of paint obtained from the most reliable makers.