Cyclopedia of Painting/Color Testing
COLOR TESTING.
Although to accurately test the quality of a color requires somewhat elaborate experiments, both chemical and practical, yet there is no reason why the painter should not determine with a sufficient degree of accuracy for his purpose the quality of the color he uses. Indeed, if this was done more generally, much of the adulterated trash would be driven from the market, and none would rejoice more at such a result than the color manufacturers themselves. The manufacturers assert that they are as desirous that the trade should use pure colors as the painters can possibly be. Even the biggest houses produce cheap grades of colors, and this they do, as a rule, almost under a protest and simply because they are compelled by painters demanding colors for certain low prices, far below that at which it would be possible to produce the pure article. Make careful comparison between pure colors and those being used. At the same time, compare the prices and see which is cheaper to use. If even they come out at the same price, remember that by using a pure color all the benefit of the purity of tone so necessary for the execution of good work is gained.
The first thing to be done in testing any paint material is to have a standard. There must be no doubt about this. Unless we have in each case something with which to compare the particular sample of color that is being examined, we shall have no useful information concerning it. Take, therefore, good decorators' colors of well known make. If necessary purchase small tubes of the best colors, such as are put up for artists' use. This will be rather a severe trial, but still it will afford a standard. Having such samples and going through the tests we are about to describe, the painter can, after some amount of trouble, arrive at results which are almost as accurate as those which could be deduced by a chemist. An expert on this question some years ago summarized the characteristics of colors which should be considered in making the examination, under the following heads:
1. Purity of the material.
2. Purity of the tone, brilliancy; richness, which indicates the amount of care in selection.
3. Fineness of grinding or preparation; this means the degree of the division of the particles and upon the completeness of such division will depend
4. Its spreading capacity.
5. Its body. This applies, of course, only to opaque or semi-opaque colors. Body is opacity, and means capacity to conceal the surface to which the paint is applied, and must not be confused with spreading. It is an inherent quality.
6. Its staining power or tinting strength with white or colors.
7. The quality of purity of the tint with white.
8. If a paste color, the consistency of the paste.
9. Transparency of transparent colors and the quality of the transparency.
10. The permanency of the color.
It will be observed that all of these tests will not necessarily be applied to every color. For instance, a transparent color would be tested for its transparency, but certainly not for its body. The one condition is the converse of the other.
Purity of the Material. This is sometimes of considerable importance, as in the case of white lead, whilst in others, for example the earth colors, it can hardly be said that there is a standard of purity. As a rule a knowledge of practical chemistry is necessary in order to determine whether a sample of paint or color is pure or not.
The purity of white lead, however, can readily be ascertained by the painter who possesses no chemical knowledge by aid of a blow-pipe. Take a piece of flat charcoal and cut out a hollow space from it into which place a small piece of white lead to be tested, about the size of a pea. Now direct the flame of a blow-pipe upon it, using an ordinary candle or a Bunsen burner, taking care that the blue portion of the flame bears upon the lead. Keep up a steady blow for a few minutes and the white lead will be converted into metallic lead, which will show in the form of a bright silver-like button. If the lead is adulterated the blowing will only have the result of making it appear like a cinder. To conduct this experiment successfully requires a little practice with the blow-pipe in order to obtain a steady flame.
Another method of testing is to place a little white lead in a crucible and place this on a hot fire, when, if genuine, it will be converted into metallic lead.
Purity of Tone. Some remarks on this subject will be given under the heads of the various groups of colors. Speaking generally, the richness of brilliancy of tone is easily discernible by placing the sample to be tested side by side with another of well known excellence. In siennas, ochres and umbers the selection of crude material by which the richness of tone is assured is of great importance.
Fineness of Grinding. The method of testing the fineness of a pigment usually employed by the painter is to rub a little on the finger nail, but this is a crude and unreliable method. If the pigment is dry and it is desired to compare it for fineness with a similar pigment or white lead, the following is as good a plan as any:
Take two tall vertical glass jars, place in them an equal quantity of turpentine and then take a small quantity of the white lead to be tested. Place it in one jar, and an equal quantity of the pigment with which it is to be compared in the other; thoroughly stir up both and then note the time it takes the sample to settle. If graduated marks are made to the two jars the observations will be taken more readily.
Another test is to weigh out equal quantities of the two leads, and then to take a very small quantity of the same color, say black, and add to each sample, thoroughly mixing. The lead that is the lightest in color will be the finest. The explanation of this is somewhat interesting. Suppose that we have a number of cubes of white lead each measuring one inch side. This will give us six superficial inches to be colored. Now suppose that we break up these inch cubes into half inch cubes, which will give eight half inch cubes to each inch cube. Now as each half inch cube has six faces measuring half an inch by half an inch, it has a superficial surface of three square inches, and as there are eight of the half inch cubes, there are twenty-four superficial inches to be colored against six in the inch cubes. It will be seen, therefore, that by increasing the fineness of a pigment a greater surface is presented to be colored, and hence more color is required.
Another test for fineness is to paint different samples thinned in turpentine on plate glass; when dry the two specimens may be compared and the difference of fineness between them will soon be apparent.
Still another test, and one frequently used by painters, is to place a quantity of the color ground in oil that is to be tested upon a level surface such as a piece of glass, and to run the blade of a spatula or palette knife over it, and then over another sample with which it is to be compared, noticing carefully the difference in appearance of the two samples. By these means the presence of grit is discovered.
Spreading Capacity or Covering Power. The spreading capacity of pigments and their body are very nearly related, although of two equal in body one may possess greater covering power or spreading capacity than the other. A practical method of testing covering power is to mix a small quantity of a standard paint and an exactly similar quantity of the pigment to be tested, taking care to use precisely the same amount of oil and thinners in each case. Then, taking a clean brush for each of the paints, paint a door, or other surface that has been primed, on two panels side by side, continuing to paint till all the pigment has been in each case used up. The one that goes farthest has the greater covering power.
In comparing the two it will be well to notice whether the body is equal in both cases, as one may go farther but not cover so well.
Body. The word body, as applied to pigments, is almost synonymous with opaqueness. It is the most important property of a pigment, and it is because white lead possesses the quality in an eminent degree that it is so much valued.
Body is sometimes called covering power, but this term is a little misleading, as some may suppose it to relate to the spreading capacity of the pigment.
If two different white leads ground in oil to an equal consistency are applied to different panels of a door, primed in the same manner, the one of the two leads that possesses the better body will be shown by it hiding the grain of the wood better. Some white leads, especially those that are manufactured by the new processes, lack this important quality of body, and three coats will only cover the work as well as about two of old process white lead.
There are numbers of methods of practically testing the body of pigments, among the simplest being the following:
Prime and paint a board with alternate black and white squares, like a chess or draughtboard. Take a sample of a pigment, similar to that to be tested, of which the body is known to be good, and paint a wide strip across the chess board; then paint a smaller strip of the pigment to be tested. When both strips are dry, by comparing them one can tell almost at a glance which has the better body, the superior pigment covering or hiding the black squares better than the other. A second coat may afterward be applied to each over a portion of the strip, if desired.
It is important to notice that in all cases of practically testing paints the results are obtained by comparisons being made, and hence it is necessary in every case to have a standard with which to compare the sample to be tested as has already been explained.
The test of painting over squares of black and white may be varied by using stripes instead. The test answers equally well for white lead, zinc or any color of which the quality of body is of importance. In some colors it is of little moment.
Tinting or Staining Strength. Any painter can test the tinting strength of any color himself in a very simple manner. All that is necessary is to have a pair of druggists' scales, some blotting paper, a palette knife, some pieces of glass or a flat piece of marble and some pieces of waxed paper. First weigh out say eighty grains of dry white lead or dry zinc. Any other white will answer equally well. Place these eighty grains on one side of the glass and the second eighty grains on the other. Now take the dry color and weigh one grain and add that to one of the little piles of white, then weigh a grain of the standard color and add that to the other pile. Now add to each pile a few drops of oil, taking care that the number of drops is the same in each case. With the palette knife thoroughly mix until no streaks can be seen and the mixture is perfectly uniform. Then by comparing the two the difference in tinting strength will at once be apparent. The same result would have been produced had ordinary white lead ground in oil been used instead of dry lead or zinc. If the color is ground in oil a little difference in the method must be observed, the reason being that one color might be ground much thinner than the other, in other words might contain much more oil than the other, and hence if equal weights of each were compared the result would be misleading. Take then each color in oil, that is the standard and the color with which it is to be compared, place on a small quantity of blotting paper and allow it to remain a few minutes so that the oil may be extracted. If it is thought necessary the sample can be washed with benzine, but for painters' purposes the extraction of the oil by means of blotting paper is sufficient for the purpose. The two samples having remained on the blotting paper for a short time one grain of each is weighed out separately on little pieces of wax paper, this being used so that the color shall not stick to the scale. Then each grain is mixed separately with the white and the result compared as before. It is not too much to say that every painter should be prepared to make this test, because it informs him not only as to the tinting strength of the color, but also gives valuable information as to the tone. Of course the quantities may be varied if necessary, and a larger amount used instead of the single grains. It need hardly be pointed out that scrupulous cleanliness is necessary for successfully carrying out this test. The palette knife must be wiped between each operation and every care taken to do justice to both samples.
The Permanence of Colors. It must be admitted that it is very disappointing to a painter to find, after taking pains to produce the exact color required, that it flies or fades after a little exposure to the weather. The tests for the permanence of a color when exposed to light are simple enough, and are to mix a little of the colors to be tested in oil and to spread them on different slips of paper, cut the paper in half, number each half with corresponding figures or letters, expose one half to a strong light for as long as may be deemed desirable and put the other half away into a safe place where the light does not penetrate. Waxed paper is the best, as it will not absorb the thinners, or, better still, glass may be used, this being cut across with a diamond after the paint has been applied. It need hardly be said that the permanence of water colors is entirely different from that of oil colors. As far as pigments are concerned, yellow ochres, siennas, umber, Vandyke brown and the earth colors generally are permanent, as are Venetian red, Indian red, chrome yellow and lemon yellow. Ultramarine, Prussian blue and vermilion are also permanent or nearly so.
Colors Fast to Light. Some colors fly or fade very quickly, while others are perfectly permanent. The following is a list of the principal permanent colors under ordinary conditions: Yellow ochre, light red, Indian red, umber, cadmium yellow, Prussian blue, cobalt blue, Vandyke brown, red ochre, sienna, red oxide, Venetian red, vermilion, ultramarine, chrome green, lamp black, and other black pigments.
Probably the simplest method of testing the durability of colors is to provide a sheet of unglazed cardboard; that known as Bristol board will do very well. It must have so slight an absorbent property that if any coat of paint is placed on the surface it will remain there, and not soak into the substance of the cardboard. This sheet of board is ruled into squares or rectangles measuring about 3x3 inches, or 2x2 inches.
A little of the color to be tested is ground up with a little gum water into a smooth paste, and a portion of one of the ruled spaces on the cardboard painted with it. It is advisable to rule and prepare two sheets at the same time. The name of the color can be written either underneath the patch of color in the square, or in a corresponding position on the back of the card. It is also advisable to grind a little of the pigment with oil, so that the relative durability as a water color and as an oil color can be tested.
One of the prepared cards is hung in a place where it is exposed to as much sunlight and air as possible, while the other card is placed in a drawer away from any such influence. After a week or two of exposure the cards can be compared to see if any chancres have occurred; they can then be replaced in their respective positions, and from time to time are compared together. Any change which may have been brought about by the action of sunlight and air on the exposed card will be observable; some colors will be changed in a few weeks' exposure, other colors require months of exposure to produce any effect.
By placing a card painted in the manner described, with different pigments in a closed cupboard, in which is placed a vessel containing some ferrous sulphide and diluted sulphuric acid, the action of sulphuretted hydrogen on the colors can be tested; if any are affected by this test it is certain that they will be similarly affected when exposed to the action of impure air.