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 number 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 draught board. 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.
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, lithopone, or any colour of which the quality of body is of importance. In some colours it is of little moment.
Tinting or Staining Strength.
—We have already explained at length how greatly the tinting strength of different colours or stains varies. Any painter can test the tinting strength of any colour himself in a very simple manner. All that is necessary is to have a pair of apothecaries’ 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 colour and weigh one grain and add that to one of the little piles of white, then weigh a grain of the standard colour 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 colour is ground in oil a little difference in the method must be observed, the reason being that one colour might be ground much thicker 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 colour in oil—that is the standard and the colour 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 colour 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 colour but also gives valuable information as to the tone, etc. 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.
If the reader will turn to the samples of colour issued in this work he will see a number of colours given in their full strength, and also when reduced with certain parts of white, as marked upon the sheets. The colours used in the preparation of this sheet were of average quality, and it will prove interesting no doubt to the student to mix the colour he has been in the habit of using in the same proportion with white and to note whether the results come out above or below those shown by our samples.
The Permanence of Colours.
—It must be admitted that it is very disappointing to a painter to find, after taking pains to produce the exact colour required, that it flies or fades after a little exposure to the weather. The tests for the permanence of a colour when exposed to light are simple enough and are to mix a little of the colours 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 colours is entirely different from that of oil colours. Some very useful experiments were made several years ago by Capt. Abney on the permanence of water colours, and these were published in the form of a blue book. As far as pigments are concerned, we consider that yellow ochres, siennas, umber, Vandyke brown, and the earth colours 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.