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The best plan for assaying the covering power and body in pigments is to place 2 grammes of the standard sample and of the pigment to be compared with it on a block porcelain tile, and to add oil to each sufficient to allow them to flow; the oil and pigment are thoroughly incorporated by means of a palette knife, and then each is spread over the plate in a layer, making each layer of paint of as uniform a thickness as possible. That sample which, when thus made into a paint and spread over the tile, covers the most surface has the most covering power.
The following may be taken as an example of the method :2 grammes of samples of the following white pigments were weighed out and mixed sufficient raw linseed oil as would enable them to be rubbed ver the surfaces of glass plates, then the extent of surface covered by each was observed, the result being as follows :White lead (Dutch process),
760 White Lead Co.'s white lead,
1,144 Orr's patent zinc white,
1,500 Zinc white,
1,152 Lead sulphate,
640 Gardner's white lead, :
From these it is seen that Orr's white has the greatest covering power, and is followed, in order, by gypsum, zinc white, barytes, the White Lead Co.'s product, Gardner's white lead, white lead, and lead sulphate.
The quality of a pigment must not be judged solely from the point of view of its covering or spreading power, as gypsum would then be considered one of the best of pigments for oil, while, as a matter of fact, it is one of the worst; there must be taken into consideration also the colour or staining power of the pigment and its body. The colour or staining power of the pigments in question is judged by how far they retain their whiteness on being mixed with oil; in this respect the samples named above ranged themselves in the following order :Gardner's white lead, Orr's white, zinc white, lead sulphate, white lead, White Lead Co.'s white, barytes, and gypsum; the latter two were rather deficient in this respect.
By body is meant the opacity of the coat of paint which is obtained in carrying out the process described above. If the painting has been done on a black ground, and one appears to obscure the black more than another, that one will have the most body. If the paint has been spread on glass, that which
appears darkest by transmitted light is the best. Judged by this standard the eight white pigments named above appear in the following order :-Orr's white, Gardner's white lead, White Lead Co.'s white, white lead, zinc white, lead sulphate, barytes ; gypsum was very transparent.
To that sample which has the most body a small additional quantity of oil is added, and the body of the mixture again compared with the other sample; if it is still the best more oil is added, and the process repeated until both samples appear to have the same covering power. Now, it may be assumed, without much error, that the covering power or body of the two samples is in proportion to the quantities of oil used to mix with them; that sample taking the most oil having the most body. Thus, of two samples of barytes one took 3 grammes of oil, and the other 3.25 grammes; taking the last as the standard or 100, the former had only 3.25 : 3 :: 100 = 92.3 or 7.7 per cent. less body than the standard sample. Or, to put it in another way, 3 : 3.25 : : 100 = 108:3; that is, 100 lbs. of the standard sample will cover as much surface as 108.3 lbs. of the other, weaker sample.
It is by comparing both covering power and body that a true idea of the value of a pigment for painting can be ascertained.
DURABILITY or PERMANENCE.—Durability is one of the most important properties a pigment can possess,
it depends the fact whether it will ever come into extensive use as a pigment, especially for artists' use, where permanence is one of the most essential things a picture must possess. Until recently our knowledge regarding the permanence of pigments, or, what is the same thing, their power of resisting exposure to light and air, was empirical and unreliable; but recent researches on the subject have quite altered its character.
Of those colours which have been shown to be permanent, the mineral pigments-ochres, umbers, sienna, Vandyke_brown, barytes, Chinese white, siennas, ultramarine, vermilion, Prussian blue, and some others—are the most important, and can safely be used on work which is required to have great permanence.
What is frequently sold under the names given to old pigments is, however, not always what it ought to be, being more or less adulterated with an inferior pigment; it is, therefore, advisable to test a sample, not only for purity, but also for durability, as the adulterants are frequently anything but permanent.
Of late years many new pigments have been placed on the market, which have been made from coal-tar colours; unfortun
ately, many of these, although of brilliant hue, are far from being permanent, and will not resist any lengthened exposure to light and air. It is desirable that a user of these pigments should make some experiments as to their durability. There is a great deal of difference in this respect among these coal-tar pigments. Some are as permanent as can be wished; others, again, are very fugitive. Then, again, the method of using has more influence on the durability of these pigments than it has on that of the older pigments; for some which are rather fugitive, when used as water-colours, will resist a fair amount of exposure when used as oil-colours.
Probably the simplest method (which is a very good one) of testing the durability of colours, is to provide a sheet of unglazed cardboard ; that known as Bristol board will do very well, 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 3 x 2 or 2 x 2 inches.
A little of the colour 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 colour can be written either underneath the patch of colour 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-colour and as an oil-colour 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 changes 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 colours will be changed in a few weeks' exposure, other colours require months of exposure to produce
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 dilute sulphuric acid, the action of sulphuretted hydrogen on the colours 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.
Another plan for testing durability of pigments is to prepare a sheet with washes of the colour in the following manner :A strip of paper or card is taken, and a light wash is spread over it; the whole length is now divided into ten parts; when dry, another wash is given over nine of the parts; this is allowed to dry, and a wash given over eight of the parts ; and this is continued until ten washes have been given. There will then be a strip with washes of colour from pale to dark. This strip is now cut into two, lengthways-one-half being exposed to the light, the other placed in a drawer or damp place. When a few days have elapsed, the two strips can be compared and the rate of fading noted. Thus a brilliant green lake gave the following results :
Original standard 10.
3. Testing pigments for durability is a very long operation, and it is no wonder that there have been few systematic researches on this subject. The most exhaustive and systematic experiments on the permanence of pigments which have ever been made are those made by Captain Abney and Dr. W.J. Russell, at the request of the Science and Art Department; these extended over a period of two years, and the results were published in the form of a Blue Book, entitled—“Report on the Action of Light on Water Colours.' This report must be consulted for details as to the method of testing adopted, &c. ; but the following will give some idea of the methods and results of these researches.
The experiments were carried out as follows :- A sheet of Whatman paper of good quality was covered with washes of the pigment to be tested in such a manner as to form a series of eight tints, varying from pale to dark. From this sheet, strips, 8 inches long by 2 inches wide, and containing all the tints were cut, and two of them placed in a glass tube, 2 feet long and i inch in diameter, open at both ends, and bent over at the upper end in the form of a hook, in order to prevent the admission of dirt; the tube was then hung in such a place as to receive as much sunlight as possible. It may be mentioned that one of the strips of paper was covered with oil-cloth so that it could not be acted on by light, although otherwise it was subjected to the same influences as the other strip. Similar tubes were filled with dry air, moist air, moist hydrogen, while one set had the air withdrawn so that the strips were exposed
in a vacuum. The experiments lasted from May, 1886, to March, 1888. The results of these experiments are given in the following table, which has been compiled from the tables given in the Report.
Prussian blue fades when exposed to light, but on placing the faded colour in a dark place the colour comes back again.
It is evident from the above experiments that moisture has a material influence on the durability of pigments; colours which fade in moist air are permanent in dry air; then, again, colours are more permanent in an atmosphere of moist hydrogen gas and in a vacuum than in air; it is evident, therefore, that the three elements of destruction which cause colours to fade are light (which may be called the determining cause), oxygen, and water. But light alone has little effect. Hence it may be concluded that when a colour is kept under conditions where moisture and air can have little action it will be permanent; so that it should always be kept under such conditions if possible.
Mr. A. P. Laurie* has investigated the effect of light on lake pigments prepared from coal-tar colours. He finds that lakes prepared from alizarine and its derivatives are very durable. Galloflavine lake on alumina base is quite permanent, the only change being that it becomes somewhat more brownish in tone. Alizarine blue lake fades slightly. Alizarine brown lake prepared from alizarine on a copper and alumina base is quite permanent. Alizarine green lake is permanent. Alizarine orange lake fades slightly. Lakes made from methyl blue, methyl violet, and magenta were fugitive, becoming nearly colourless in a short time. A lake made from methylene blue was slightly more permanent. Scarlet lake and geranium lake (which are made from eosine) fade and soon show a spotty appearance. A crimson lake made from erythrine fades but slightly, an unexpected result, considering the connection of erythrine with the eosines. Green pigments made from brilliant green are very fugitive. It may be pointed out that the method of making the lakes has a material influence on durability of pigments. Then, again, the mode of using will also have some influence. Used as water colours the lakes will be more fugitive than if used as oil colours. Thus geranium (eosine) lake when used as a water colour will fade in about 5 to 6 weeks, or even less, according to method of making; when used as an oil colour it is by no means so fugitive, and only becomes duller after twelve months' exposure. * Journal Society of Chemical Industry, March, 1895.