« VorigeDoorgaan »
then introduced into a solution containing 3 lbs. of alizarine, 1 lb. of Turkey-red oil, and 1} oz. of tannic acid, and the mixture heated for half an hour to about 70° C., when it will be found that the alumina has taken up all the alizarine, and become dyed thereby. It is now boiled for one hour longer, and then finished in the usual way.
5. Alizarine Red Lake.—Muller Jacobs has patented, in Germany, the following process for making an alizarine lake: 50 grammes of alizarine oil are dissolved in 1,400 cc. of water, 15 grammes of alizarine, and 0.2 grammes of tannic acid; the mixture is heated to boiling, when 60 cc. of a solution of alumina sulphate of 1.101620:3° Tw.), specific gravity, which has been previously mixed wit 22 per cent. of soda crystals, are added. The lake soon forms, especially on boiling for some time; it contains some oil which can be extracted with ether. The use of such a large proportion of oil is objectionable, as it makes the resulting lake very greasy, and prevents its use for certain purposes. Moreover, it is not practicable on a commercial scale, owing to the cost involved in extracting this excess of oil.
6. Alizarine Scarlet Lake. -A very fine lake is made in the following manner :-6 lbs. of alumina sulphate are dissolved in 20 gallons of water; to this is added the solution of 1 lb. of calcium chloride in one gallon of water, and immediately after a solution of 44 lbs. of soda ash dissolved in 10 gallons of water. cipitate of alumina is obtained, mixed with some sulphate of calcium ; this precipitate is collected and well washed. It is now diffused through 10 to 15 gallons of water, and there is added 3 lbs. of alizarine, 1 lb. of Turkey-red oil, and 1} oz. tannic acid, the mixture being heated to from 160° to 165° F. and kept at that heat for about half an hour, when 1 lb. more of Turkey-red oil is added. Then the whole mass is boiled for one hour, after which the lake is ready for washing and drying. It is important that the sulphate of alumina used be free from iron, and that, during the process of making, the materials be kept free from contact with that metal, or the colour of the lake will be deteriorated.
In making alizarine lakes it is important to use an alumina sulphate which is free from iron, as this latter ingredient has a great and deteriorating influence upon the colour of the resulting lake; very small traces of iron are sufficient to give a brown hue to the lake.
The methods of making lakes from alizarine, just described, are equally applicable to the preparation of lakes from the other alizarine dyestuffs.
SCARLET LAKES may be made from a mixture of alizarine and alizarine orange.
YELLOW LAKES can be made from alizarine yellow, galloflavine, gambine yellow, and flavasol, by any of the above processes, using alumina salts as precipitants. Generally a very satisfactory lake, of good colour, can be readily obtained from any of the dyestuffs named.
ORANGE LAKE can be made from alizarine orange, or by using a mixture of alizarine and one of the yellows just named.
MAROON or CLARET LAKES can be obtained from alizarine, by substituting acetate of chrome for acetate of alumina, in process No. 3 abov by employing 13 lbs. of chrome alum in place of alumina ulphate in process No. 6 above.
BLUE LAKES can be obtained from alizarine blue, alizarine cyanine, or chrome blue, by using chrome acetate as a precipitating agent, and the process described in No. 3 of alizarine lakes, or by substituting 13 lbs. of chrome alum for the sulphate of alumina in process No. 6 above.
GREEN LAKES can be made from nitrosoresorcin and gambine by using iron sulphate as the precipitating agent ; the lakes from gambine are rather brighter than those from the resorcin product. Dark green lakes can also be obtained from coerulein by using chrome acetate as the precipitant.
VIOLET LAKES can be made from chrome violet by using chrome acetate as the precipitant; or from gallein and gallocyanine, by using either chrome or alumina salts as precipitants.
BROWN LAKES can be made from anthracene brown with chrome acetate as the precipitant.
It has not been thought needful to give full details of the method for converting all the alizarine colours into lakes, as the process or processes, and the proportions of materials used are very similar to those which are used for making alizarine lakes, and these have been fully dealt with. Any intelligent colour-maker can easily apply the proportions and process for making an alizarine red to making an alizarine blue-lake.
The great merit which distinguishes the lakes made from the alizarine group of dyestuffs is that of being permanent; they resist a considerable amount of exposure to air and light without becoming faded or dull, they can, therefore, be used for decorative or artistic purposes where some degree of permanence is essential; still, even in this property, they are by no means equal to such pigments as vermilion or yellow-ochre or chromeyellow.
Lake pigments prepared from various coal-tar dyes by the methods described in the preceding pages are now largely used in the manufacture of letterpress and lithographic printing inks, and, on the whole, they are found to answer very well for this purpose, working well in conjunction with the other constituents of the ink. The lake pigments are sold under a variety of names and brands or marks, varying with different makers, and affording no clue to their composition or to the dye from which they are made. Where brilliant coloured inks are required without any great fastness, the Eosine and Erythrosine red lakes can be used for red inks; when fastness is desired, then the Alizarine and Azo rea hakes should be used. The methyl and other violets give lakes of great brilliance, but are not fast to light. Fast greens of any brilliance are not known, although the lake pigment from Naphthol green B can be used for making a green ink of some degree of fastness to light.
ASSAY AND ANALYSIS OF PIGMENTS.
BESIDES the analysis required scertaining the chemical purity of a sample of a pigment, it is also necessary, with many of the pigments, to make an examination or assay for other properties—such as colour or hue, brilliancy, colouring power, covering power or body, durability, fineness, and what may be
lled miscibility. In some pigments they are fairly constant in degree, while in others—such as the chromes, ochres, umbers, blacks, &c.—they are very variable, hence due precaution should be taken, both by purchasers and makers, that every lot of pigments is uniform, both in quality and intensity.
1. COLOUR HUE.-The tint or shade of a pigment is a matter of the greatest importance.
The terms, as used by colourists, are, however, rather confusing; thus
persons consider tints to mean the standard colour mixed with white so as to obtain lighter colours ; shades they consider to be those produced by mixing the standard colour with black; while other persons use these terms as if they were synonymous, and speak of the shade or tint of a colour without any reference to a standard. This, perhaps, is the custom more particularly in the dyeing trades. Under these circumstances it will be best to use a term which is free from any liability of confusion, viz., "hue." The hue of a colour may be defined as the optical effect produced on our colour-sense by a pigment.
The hue of a pigment is a variable quantity ; in some cases, such as vermilion and antimony orange, where the chemical composition is a definite one, and does not vary with different makers and batches, the hue only varies within small limits. In other cases, such as the chrome-yellows, Brunswick greens, and many others, where the composition is liable to vary with different makers, and even with different batches of the same maker-as also the ochres, umbers, and other natural pigments--the hue varies very much. In such cases, the terms-pale,
medium, dark, &c.—are arbitrary distinctions which are by no means uniformly indicative of the same precise intensity of hue. Hence, in all such pigments, special examination is required in order to determine what the actual intensity is.
The assaying of a pigment for hue can be done in two wayscomparatively or absolutely; the former is the one usually adopted.
Comparative Method of Assaying Pigments. In this method the hue of a sample is simply compared with that of a standard sample and the results stated in terins of this standard. For this purpose a sample of the best quality is selected, and a fair quantity of it is placed in a bottle. Colours which are acted on by ligat, sich as the chrome-yellows, Brunswick-greens, and others, are placed in orange or amber coloured bottles to protect them from such action; while white pigments, black pigments, ochres, umbers, and the permanent colours generally, are kept in ordinary bottles. These samples form the standards.
The method of assaying the hue of a pigment by comparison with a standard sample is simple, but experience and a good eye for colour are essential requisites; some persons can detect very small differences in the hues of pigments, while others are deficient in this respect, and consequently do not make good assayers of hue. The colour sense can be improved by cultivation.
A sheet of black paper for pale colours, or of white paper for dark colours, is placed in front of a good window and in diffused light, this being the best for assaying hue. The paper must have a dead surface, as a bright one interferes with the observations too much. A small heap of the standard colour is placed on the paper by means of a palette knife, and beside it a similar heap of the colour to be compared; then, by a gentle but steady downward pressure with the knife, the surfaces of the two heaps are flattened in such a way that a distinct line separates the two colours; if the knife is moved at all laterally, the two heaps of colours are partially amalgamated, and a perfect comparison thereby precluded. The observer now carefully compares the two heaps of colour; this should be done in several ways, viz., by looking down on them, by holding the paper on a level with the eyes and looking sideways at the colours, &c.; by this means and with a little experience the differences in the hue of colours can be readily observed.
There are two points in connection with the hue of a colour which can thus be compared with a standard colour : 1st, depth; 2nd, tone. The two samples being compared may be equal in