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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 above, or by employing 13 lbs. of chrome alum in place of alumina sulphate 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 red lakes 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.

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CHAPTER X.

ASSAY AND ANALYSIS OF PIGMENTS.

BESIDES the analysis required for ascertaining 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 called 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 or 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 some 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 light, such 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

depth of hue or colour, yet different in tone. For instance, in two vermilionettes, the standard sample may have a bluish tone, while the sample compared with it may have a redder tone. Again, two samples of chrome-yellow when compared together in this way may show differences of tone; one may be a greenishyellow, while the other may have an orange tone. These differences in tone are of quite as much importance as differences of depth of hue, as occasionally they will have an influence on the use of a pigment; thus, an orange-toned chrome-yellow does not mix with Prussian blue to make greens as well as a greenish toned chrome-yellow does.

Absolute Method of Assaying Hue.—An absolute method of assaying pigments for hue may be founded on the use of an instrument devised by Captain Abney, named by him a "colour patch" apparatus. It is described in his book on Colour Measurement and Mixture. This method, while of some interest from a scientific point of view, is, however, scarcely one which will come into practical use in a colour shop, owing to its rather complex construction and to its requiring powers of experimenting beyond those of colour-makers generally. However, a brief description of the process and apparatus may be useful. The colour patch apparatus consists, first, of a spectroscope, with which a spectrum of the light from an arc electric lamp can be formed on the screen of a camera; by substituting a slide having a narrow slit in it for this screen and passing the light which comes through this slit through a lens, a patch of coloured light can be obtained on a screen placed behind the camera; the colour of this patch will depend upon the position of the slit in relation to the spectrum which falls upon it and will necessarily be monochromatic. The arc electric lamp is preferred as the source of light, because it can be more depended upon than any other kind of light for uniformity in amount and quality, which feature is of importance where light measurements have to be made.

Another part of the apparatus consists of an arrangement by which a disc of coloured card can be rotated; the same apparatus also carries a larger pair of overlapping black and white discs, the amount of overlapping being capable of variation at will. This is placed so that the colour patch falls partly upon the coloured disc and partly upon the black and white discs; these are rotated, and the slit of the colour patch apparatus moved along the spectrum until a point is reached when the luminosity of the patch on the two discs is equal. Then a note is made of the position of the slit, as given on a scale attached to the colour patch apparatus,

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