alizarine* with 14 galls. of water; then add 10 ozs. of alumina sulphate previously dissolved in water, and 2 ozs. of calcium acetate dissolved in water; boil the whole together for about an hour; then add 10 ozs. of soda crystals dissolved in water in small quantities at a time, at intervals long enough to allow of the subsidence of the effervescence thus set up. The whole mass is now boiled for about an hour, then allowed to stand for 24 hours, filtered, washed, and dried. This makes a dark red lake of good body and staining power. upon the kind of alizarine used.

The shade or tint will depend Excess of soda crystals should

be avoided, as it causes the lake to be of a dark colour.

2. Dark Red Alizarine Lake.-Diffuse 100 lbs. of barytes through 50 gallons of water, add 20 lbs. of alizarine, 10 lbs. of alumina sulphate, and 2 parts of calcium acetate; stir well together, and then allow to stand for two or three hours, stirring at intervals to keep the ingredients well mixed. Heat slowly, so as to take about two hours to reach the boiling point, and, at intervals, add portions of a solution of 10 lbs. of soda crystals. Much of the beauty of the resulting lake depends on the care exercised during this stage of the process; too rapid heating, and too rapid addition of the soda crystals, has a tendency to cause the shade of the lake to be darker than it should be. The lake obtained by this process is of a fine ruby red colour. After it is formed, as described above, it is finished in the usual way.

3. Alizarine Lake.-Diffuse 100 lbs. of barytes through 50 gallons of water, add 10 lbs. of oleine or Turkey-red oil, and boil for one hour; allow to stand for twenty-four hours, stirring up at intervals, add 20 lbs. of alumina acetate (12° Tw.), and 2 lbs. of calcium acetate, stir all well together, and allow to stand for two days; then boil well for two hours, adding at the same time 5 lbs. of soda crystals. When the lake has formed, filter off, wash, and dry.

4. Alizarine Red Lake.—6 lbs. of alumina sulphate are dissolved in water, and to this solution one of 1 lb. of calcium chloride is added; a precipitate of calcium sulphate is obtained, but, before this has had time to settle out, a solution of 44 lbs. of soda crystals is added; the precipitate of alumina and calcium sulphate thus obtained is collected on a filter and washed. It is

* Nearly all the so-called alizarine colours are sold in the form of a paste, containing about 20 per cent. of actual colouring matter. The reason for this form is that, as a rule, these dyestuffs are insoluble in water, and it has been found by practical experience that if sold in a dry form that the dyestuff does not mix well with water, and that they thus give rise to uneven dyeing, while the paste form mixes very well with water, and uneven dyeing rarely occurs.

then introduced into a solution containing 3 lbs. of alizarine, 1 lb. of Turkey-red oil, and 14 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 1-1014 (20-3° Tw.), specific gravity, which has been previously mixed with 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.

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 flavazol, 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.

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.

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.

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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 pigmentsthe 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 ways— comparatively 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