In the manufacture of pigments from dye-woods or other organic substances, the value of the raw material is in proportion to the amount of colouring matter it contains, other things being equal. It is specially desirable to estimate accurately the colouring matter in expensive materials such as indigo and cochineal.

There are a number of methods which permit an accurate estimation of the indigo blue in indigo. One good process is founded upon the decomposition of indigo blue by chlorine, when the colour of the solution changes from blue to yellow. Since a definite amount of chlorine is required to decompose indigo blue, from the quantity of chlorine required by a sample of indigo, its content in indigo blue can be ascertained.

Whilst the percentage of indigo blue contained in indigo can be found with tolerable accuracy, though by a rather elaborate process requiring special apparatus, there is no convenient method for examining the other organic colour materials by which their content in active constituents can be readily found. In practice a process is particularly valuable which requires little time and no complicated apparatus. Colouring materials can be rapidly tested by a physical process which requires little time and an inexpensive apparatus. Under similar conditions the extract of a dye-wood is deeper in colour in proportion to the colouring matter it contains. If therefore the intensity of the colour of the extract can be accurately measured, there is no difficulty in drawing a certain conclusion as to the amount of colouring matter in the raw material.

The Colorimeter is the apparatus adapted for the purpose in question. There are many forms of colorimeter. The instrument devised by Dubosq is distinguished by simplicity and accuracy of results before other apparatus of similar construction. Dubosq’s colorimeter consists of the following parts (Fig. 43): two glass cylinders, C and C₁, the bottoms of which must be perfectly plane both inside and outside (the accuracy of the results depends upon this), stand upon a sheet of plate glass. Two glass cylinders of smaller diameter, T and T₁, are suspended in C and C₁. The bottoms of these cylinders must also be quite plane. It would be very expensive to make glass cylinders of this kind in one piece. The same result is obtained by providing each cylinder with a metal ring upon which screws another ring in which is cemented a circular piece of plate glass.

Light should only reach the eye of the observer in a direction parallel with the axis of the cylinders. C and C₁ are therefore blackened on the outside. The inner cylinders, T and T₁, are fastened to racks moving vertically. The distance through which the cylinder is moved is measured by a scale on one of the racks. Above the cylinders T and T₁ are Fresnel’s prisms. Below C and C₁ is a mirror, S, which can be adjusted to throw light vertically upwards. Beams of light pass through the plate glass and the bottoms of the cylinders C and T, C₁ and T₁, unrefracted, they are then deviated by the Fresnel’s prisms so that the observer looking down through the telescope, F, has a circular field of view, one half of which is illuminated by the light passing through the cylinder C, and the other by the beam passing through C₁. The intensity of the light which has passed through the two cylinders can thus be accurately compared.

In order to use this apparatus to compare the intensity of colour of two liquids, the following process is performed: A liquid is made, the colour intensity of which is taken as 100. The colour intensity of the liquid under examination will then be represented by a number indicating the relation between its intensity and that of the standard liquid. A solution of caramel in water is generally used as the standard, since this substance has very great intensity of colour. The preparation of absolutely pure caramel is difficult, and it is therefore advisable, in order always to have the standard solution of the same intensity, to prepare a large quantity of caramel solution at once, to add carbolic acid to prevent it from decomposing, and to keep it in well-closed bottles. When the standard solution is almost used up the colorimeter is employed to prepare a fresh quantity of equal intensity.

To render possible an exact comparison of two substances they must be tested under the same conditions, that is, the solutions of the colouring matters must be made in exactly the same manner. Finely-powdered materials dissolve more readily than coarse powders. In making the solutions which are to be examined for intensity of colour, the raw materials must be brought into a condition of fine division by the same instrument, for example, a rasp. The colour solution is made by boiling 100 grammes of the dye-wood with exactly a litre of distilled water for precisely thirty minutes. The liquid is then filtered into a 1-litre flask. The dye-wood absorbs water, and some is lost by evaporation, so that considerably less than 1 litre of liquid is collected in the flask. Distilled water is added to make up the volume to 1 litre.

If two samples of logwood are treated in this manner, solutions are obtained which contain the colouring matter in the same proportions in which it exists in the two samples. The intensity of colour of the solutions is then estimated in the following manner: The cylinder C is filled with the standard caramel solution up to a mark on the outside of the cylinder. The cylinder C₁ is filled with the decoction to the same height. The distance between the bottoms of the cylinders, R and T, R₁ and T₁, must be made smaller in proportion to the intensity of colour of the liquid between them. If now the amount of light which penetrates a layer of caramel solution of a certain thickness be taken as unity, the depth of a layer of the decoction must be greater, the smaller the quantity of colouring matter it contains, in order that the two halves of the field of view may be equally illuminated. The cylinder T₁ must be raised to a greater height the smaller the quantity of colouring matter in the liquid, in order that the two halves of the field may be illuminated to the same extent. If the colouring power of the caramel solution is taken as 100, the colouring power of the decoction can be readily calculated from the height to which the cylinder, T₁, is raised. The heights of the layers of liquid between the bottoms of the cylinders C and T, C₁ and T₁, are inversely as the quantities of colouring matter contained in the respective cylinders.

When caramel solution is used as the standard the intensity of the light in the two halves of the field can be judged, but not the intensity of colour. In order to estimate the latter a solution of that colouring matter must be used as a standard which is the principal constituent of the decoction under examination. Thus, in a careful examination of logwood a solution of hæmatoxylin would be used, and in the examination of red wood a solution of pure brasilin, as the standard. In this case a saturated solution of the colouring matter would be taken as the standard. If the intensity of its colour were represented by 100, the colour intensity of the wood under examination would always be less than 100, and would, with tolerable accuracy, represent the percentage of colouring matter in the wood. The result would not be quite accurate, because the dye-wood contains other substances which dissolve in water on boiling and affect the colour of the decoction, but the results are of such accuracy that for practical purposes no material mistake will be made by taking them as percentages of colouring matter.

Although at first sight the estimation of the colouring matter in a dye-wood by means of the colorimeter appears somewhat complicated, yet it yields the most accurate results with the smallest expenditure of labour and time. The value of a colouring material may also be estimated by preparing the pure colouring matter from a weighed quantity. This process is lengthy, demands considerable practice, and only gives good results when it is carried out with the most painful accuracy. The colouring matters in question are precipitated by lead salts. If the dye-wood extract contained colouring matter alone its amount could be found by observing the volume of a lead solution of known strength required to precipitate the colouring matter completely. The decoctions, however, contain other substances which form lead compounds, and are precipitated together with the colouring matter, so that if the precipitate were regarded as the pure lead compound of the colouring matter a very inaccurate result would be obtained. In order to obtain results with some pretensions to accuracy the lead compound of the colouring matter must be purified. The impure precipitate is washed and suspended in water, through which sulphuretted hydrogen is passed until all the lead is precipitated as lead sulphide, which is filtered off, excess of sulphuretted hydrogen expelled by boiling, and the solution again precipitated by a lead salt. This precipitate may be regarded without considerable error as the lead compound of the colouring matter. The weight of colouring matter contained in the quantity of wood used can be calculated from the weight of the dry precipitate. This method is somewhat complicated and tedious; the results are inferior in accuracy to those obtained by means of the colorimeter, which instrument furnishes the most suitable method for testing dye-woods for practical purposes.

A thorough knowledge of chemistry is indispensable to the colour manufacturer who wishes to carry on his business on any extensive scale. It enables him to match any sample of colour submitted to him and to test his raw materials with ease. We have indeed given in a section of this book simple methods by which the majority of commercial pigments can be tested with tolerable accuracy by means of a few reagents, and for ordinary purposes these methods are sufficient. But when an accurate examination of a pigment is required, it must be conducted by the ordinary processes of analytical chemistry. The colour manufacturer has not only to carry out these occasional examinations, but has frequently to test certain raw materials which he uses in large quantities. Soda may be taken as an example. An estimation of the percentage of sodium carbonate in this substance is an exceedingly simple matter to the chemist, but can hardly be carried out without a knowledge of chemistry.

A manufacturer without chemical knowledge, who is carrying on an industry which, like the manufacture of colours, rests entirely upon a chemical foundation, will constantly be compelled to seek advice from a scientific chemist. Many pigments can be made according to a settled formula, but the results of working strictly according to the formula, without a knowledge of the reasons for the operations, can only be satisfactory whilst no irregularity occurs. The least irregularity places those who work blindly in a completely helpless position, for they do not know what is wrong and cannot remove the hindrance.

In the manufacture of pigments certain by-products are produced. These can be utilised by a manufacturer possessed of chemical knowledge, whilst they are simply thrown away by many, thus making the manufacture of the particular pigment far more expensive than when the by-product is also made valuable. Strictly speaking, there are no worthless by-products in making pigments, every liquid obtained in precipitating a colour might be further utilised. Salt solutions can only be regarded as worthless by-products when the cost of separating the salt from the solution would be greater than the value of the product. Thus we cannot conclude this section of the work without again insisting that the study of chemistry is indispensable to the colour maker, since his industry is chemical from beginning to end. The colour maker who works simply by recipes will never raise himself above the position of an ordinary workman, who does what he is told without thinking of what he is doing. The smallest mistake in carrying out the process generally results in the complete failure of the whole operation and thus causes the manufacturer material loss.