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.
CHAPTER LXXIII.
THE DESIGN OF A COLOUR WORKS.
In the establishment of a colour works several conditions are necessary. The most important is the supply of water in sufficient quantity and purity. It has already been stated that many pigments cannot be made with water containing much organic matter or salts, since the dissolved substances affect the shade. This action not only takes place in the formation of the colour, but is also unpleasantly manifest when it is washed. The delicate lakes are discoloured by organic matter in the water, and are so changed by any considerable quantity of lime that the alteration in shade is clearly perceptible after continued treatment. If the water contains but a very small quantity of iron, the preparation of some pigments is made quite impossible, since—and this is especially the case with the lakes—the ferric oxide is precipitated together with the pigment, and in consequence of its characteristic colour imparts to it an ugly shade. Thus in choosing a site for a colour works the available water supply must be carefully examined. If it is not sufficiently pure or in sufficient quantity, the position must be regarded as unsuitable.
No colour maker, although working on the largest scale, is in a position to make all the materials he requires. With the continual development of the chemical industries, the number of these substances which can be made economically in the colour works continually diminishes. It is far more advantageous to obtain them from works in which they are made on the large scale. Many of these substances are required in large quantity, so that a site should be chosen in direct communication with the railway, so that the cost of carriage is diminished, and also the cost of distributing the manufactured materials. This is especially necessary for materials which have a low value and consequently can bear no high cost of carriage.