The manufacture of mineral and lake pigments

by Josef Bersch

Reactions of the Organic Colouring Matters.

Colouring Matter.	Dilute Acids.	Caustic Soda.	Lime Water.	Strong Nitric Acid.	Alum.
YELLOW.
Buckthorn berry extract	Slight turbidity.	Yellowish red.	Slight greenish precipitate	Reddish brown.	Pale yellow.
Dutch pink	Yellow solution.	Brown.	Brownish	Reddish brown.	——
Weld extract	Slight turbidity.	Golden yellow.	Greenish yellow.		Slight yellow precipitate.
Weld lake	Yellow solution.	Unaltered.	Greenish yellow.	Golden yellow solution.	——
Gamboge	Unaltered	Deep reddish yellow.	Deeper yellow.	Deep yellow.	——
Gamboge lake	Yellow solution.	Deep reddish yellow.	Deeper yellow.	Deep yellow.	Yellow precipitate.
Fustic	Becomes paler.	Dirty brownish red.	Deeper.	Slight precipitate.	Pale yellow precipitate.
Quercitron	Becomes paler.	Deeper.	Deeper.	Paler.	Paler, slight precipitate.
Young fustic	Slight greenish precipitate.	Purple red.	Reddish.	Greenish.	Slight greenish precipitate.
Turmeric	Carmine red.	Brown.	Brown.	Carmine red.	Yellow.
BLUE.
Indigotin	Unaltered.	Yellowish brown solution.	——	Yellow.	——
Indigo carmine blue lake	Unaltered.	Yellowish brown solution.	——	Yellow.	——
Litmus	Turns red.	Unaltered.	——	——	——
Logwood	Red.	Violet.	Blue.	Yellow.	Dirty violet.
RED.
Cochineal decoction	Becomes paler.	Violet.	Becomes darker.	Yellow.	Red flocks.
Carmine	Red solution.	Violet solution.	Becomes darker.	Yellow.	——
Lac, lac dye	Yellowish red.	Violet solution.	——	Yellow.	Red precipitate.
Safflower red	Unaltered.	Red solution.	——	——	——
Madder lake	Red solution.	Red precipitate.	——	Yellow.	Red precipitate.
Red wood	Pale red.	Violet.	Carmine red precipitate.	Yellow.	Red precipitate.
Red wood lake	Red solution.	Violet solution.	——	Yellow.	——
The remaining red colouring matters give such uncertain reactions that it is almost impossible to distinguish between them by ordinary means; they are therefore not included here.
GREEN..
Sap green	Reddish brown.	Brownish green.	Becomes deeper.	Reddish yellow.	——
Leaf green	Green solution.	Green solution.	——	Reddish yellow.	——
BROWN..
Sepia	Unaltered.	Brown solution.	——	——	——
Catechu solution	Paler.	Reddish brown.	Reddish brown.	Yellowish brown.	Reddish yellow coloration.

Colouring Matter.	Stannous Chloride.	Ferric Chloride.	Copper Acetate.	Gelatine Solution.	Remarks.
YELLOW.
Buckthorn berry extract	Slight greenish yellow precipitate.	Olive green.	Dirty yellow.	Slight precipitate.	——
Dutch pink	——	——	——	——	Chars when heated on platinum foil.
Weld extract	Considerable yellow precipitate.	Greenish	Reddish yellow precipitate.	Slight turbidity.	Gives yellow scales some time after adding potassium bichromate.
Weld lake	——	——	——	——	——
Gamboge	——	——	——	——	Readily melts and burns.
Gamboge lake	Yellow precipitate.	——	——	——	——
Fustic	Yellow precipitate.	Blackish green precipitate.	Brownish yellow precipitate.	Considerable precipitate.	Lead acetate gives an orange red precipitate.
Quercitron	Yellowish red precipitate.	Becomes green, green precipitate.	Yellowish green precipitate.	Reddish precipitate.	——
Young fustic	Orange red precipitate.	Brownish green precipitate.	Brown flocculent precipitate.	Yellowish red precipitate.	Ammonia gives red coloration.
Turmeric	Yellow.	Brown.	——	Fairly large precipitate.	Very sensitive to alkalis.
BLUE.
Indigotin	——	——	——	——	On heating gives peculiar odour and purple red vapours.
Indigo carmine blue lake	——	——	——	——	On heating gives peculiar odour and purple vapours.
Litmus	——	——	——	——	Unpleasant smell on heating, white ash.
Logwood	Dirty violet.	Dark coloured solution.	——	Very turbid.	Black with potassium bichromate.
RED.
Cochineal decoction	Reddish violet flocks.	——	——	——	Completely soluble in ammonia.
Carmine	——	——	——	——	——
Lac, lac dye	Red precipitate.	——	——	——	——
Safflower red	——	——	——	——	Rapidly decomposed by boiling with alkalis.
Madder lake	Red precipitate.	——	——	——	——
Red wood	Deep red precipitate.	Violet precipitate.	Violet red precipitate.	——	——
Red wood lake	——	——	——	——	Much ash on ignition.
The remaining red colouring matters give such uncertain reactions that it is almost impossible to distinguish between them by ordinary means; they are therefore not included here.
GREEN..
Sap green	——	——	——	——	White ash on ignition.
Leaf green	——	——	——	——	White ash on ignition.
BROWN..
Sepia	——	——	——	——	Burns with peculiar smell.
Catechu solution	Brownish precipitate.	Brownish green precipitate.	Brown precipitate.	Considerable precipitate.	——

It is not easy to decide which colouring matter is united with the metallic oxide, because the organic colouring matters do not give, as a rule, decided reactions. In examining pigments of this kind it is always advisable to test at the same time a pigment of known composition, and to compare the reactions of the two. The colouring matter of the lake is first brought into solution. This is accomplished by the action of dilute hydrochloric acid, which decomposes the compound of metallic oxide and colouring matter, the latter dissolving. A portion of the lake should remain undecomposed, so that the solution has no strong acid reaction by which the action of the reagents to be used would be modified. When the lake has been treated for some time with dilute hydrochloric acid, water is added and the solution of the colouring matter filtered from the residue. Small quantities of this solution are then treated with the different reagents. By a comparison of the reactions given by the solution and by a solution of the pure colouring matter, the nature of the colouring matter under examination may be decided.

In deciding in the first place if a colour is of organic origin, it is treated with hydrochloric acid in the above manner. If a coloured solution results, an organic colour is probably present. Chlorine water is then added to a portion of the solution; if the latter is quickly decolourised an organic colouring matter is certainly present, for they are all decomposed by the continued action of chlorine.

The following reagents are used in testing organic colouring matters: dilute sulphuric or hydrochloric acid, caustic soda solution or lime water, strong nitric acid, which, in consequence of its oxidising properties, gives different reactions to the other acids. Of the metallic salts, alum, stannous chloride and ferric chloride are used, and occasionally copper acetate. Glue solution is also used in the pure form of isinglass or gelatine solution, with which several colouring matters give characteristic precipitates. The foregoing tables give the behaviour of the colouring matters contained in the organic pigments towards the reagents mentioned above.

CHAPTER LXXII.
THE TESTING OF DYE-WOODS.

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.

Fig. 43.

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.

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.

In regard to the space required by a colour works, no actual dimensions can be given, since these vary with the extent of the business, and with the pigments produced. In the price lists of large colour works all the commercial pigments are generally quoted, but they are rarely if ever actually made in one works, but are obtained at a lower price from other establishments, which make a speciality of certain pigments, and by working on a large scale can produce them at such a cost that the smaller manufacturer is not in a position to compete with them. Thus there are works in which only white lead or ultramarine is made.

If a colour manufacturer is in the fortunate position of placing his works on a river, he has not only an unrestricted water supply, but may also be able to use water power, the cheapest of all powers. When water power is available, it will be used to raise the large volumes of water required and to move the machinery for grinding the raw materials, rasping dye-woods, etc. In a colour works of any size a boiler is required; if water power is not used, it must be of sufficient power to give steam for driving the engine, for boiling liquids and for heating the drying stoves. The boiler is of great advantage in providing steam for dissolving salts, extracting dye-woods and boiling liquids. When liquids are boiled by steam there is considerable economy in that the majority of the boiling tubs can be of wood, which is provided with a protective coating for liquids which attack wood. Thus there is economy in dispensing with large metal pans and with the fireplaces in which they would be built, and also in the course of time there is considerable saving in fuel.

For a well-equipped colour works a drying stove, in which the pigments can be thoroughly dried, is important. It is most convenient to heat drying stoves with steam. The temperature can be easily regulated by increasing or diminishing the supply of steam. For drying pigments which would not be injured by considerable increase of temperature, the drying stove may be heated by a fire.

In a colour works in which many pigments are made sulphuretted hydrogen is frequently required. Since all lead pigments are blackened by this gas, the greatest care is required in using it. Also the poisonous nature of sulphuretted hydrogen is generally under-estimated. In working with sulphuretted hydrogen, the apparatus depicted in [Fig. 3] should be used. It should be placed in a position, such as a closed yard, in which escaping gas will be harmless. If this cannot be done the precipitation should be accomplished in closed vessels, from which a pipe should carry the gas to a fire, where it will be burnt to sulphur dioxide and water.

In most cases the manufacturer of colours makes but a certain number of pigments and rarely or never all which are mentioned in his price lists. The dimensions of the establishment will be in accordance. In commencing a new colour works it is advisable from purely financial reasons to produce at first a limited number of colours, but these in perfection. By many experiments and diligent study of chemistry a colour manufacturer may hope to compete successfully in so difficult a branch of chemical technology as the manufacture of colours.

CHAPTER LXXIV.
COMMERCIAL NAMES OF PIGMENTS.

In commerce the pigments are found under the most different names, the most common of which have been given together with the description of the pigment. No regularity can be found in the names chosen for the different pigments; quite arbitrary designations have been taken. Pigments are most commonly named after places—for example, Prussian blue, Paris blue, Bremen green; also after the discoverer—Turnbull’s blue, Hatchett brown. Whilst these names give the place of production or the name of the discoverer, and thus have some foundation, there are many others for which no reason can be assigned, e.g., King’s yellow. Certain names are based upon the chemical composition of the pigment. These should be used by preference, but now that the expressions white lead, chrome yellow and Chinese blue have become common no one would think of speaking of basic lead carbonate, lead chromate or ferric ferrocyanide. The confusion in the nomenclature of colours is increased by placing pigments which possess English names upon the market under French, German or Latin names, which are often sadly mutilated. This is more the case in Germany than in England.

It may easily happen that a reader of a work on colour making might search in vain for a pigment whose name he had somewhere heard, whilst the book contained a description of the colour and its properties, but under another name. To remove this difficulty it has been thought necessary to collect the names of the different pigments, which are contained in the following table. The French and German names are also given. The most usual names are printed in italics.