Josef Bersch

“ When the clay has been derived from the weathering of such rocks, it is easy to understand that it may contain admixtures of quartz and mica, which are frequently visible to the naked eye, or at any rate under the microscope. Whereas clay forms a white, amorphous mass, the grains of quartz sand are decidedly crystalline, transparent and of vitreous lustre; the scales of mica, on the other hand, appearing as thin tabular crystals, mostly of a green or brown colour and exhibiting, when viewed at certain angles, a brilliant metallic sheen. ”

“ As a pigment, chalk possesses many valuable properties. The organic structure of chalk gives it high covering power as a wash, a thin layer applied to a surface sufficing to mask the colour of the underlying ground completely. The lime in chalk being combined with carbonic acid, its basic properties are so extensively weakened that chalk can be mixed with even the most delicate colours without fear of their shade being affected. A coating of pure chalk paint on any surface will never change colour in the air ”

“ The method of formation of terra di Siena was probably on the same lines as that already described in the case of ochre, namely by the breaking down of minerals—in this case brown ironstone—and natural levigation, the powder being deposited in places where the water containing the ferric hydroxide in suspension came to rest and allowed the solid particles to settle down.
The best lumps of terra di Siena in point of purity and colour can be used as pigments without any preparation; but in most cases the earth is lightly calcined, in order to improve the colour. ”

“ The employment of barytes in the colour industry is often regarded as adulteration, which, however, it is not when the case is considered from the right point of view. For instance, the only preparation which can properly be termed white lead consists of basic lead carbonate. This, when pure, is a rather expensive pigment, whereas, for certain purposes, the consumer requires a product that can be obtained at a low price. ”

“ The somewhat expensive pigment, Indian red, is—when pure—really nothing but a very pure ferric oxide of Indian origin. Ferric oxide, however, often contains impurities which considerably influence the colour of the product.30 Owing to the fact that large quantities of ferric oxide are formed as by-products in certain chemical processes which are carried out on a very extensive scale, this oxide, which is very pure, can be advantageously used for making iron pigments, especially as its application for other purposes is very restricted, and it can therefore be had at a very low price. ”

“ Even with the greatest care, however, it is impossible by this means to obtain sufficient fineness of division for mixing with fine colours, the only way in which this can be accomplished being by levigation.
Given a fairly pure white barytes to begin with, levigation furnishes a handsome white pigment that can be mixed with colours of any kind; but when used by itself in association with oil or varnish, its covering power is very low and the colour never perfectly white. Native barytes is therefore unsuitable, as such, for paint. ”

“ Many that are rather dark in colour will, nevertheless, burn perfectly white, whereas others, much lighter in shade, always give a product that is not quite pure in tone. This divergent behaviour seems to have some connection with the chemical composition of the organic matter in question. If it consists of coal, or substances analogous thereto, no really pure white lime can be obtained from a light grey limestone, it being impossible to burn off the finely divided carbon completely. ”

“ Most ochres consist of varying mixtures of clay, ferric hydroxide and lime; and, as a rule, the higher the proportion of ferric hydroxide, the deeper the colour. Thus, for example, the ferric hydroxide may amount, in the dark grades, to 25% of the entire mass, whilst in the lighter kinds it may be as low as 3%. It is very rare that ochre is put on the market in its native condition, being mostly subjected to chemical treatment enabling a definite shade of colour to be obtained. ”

“ On exposure to the air, green vitriol turns an ugly brown colour, and is no longer completely soluble in water, passing gradually into the condition of basic ferrous sulphate. This is because ferrous oxide is a highly unstable substance, which attracts oxidation and changes into ferric oxide. This latter, however, requires for the production of soluble salts a larger quantity of acids than does ferrous oxide, and therefore the oxidation of ferrous sulphate in the air leads only to the formation of salts that are imperfectly saturated with acid, namely basic salts. ”

“ Although it has hitherto been the custom to confine the term earth colours to such as occur ready-formed in Nature and only require simple mechanical treatment to make them ready for immediate use as pigments, the author is nevertheless of opinion that a book dealing exhaustively with earth colours should also make some mention of all the mineral colouring matters which can be easily made into pigments by simple processes, such as calcination or bringing into association with other substances. ”

“ In judging the quality of raddle as a pigment, the presence of impurities is of less account than their nature; and in some cases a very highly contaminated raddle may be worth far more, as a pigment, than one containing only very small admixtures of extraneous substances.
As stated above, the ordinary impurities in raddle are clay, lime and quartz sand. An admixture of clay, even if fairly large, is no great drawback, since the material can be used in its natural state, and also be toned by burning. ”

“ In chemical composition, true chalk is calcium carbonate, but of a fossil character, for if chalk dust be examined under a high-power microscope, it will be seen to consist of the shells of minute animals, and is therefore to be regarded as fossil. The organic matter of the animals has long disappeared, leaving the inorganic material, a very pure calcium carbonate, behind. ”

“ The treatment of the crude earth is best carried on in the same vessels as are to be used in the subsequent levigation process. After the acid liquor has been drawn off, the earth is brought into contact with water, stirred up well, and the water run off, by opening tapholes in the side of the vessel, into settling-tanks, where it is left until all the suspended matter has completely subsided.
The colour of green earth can also be toned by the addition of yellow ochre, thus producing a range of greens with a yellowish tinge. ”

“ The testing of a mineral for its suitability as pigment is a very simple matter, all that is required being to subject a small quantity to the same treatment that is applied to the earth colours on a large scale. For this purpose a few pounds of the mineral are levigated, and the residue is dried. To ascertain the tones obtainable by calcination, small samples—of about 100 grms.—are placed in crucibles, and gradually heated in a furnace. When the masses have attained a sufficient temperature, the samples are taken out of the furnace, at intervals of ten minutes, and left to cool. ”

“ There is scarcely any need to mention that all substances containing ferric oxide can be used for making any of the pigments obtainable from the oxide itself, the only difference33 between the various raw materials being their degree of purity, so that it is not always so easy to obtain a certain desired shade from a given material in such beauty as is furnished by another material, the small quantities of impurities associated with the ferric oxide having, in many instances, an important influence on the colour. ”

“ A fundamental condition for the preparation of a good Vienna white is the employment of pure raw material, which must be free from ferric oxide or earthy impurities, and fully burned. An excellent material for this purpose is calcined mussel shells, which furnish a loose, and at the same time very pure, lime, and are very largely used for lime-burning in places such as Holland, where they are available in large quantities.
Vienna white is not much used as a paint colour, owing to its powerful alkaline properties which have a destructive effect on many colours. ”

“ Marl that is fairly free from ferric oxide, however, can very well be used as pigment; and many white pigments sold as “chalk” are really finely ground marl. In accordance with the general practice, in the colour industry, of giving colours a great variety of names, and suppressing the real names, which, so far as the artificially prepared colours are concerned, should bear some reference to their chemical composition, numerous white earth colours bear fancy names, though really consisting of chalk, lime (generally marl) , or white clay. ”

“ Green earth is a valuable pigment for all kinds of painting, on account of its extreme permanence. It may be applied directly over lime without suffering any change, whereas most of the cheap green colours are destroyed in like circumstances. This behaviour renders green earth specially valuable in fresco work, although it is also largely used as an oil colour.
Augite is of frequent occurrence in volcanic districts; and in such localities, deposits of green earth are certain to be found. The test for the suitability of a green earth consists mainly in treatment with dilute hydrochloric acid. ”

“ Any sandy residue left when calcium carbonate is dissolved, mostly consists of quartz sand. In dissolving dark-coloured limestones, grey, or even black, flakes are left, which consist of organic material very high in carbon. On limestone being subjected to fairly strong calcination, all the carbonic acid is expelled, leaving behind the so-called quick or burnt lime, which is, chemically, calcium oxide:—
If burnt lime be left exposed to the air for some time, it again gradually absorbs carbon dioxide and is reconverted into calcium carbonate. ”

“ The application of the term “umber” to this earth can only have been based on a certain similarity in colour to true umber. In chemical composition, however, the two are quite different, Cologne earth really consisting of a mixture of humic substances. It is well known that the rotted wood found in the interior of decaying trees is often a handsome brown colour; and all woody matter, after lying a very long time, finally acquires this colour, owing to the transformation of the wood into dark-coloured compounds richer in carbon. ”

“ On account of the large quantity of mechanically associated water, freshly dug green earth is greasy in character, like wet clay. In partial drying, most of this water evaporates, the mass becoming earthy and adherent to the tongue. Sometimes the colour is an ugly brownish-green, owing to the presence of a considerable amount of ferric oxide formed as the result of changes set up by exposing the mineral to the air. Ferrous oxide is a very unstable compound, having an energetic tendency to combine with more oxygen and thus undergo transformation into ferric oxide ”

“ Ferric oxide is revealed by its reddish colour, in both the limestone and burnt lime.
Magnesia, which is present, for example, in dolomitic limestone, cannot be detected by the colour, either before or after burning, this oxide being itself perfectly white; but its presence is a drawback because if in large quantity it makes the lime very difficult to quench, and such lime is never of a fatty character.
Organic matter betrays itself by the colour, the lime being dark tinted, varying from grey to black. ”

“ Although the number of green-coloured minerals is large, but few of them are suitable for painters’ colours, because they occur so rarely in Nature that their employment for this purpose is out of the question, more especially since a very large number of green pigments can be obtained by artificial means. The most important of the earth colours in this category are Celadon green, or green earth, and malachite green—the latter, however, less so, because the substance of which it is composed can be prepared artificially. ”

“ Many clays that were originally white gradually assume a yellow tinge on prolonged exposure to air and moisture, because the clay contained ferrous oxide, which changes, in the air, to the stronger pigment, ferric oxide.
Many kinds of clay merely require a simple levigation to fit them for use as pigment. The lumps of freshly dug clay are placed in large tanks, etc., filled with water and stirred up continuously in order that, instead of forming a plastic mass which is very difficult to distribute in water, the particles detached from the lumps may pass at once into suspension. ”

“ The lumps of chalk are reduced by mechanical means, such as a stamp-mill, or, more frequently, in a mill of the same type as for grinding flour, since it is impossible to get the lumps so dry as to produce the degree of brittleness necessary for a thorough reduction in a stamp-mill. The millstones are enclosed in a wooden casing, and the chalk is ground in admixture with water, the ground mass escaping, through an opening in the casing, as a thick pulp which is stored for a considerable time in large tanks. ”

“ Still more frequently, magnesia occurs in association with calcium carbonate, in the mineral dolomite, which contains up to 20% of magnesia.
A less abundant native mineral is hydromagnesite, which consists of basic magnesium hydrocarbonate. Hydromagnesite is a very light, chalk-white mass, with a non-greasy feel, which, when reduced to a soft powder, forms an excellent material for paint. It is highly inert, in a chemical sense, and can therefore be mixed with the most delicate colours, having no other effect thereon than to render them lighter in shade. ”

“ The character of clay is just as varied as its colour. In some places, large deposits of extremely fine clay are found, the material, when mixed with water, forming a highly plastic mass which, when dried and subjected to slight pressure, furnishes a very soft powder. ”

“ Barytes, or Heavy Spar
The mineral known as heavy spar occurs in very large quantities and in numerous localities. It forms rhombic crystals, which are very often extremely well developed and form flat plates of considerable size. A remarkable peculiarity of this mineral is its high specific gravity, which is due to the barium content. It is found native in all colours, white being the most common.
Chemically, heavy spar is barium sulphate, BaSO4. It can be used as a pigment per se, but only when prepared artificially, the trade name for the product being permanent white, or blanc fixe. ”