Chapter VI: Red Earth Colours

The number of minerals that can be directly used as red earth pigments is comparatively small, and by far the greater proportion consist of ferruginous colours, a few of which are obtained by the mechanical treatment of native iron ores or clays coloured red by ferric oxide, the majority, however, being formed by burning certain materials of another colour. To these belong nearly all the materials mentioned in connection with the ochres and the brown iron colours, together with a few by-products of the chemical industry.

In addition to the foregoing, which have ferric oxide for their pigmentary principle, is the native mercury sulphide, occurring, as scarlet, crystalline masses, under the name of cinnabar (vermilion). The only reason for including natural vermilion with the earth colours is to make the list complete, the largest proportion of this pigment being prepared by artificial methods. The product sold as “Chinese” vermilion may, in former times, have really been introduced from China into Europe, and prepared there by grinding and levigating the best-coloured lumps of the natural cinnabar; but, at the present time, all the vermilion made--in Europe at least--is from sulphur and mercury, by artificial processes, and the name Chinese vermilion is merely retained to designate a particularly fine grade.

On the basis of occurrence and chemical properties, the red earths can be classified into several groups. The first comprises natural products requiring only mechanical preparation, such as the minerals known as hematite, micaceous iron ore, Elbaite, etc., and the special modification of red ironstone termed raddle. All these minerals consist almost entirely of ferric oxide in a pure state. The mineral, bole (red chalk, terra sigillata, Lemnos earth), is chemically allied to the ochres, being, like them, composed of alumina, frequently accompanied by lime and small quantities of magnesia, but differing in that ferric oxide is always present in bole, whereas the ochres always contain ferric hydroxide.

The second group consists of the artificial reds obtained by burning or calcining raw materials, whose ferric hydroxide is more or less transformed by heat into ferric oxide, such as vitriol ochre, pit ochre and alum sludge.

Of late years the artificial earth colours have attained a high degree of importance. They are obtained in large quantities in the manufacture of sulphuric acid from green vitriol. Formerly, it is true, they were also used as pigments under the name of caput mortuum or colcothar, but were not held in much esteem; and it is only within recent times that it has been discovered that these inferior by-products can be converted into very handsome and brilliant colours, which now form important articles of commerce.

BOLE

Bole, Lemnos earth, terra sigillata, etc., is, for the most part, a product of the decomposition of highly ferruginous minerals, and occurs, in the form of lumps, having a conchoidal fracture, in pockets or detritus. The lumps have a sp. gr. of 2·2–2·5, are Isabella brown to dark brown in colour, and give a slightly greasy-looking streak. There are two distinct varieties of bole: the one adhering firmly to the tongue, whilst the other lacks this property and, when placed in water, crumbles down to powder in emitting a peculiar noise.

The composition of the boles varies, but all of them may be regarded as alumino ferric silicates combined with water. Most of the specimens examined from different deposits contain 24–25% of water, 41–42% of silica, and 20–25% of alumina, the remainder consisting of ferric oxide with small traces of manganese oxide.

Some varieties, however, are exceptional and contain only 30–31% of silica and 17–21% of water, _e. g._ those from Orawitza and Sinope. Lemnos earth, the true terra sigillata, is mostly silica (66%) with 8% of water, and contains a smaller percentage of ferric oxide than the others. It is also of a distinct colour, lighter than the true boles and having a greyish or yellowish tinge.

The behaviour of the different kinds on burning is just as diverse as their chemical composition. Whilst some kinds are infusible at even the highest temperatures, and merely change into hard, red masses; others, again, fuse at a moderate heat. This difference is due to chemical composition, those high in silica being generally less refractory than those in which alumina preponderates.

In order to render the boles suitable for painting, they are put through a somewhat different treatment than the other earth colours. The freshly dug material is first sorted, the uniformly coloured lumps of fine texture being set apart and suffused with water, with which they form a pasty mass of low plasticity, which is kneaded by hand to make it homogeneous, and is then stirred up with more water. When the lumps have distributed in the water, the latter is drawn off into a second tub, and the residue is stirred up with fresh water, the treatment being repeated until the effluent no longer shows any signs of colour.

The liquid in which the finely divided bole is suspended is left to settle, and the bole subsides as a fine powder, which is dried to the condition of paste, pressed into moulds and dried completely.

Owing to its low content of ferric oxide, the colour of bole is not particularly bright, but is very permanent--a property equally shared by all the other ferric oxide pigments.

NATIVE FERRIC OXIDE AS A PIGMENT

In nature, ferric oxide forms extensive deposits, which, by reason of the light red colour characteristic of certain varieties of ferric oxide, are largely employed in painting. These colours may be classed among the oldest known to mankind, ferric oxide pigments having been used frequently in the most ancient paintings.

The most important varieties of ferric oxide for our purpose are: iron glance, with its modifications, micaceous iron ore and frothy hematite; red hematite, and raddle.

IRON GLANCE

This substance forms handsome black crystals of very high lustre, which, when small and scaly, constitute micaceous iron ore. Both, when rubbed down, furnish a dark red powder of no particular beauty. Micaceous iron ore forms the transition stage into frothy hematite, or iron cream, the sole difference being that the crystals of the latter are much smaller, and the scales finer, the iron-black colour passing gradually into cherry red. At the same time, the lustre, though still high, loses most of the metallic sheen exhibited by micaceous iron ore.

HEMATITE

The variety known as hematite or bloodstone, sometimes occurring as shiny nodules, is distinguished by its handsome red colour. Some of the lumps are composed of long, thin crystals grouped about a common centre so as to form a globular mass. Despite its bright colour, the hardness of hematite (between 3 and 5) prevents it from being used as a pigment, the value of the product not being commensurate with the cost of reduction.

RADDLE

There are numerous deposits of red ironstone, in the state of fine earth, where the operations of grinding and levigation have, to a considerable degree, already been carried out by Nature. These deposits form the mineral which, under the name of raddle, is often used as a pigment for ordinary paints. It may be considered to have originated in the transformation of red ironstone, by the natural forces that can everywhere be seen disintegrating rocks, namely water and frost, into a fine powder, which has been transported, often over long distances, by water, and has finally settled down.

In places where the process has been carried out in this manner, the raddle will be in a condition, as regards fineness of division and beauty of colour, that leaves nothing to be desired, and the material itself is ready for use as a very valuable pigment. Large deposits of this kind, however, are of rare occurrence; but there are plenty in which the ferric oxide is associated with varying quantities of clay, sand, and sometimes lime.

The conditions here are on all fours with those of clay, which, too, has been formed in a similar way. Pure clay, the so-called kaolin, is a highly valuable material, whereas ordinary loam--highly contaminated clay--is only of low value. 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. Lime is less favourable, for though a calcareous raddle can be used as it is, the lime parts with its carbon dioxide on calcination, becoming changed into caustic lime and imparting to the product qualities which preclude its employment for a number of purposes, especially for mixing with delicate organic colours.

The presence of quartz sand is immaterial when the raddle is to be burned, inasmuch as sand is unaltered by calcination. But it constitutes a drawback because it makes the fine raddle gritty and unsuitable for fine paint work. The only way to eliminate this impurity is by levigation--an expensive operation which should, as far as possible, be avoided for these native ferric oxides, because they must be sold very cheaply, and have to compete with the large quantities of oxide obtained as a by-product of the chemical industry.

The suitability of a given specimen of raddle for use as a pigment may be easily ascertained by weighing out exactly 100 grams and heating to about 120° C. The loss of weight will give the amount of water in mechanical retention. The residue is suffused with strong vinegar, and left for several days, being stirred at frequent intervals. The carbonates of lime and magnesia present will dissolve in the acid, the ferric oxide remaining untouched. The liquid is decanted, and the residue washed several times with water and dried, the diminution in weight being a measure of the carbonates in the sample. If the vinegar has turned a yellow colour, the presence of ferric hydroxide in the mineral is indicated, this hydroxide being readily soluble in acetic acid. If the residue feels gritty, it contains quartz sand, the amount of which can be found with sufficient accuracy by levigating the mass and weighing the sandy residue after drying.

Deposits occur, in many places, of a mineral similar to raddle, but formed under peculiar conditions. Thus, there are found, in the vicinity of brown-coal deposits that are rich in pyrites, earthy masses which are occasionally of a handsome red colour and consist of a variety of minerals admixed with a considerable proportion of ferric oxide.

These masses probably originated in fires in the coal seams, whereby the pyrites became transformed into ferric oxide and basic ferric sulphate; and where the deposits are of sufficient size, they may be advantageously utilised in the production of cheap reds. In most cases, however, the minerals must be levigated, owing to the frequency with which they contain large proportions of extraneous minerals in a gritty condition.

BURNT FERRIC OXIDE AND OCHRES

It has already been stated, in dealing with the yellow ochres, that these colours can be toned by burning, part of the ferric hydroxide losing its water and changing into red ferric oxide. The more severe the burning, the larger the amount of ferric oxide formed and the nearer the colour of the product approximates to red. According, however, as the original ochre was yellow or brown, the tone of the burnt colour will lie between orange and brownish red. If the heating be pushed so far as to transform all the ferric hydroxide into oxide, the red will come more and more into prominence in proportion to the amount of hydroxide in the original material. If the product consists entirely of ferric oxide, as is the case with that obtained, as a by-product, in the manufacture of English sulphuric acid, a pure red ferric oxide (caput mortuum, colcothar, English red, etc.) will be obtained. If the heating be increased above a certain point, the pure ferric oxide will change colour, assuming a brown to violet tone according to the temperature employed.

(_a_) _Burning in the Muffle_

Since, as a rule, the quantity of material treated in the preparation of these brown, violet to black ferric oxide pigments for the purposes of the painter on porcelain is not large, the same kind of muffle furnace (Fig. 29) as serves for making enamels can be used. The fire-clay muffle _M_ is inserted in a reverberatory furnace _O_, with a good draught, and is raised to a white heat. The finely powdered material to be burned is spread out evenly on plates of sheet-iron or fire-clay, and introduced into the white-hot muffle, where it is left for a period corresponding to the colour desired. To save time, the plates may be pre-heated in a second muffle arranged above the first.

By this means a large range of tones can be obtained from one and the same material, by heating it to different temperatures; and the colours so produced are distinguished, not only by their warmth of tone, but also by very high stability. In fact, they may be regarded as permanent, because very strongly calcined ferric oxide only passes very slowly into solution even under prolonged boiling in the strongest acids. Owing to this excellent property, which is equalled by very few other pigments, and the low cost of preparation, these colours deserve the most careful consideration by all manufacturers who are in a position to obtain suitable material in sufficient quantities.

(_b_) _Caput Mortuum, Colcothar_

Previous to the English method of making sulphuric acid by the oxidation of sulphur dioxide with nitric acid, this acid was manufactured by heating dehydrated ferrous sulphate (green vitriol); and even now, fuming sulphuric acid--oil of vitriol, or Nordhausen sulphuric acid--is largely obtained by the same process.

When anhydrous ferrous sulphate, FeSO{4}, is exposed to a very high temperature--strong white heat--it is decomposed into sulphur trioxide, SO{3}, sulphur dioxide, SO{2}, and a residue, mainly composed of ferric oxide and a little basic ferric sulphate, which remains behind in the heating-pan. In fact, even at the highest possible temperatures obtainable in the furnaces used for the distillation of the green vitriol, it is impossible to recover the whole of the sulphuric acid, a small portion being tenaciously retained by the iron.

This red residue is sold under various names--colcothar, caput mortuum, English red, Indian red, etc.--and is used as a low-grade pigment, and also as a polishing agent. The name caput mortuum is a survival from the time of the alchemists, and was probably applied to indicate a dead-burned product, from which all the active ingredients had been removed.

Although, in former ages, this substance was held in low estimation as a pigment, attempts have been made in recent times to convert it, by suitable treatment, into a more valuable product; and these attempts have been crowned with success, affording another instance of how a high commercial value can be imparted to a waste product by proper manipulation.

(_c_) _Calcining Ferric Oxide_

In order to obtain a series of tones of colcothar, it is subjected to repeated calcination, but not by itself, since it would require an extremely large quantity of fuel to effect any change of tone in view of the very high temperature the material has already been exposed to in the sulphuric acid plant. If, however, salt be added, then a variety of tones can be obtained without recourse to any particularly high temperature. It is frequently stated that the only effect of the presence of salt is to keep the calcining temperature uniform, inasmuch as the salt volatilises at a strong red heat, and when that temperature is reached, the whole mass cannot get any hotter until the whole of the salt has passed off, all the heat applied being consumed in transforming the salt into the state of vapour.

As a rule, however, the amount of salt added does not exceed 6% of the weight of the charge to be calcined; and this quantity does not seem to be sufficient to keep the temperature at a uniform level through the several hours required for the calcining process. The author is therefore of opinion that the salt also has a chemical action on the material during the calcination.

As already mentioned, colcothar is by no means pure ferric oxide, but always contains basic ferric sulphate. Now, it is feasible that some reaction may take place between the basic sulphate and the sodium chloride at calcination temperature, with the formation of caustic soda, which, being a far more powerful base than ferric oxide, deprives the latter of sulphuric acid, sodium sulphate being formed. The chlorine of the salt combines with the iron to form ferric chloride, which volatilises at a glowing heat.

According to this hypothesis, therefore, the addition of common salt in the calcination of colcothar is less for the purpose of maintaining a uniform temperature within certain limits than for decomposing the basic ferric sulphate present and inducing the formation of a product consisting entirely of pure ferric oxide. The various tones obtained are due to the varying length of exposure to the heat.

The following method is pursued in the conversion of colcothar into iron pigments on a manufacturing scale. The crude colcothar from the sulphuric acid plant is ground, as finely as possible, in ordinary mills, and the resulting soft powder is intimately mixed with salt, 2, 4 or 6% being the usual proportions added. The calcination is ordinarily continued for six hours in the case of the mixture containing the largest amount of salt; but only two hours, or even one, for the other mixtures.

The operation is carried on in earthenware pipes, a large number of which (up to sixty) are built into a furnace. The latter is fired very carefully, the temperature being raised only very gradually, since experience has shown that much better coloured products are obtained in this way than by raising the mass quickly to a high temperature.

When incandescent ferric oxide is allowed to cool down with unrestricted access of air, the colour is not nearly so bright as when air is excluded during the cooling. Since air has no action on ferric oxide, this remarkable phenomenon cannot be due to the presence of the air, but probably to the influence exerted by the rapid change of temperature on the arrangement of the finest particles of the oxide. Nevertheless, some manufacturers hold that rapid cooling, with restricted access of air, improves the colour.

To exclude air from the ferric oxide during calcination, the open ends of the pipes are flanged and covered with close-fitting plates, which are luted with clay. The expansion of the internal air as it grows hot would burst the pipes unless a means of escape were provided, which consists in leaving small vent holes in the cover plates.

As previously mentioned, calcined ferric oxide is very inert, chemically, so that, when the calcination has been strong, prolonged boiling with the most powerful acids is needed to bring the oxide into solution. If the heating has been continued up to the strongest white heat, and the ferric oxide maintained in that condition for several hours, even hot sulphuric acid will have only a slight effect on the oxide, and the only way to make it more readily soluble is by fusion with potassium bisulphate.

Now indifference to chemical action is just the property required of a pigment for fine work; and in this respect, the ferric oxide colours are superior to all others. The gradations of tone that can be obtained from ferric oxide by varying the calcination are very numerous, comprising all between iron red, red-brown and pure violet.

The author has tried heating ferric oxide for a considerable time at a very high temperature, equivalent to the strongest white heat, and obtained a product which was no longer pure violet, but had a decidedly blackish colour. Perhaps, by greatly prolonging the heating, it might be possible to get a pure black; but, even if this were so, the matter would be of no special interest, because black pigments for paints can be prepared in a much cheaper manner. All that would be accomplished would be the proof that ferric oxide actually undergoes an extensive molecular modification when heated.

FERRIC OXIDE PIGMENTS FROM ALUM SLUDGE

Alum is manufactured from alum shale and alum earth, the former being a carbonaceous clay shale interspersed with pyrites, and the latter a clay charged with pyrites and bitumen. The raw materials are left in heaps for several years, the pyrites being thereby oxidised with formation of free sulphuric acid and ferrous sulphate. This free acid reacts further on the clay, which it transforms into sulphate of alumina; and by leaching the heaps with water, a solution is obtained which contains the sulphate of alumina and the ferrous sulphate. On the liquor being concentrated, a basic ferric sulphate is deposited, which is worked up into red pigment.

For this purpose it is first levigated in a special manner, the sludge from the pans being placed in a large vat, suffused with water, and kept in slow circulation by stirrers, which distribute the particles in the water, forming a turbid liquid. This liquid is conducted into a gently sloping shute, the sides of which are perforated with openings at certain intervals, to allow part of the liquid to run off into large collecting vessels underneath.

The heaviest of the suspended particles settle down first and are flushed out by the water escaping through the first opening. The finer the particles, the longer they remain in suspension, so that the liquid escaping through the last holes carries off only a very fine powder. The liquid collected in the different vessels is allowed to subside and is then drawn off from the firm deposit. The operation is repeated with fresh quantities of sludge until sufficient sludge has been collected for further treatment. The collecting vessel furthest away from the intake of the shute contains the finest levigated material, and this is used for making the best ochres.

The levigated mass is dried in a very simple manner, being usually spread out on boards, which are exposed to the air in open sheds, covered with a roof to keep out the rain. Here the sludge is left until it forms a pasty or earthy mass, and is then calcined.

The best calcining furnace is of the type used for colcothar; but the pipes must be connected to an exhaust pipe for carrying off the vapours disengaged during calcination.

However, since alum manufacturers do not usually go in for making the highest-grade pigments, simpler calcining furnaces are used, consisting of reverberatory furnaces in which the heating-gases are allowed to act directly on the materials of the charge. A front elevation and section of such a furnace are shown in Figs. 30 and 31. The furnace is constructed with several arches, one above another, marked _c_, _k_, _d_. The charge is introduced through the openings _b_ and _b’_. The furnace chamber is at _a_, and the ashpit at _g_. The gases of combustion flow over the charge on the hearths of the several arches and escape, at the top, into the stack, along with the acid vapours liberated from the glowing mass.

The further the hot gases get away from the fire, the cooler they become, and therefore the less strongly heated the charge on the upper hearths. Consequently, the resulting product (ferric oxide) from the different stages of the furnace differs in colour; and a number of gradations can be obtained by blending. The ferric oxide pigments prepared in this way are not pure oxide, but also contain small quantities of sulphuric acid and metallic oxides which were present in the original crude sludge. However, by reason of the simple process of preparation employed, these pigments are usually sold at lower prices than those from colcothar; and for less fine work they are excellent.