Chapter V: Yellow Earth Colours

All the yellow earth colours, without exception, have ferric oxide as their colouring principle, the differences in shade being entirely due to the varying proportion in which that oxide is present. The various names under which they are known date back to a period when the chemical nature of these colours was still unknown, and have been mostly derived from the locality of origin.

The yellow earths can therefore be divided into two groups, according to their chemical character. The first group, in which the ferric oxide is present as hydroxide, comprises all the ochres, Siena earth, and a number of others which are obtained from native ochre by special treatment. In the colours of the second group, ferric oxide is still the colouring principle, but is combined with other substances in place of water.

It is, as a matter of fact, incorrect to rank the ochres in general as yellow earths, because they can be made to yield nearly every variety of colour from the palest yellow to the deepest red, brown and violet. These colours merit the particular attention of the colour-maker and the painter, being distinguished by very low cost of production, unusual permanence and beauty of tone. In the interests of that highly important matter to the artist, namely the production of colours of unlimited permanence, it is desirable that colour manufacturers should bestow greater care on the manufacture of these colours than has hitherto been the case. An extremely favourable point about nearly all these pigments is that they can be very cheaply prepared by artificial means, so that the manufacturer is in a position to turn out a large number of the handsomest and most durable colours with a small amount of expense and labour.

THE OCHRES

Ochres are found in many localities, most frequently in stratified rock and rubble. The deposits are rarely extensive, mostly occurring in pockets or beds. Wherever found, ochre may be termed a secondary product, that is to say, one that has been formed through the destruction of other minerals. The analysis of ochres from different deposits shows great divergence in composition; and some consist almost entirely of pure ferric hydroxide, that has already undergone natural levigation and can be used as a pigment as soon as dug.

Such a form is, however, rare, and most ochres are intermixed with smaller or larger amounts of extraneous minerals, the contamination being sometimes so great as to preclude the use of the ochre as pigment by reason of the high outlay required for extracting the colouring constituents.

Occasionally, the ferric hydroxide is associated with a certain proportion of clay, and as this increases, the ochre passes over into ferruginous clay. This class can also be used as pigment, in certain circumstances, that is to say when it is sufficiently rich in ferric oxide to furnish a deep red mass on calcination. When, however, the proportion of ferric oxide is low, its pigmentary power is no longer sufficient, and the clay has not the requisite beauty of colour. The ordinary earth used for making tiles is an example of this class, its colour in the raw state being an ugly brownish-yellow, but turning a dull “brick” red when fired.

In some deposits the ferric oxide is accompanied by lime. Unless the latter exceeds a certain proportion, such ochres, too, are suitable as pigments, the lime being easily removed by simple levigation; but when the amount of lime is high, it is difficult to obtain certain highly coloured shades of ochre from such material. These shades entail the calcination of the ochre, and the temperature required is oftentimes insufficient to transform the lime into the caustic state. Moreover, the presence of caustic lime would be a drawback in some cases, it being then impossible to mix the ochre with other colours without endangering the shade through the action of the lime on these latter.

The following analyses will show the percentage composition of ochres from various deposits:

Ochre from--
+-------------+---------+----------+--------------+
| | Comoal | Vierzen. | St. Georges. |
| |(Savoy). | | |
+-------------+---------+----------+--------------+
|Ferric oxide | 19 | 23·5 | 25 |
|Lime | 2 | -- | -- |
|Alumina | 20 | 69·5 | 70 |
|Magnesia | 1 | 69·5 | 70 |
|Silica | 44 | 69·5 | 70 |
|Water | 7 | 7 | 5 |
+-------------+---------+----------+--------------+

In the majority of cases the mineralogical characteristics of an ochre enable conclusions to be formed as to its suitability as pigment. Good ochre is more or less yellow to dark brown in colour, and can easily be crushed between the fingers to a soft, fine powder which feels like powdered steatite and does not produce a sensation of grittiness, this latter indicating the presence of fine grains of sand in the ferric oxide. The behaviour of the ochre in presence of water is specially important. If it adheres firmly to the tongue, and forms a fairly plastic paste when mixed with a little water, the mineral contains a large percentage of ferric oxide, and as a rule will yield ochre of good colour.

In general it may be said that the value of an ochre varies directly with its content of ferric hydroxide or oxide, because when this is large the ochre will furnish a wide range of colours under suitable treatment.

A simple test for quality consists in weighing out an exact small quantity (10 grms.), and heating it to a temperature not exceeding 110° C., until the weight remains constant. A simple calculation then gives the amount of uncombined water in the sample. Since the proportion of such water varies in different parts of one and the same deposit, the test must be repeated, in order to obtain accurate results, on samples taken from different points, or, preferably, on a properly prepared average sample.

Even drying changes the colour of ochre considerably. To ascertain the behaviour of an ochre on calcination, a large sample is dried at 110° C. until the weight is constant, and divided up into a number of small samples weighing, say, 10 grms. each. The samples are then heated to different temperatures, one to the melting-point of lead, another to that of zinc, and so on.

The higher the temperature employed, the more will the colour of the ochre approximate to red; and specimens very rich in ferric oxide will give bright red colours. Beyond this range, a further increase in temperature will give violet shades, varying with the temperature and the duration of heating. After this preliminary test, it is desirable to make another on a larger scale, with quantities up to about 1 lb. For this test, the different kinds of ochre frequently found in the same deposit should be mixed together, in order to obtain an idea of what the mean product, obtained in working on the large scale, will be like.

On the whole, the results of this second test will be the same as in the first series, the only object of the second test being to gain information which may be particularly valuable in practical work. The bottles in which the calcined samples are stored should be marked with the temperature and length of heating, so that, when it is subsequently desired to obtain an ochre corresponding to a particular sample, all that is necessary will be to heat it to the same degree from the same length of time. The performance of this simple test will be of great assistance in standardising the work with a minimum loss of time.

When it is desired to ascertain the composition of an ochre superficially its behaviour towards hydrochloric acid may be noted. A weighed quantity of the freshly dug (undried) ochre is treated with pure acid, free from iron, which will dissolve out the ferric oxide and lime, leaving clay and quartz sand behind. The presence of lime is indicated by effervescence on contact with the acid; and if there is no effervescence, lime is absent. At the end of several hours the acid is carefully decanted from the undissolved residue which is then stirred up with water, left to subside, and weighed when dry. This method will give the amount of substances, other than ferric oxide and lime, in the sample. These substances usually consist of clay or sand.

For a quantitative determination, a small quantity--usually 1 grm.--is weighed out, treated with a corresponding amount of hydrochloric acid, and the solution filtered into a glass. The residue on the filter is washed with distilled water, the washings being united to the acid solution.

This solution is treated with ammonia so long as a precipitate of ferric hydroxide continues to form, this being collected on a tared filter and dried at 110° C. The precipitate may be regarded as pure ferric hydroxide, and its weight will indicate the proportion of hydroxide in the ochre with sufficient accuracy for technical purposes.

In reality, however, it is not pure ferric hydroxide, but contains in addition all the oxides that are precipitable by ammonia, lime being always carried down as well. It is therefore desirable to dissolve the precipitate with a little hydrochloric acid, and reprecipitate with ammonia.

CALCINING (BURNING) OCHRE

In many places ochre is only put through a very simple mechanical preparation before being sold for pigment, namely left to dry in the air so that most of the uncombined water evaporates. No matter how this drying process is protracted, however, it is impossible to get rid of all the water in this way, a certain proportion being retained by the hygroscopic action of the ferric hydroxide, and to expel this the mass must be heated to above 100° C. Drying is usually succeeded by pulverising and sifting the loose earthy mass, which is then ready for sale.

When the ochre contains sand or stones, this treatment is not sufficient, and levigation is necessary. No particular trouble is involved, the mineral being fairly heavy as the result of its content of ferric hydroxide. A simple method of treatment suffices to improve the value of the ochre considerably, and enables a grade that is not particularly bright-coloured in its natural condition to be converted into products of very handsome tone and various shades. This treatment consists in heating the raw ochre to a definite temperature, during which process the colour changes progressively, and any desired tone can be obtained by suddenly cooling the hot mass.

The reason for this phenomenon is that the higher the temperature, the larger the amount of water driven off from the ferric hydroxide, until finally, when a very high temperature has been reached, the whole of the water is expelled, and the ferric hydroxide is transformed into ferric oxide. The hydroxide is brown, whereas the oxide, provided the temperature has not been raised too high, exhibits the characteristic colour known as “iron red.”

Consequently, the colour of moderately calcined ochre ranges through a whole scale from brown to red; and the higher the temperature employed, the redder the tone. If the heating be protracted after all the hydroxide has become oxide, the latter undergoes molecular change, increasing considerably in density and altering in colour; and after very prolonged heating, the colour finally becomes violet.

The calcination, or burning, of ochre is ordinarily performed in a very crude manner. The mineral is crushed to the size of peas, and spread out on an iron plate which is made red-hot. As soon as the ochre has reached the desired shade of colour, it is dropped into a tub of water and then crushed to powder. The calcination requires great experience on the part of the operator, because so long as the product is hot, it has quite a different colour from that assumed on complete cooling. Since only comparatively small quantities of ochre can be treated in this way, and the operation unnecessarily increases the cost of the product, owing to the large consumption of fuel, it is highly desirable to employ a simple calcining apparatus capable of treating large quantities.

Such an apparatus may consist of an iron drum, mounted with a gentle slope inside a furnace, from which it projects at both ends. A shaft carrying a sheet metal worm is rotated inside the drum; and the whole apparatus is very similar to an Archimedean screw.

When the iron drum is raised to a strong red heat, and small quantities of ochre are fed continuously into the upper end of the drum, the rotation of the worm will push the material forward, and contact with the glowing sides of the drum will produce the necessary calcination, the degree of which can be modified by altering the speed at which the worm is turned. The calcined product is discharged at the lower end of the drum, either into a vessel of water, or, if only moderate heating has been applied, direct into a collector.

Fig. 28 represents an apparatus designed by Halliday for the dry distillation of wood waste; but, with slight structural modifications, it can also be used for calcining ochre. The material to be heated is introduced, in small pieces, into the feed hopper _B_, and is carried downward, by the worm _C_, into the red-hot drum _A_, through which it is propelled by the worm _D_ until it drops out, at _F_, into the tank _G_. The length of time the material is subjected to calcination depends on the speed at which the worm _D_ is run. The pipe _E_ carries off the water vapour expelled from the charge.

In order to obtain a uniform product when ochre is calcined in an apparatus constructed on this principle, it is necessary that the material introduced should be fairly regular in size, a condition which is easily fulfilled by squeezing the freshly dug ochre between fluted rollers, and then passing it over a series of screens, each grade being then calcined separately.

Moreover, the apparatus is only suitable for calcining at medium temperatures; and when highly calcined products are in question, the operation is best performed in fire-clay cylinders, or in thick cast-iron drums, similar to gas retorts, built into a furnace.

Other devices for calcining ochre will be described later.

OCHRES FROM VARIOUS DEPOSITS

As previously stated, ochres are frequently met with in Nature, both in the immediate vicinity of iron ore, and also at considerable distances from such deposits. In the latter case, the ochre must be assumed to be the decomposition products of ferruginous minerals and to have been carried off by water until the latter became stagnant and allowed the ochre to settle down. In their method of deposition these ochres are therefore analogous to clay, and they, too, often contain large quantities of extraneous minerals, which have given rise to the diversified substances grouped under the name of ochre.

Although ochres are so widespread in Nature, only certain kinds, found in certain localities, have acquired a high reputation. For the most part, these ochres are such as have already been prepared in a high degree, by Nature, for the purpose for which they are employed.

Thus, we find that all the ochres which have acquired a high repute among painters for particular beauty of tone and permanence, are distinguished by two properties: a high content of ferric hydroxide and great purity.

The former of these properties imparts brightness of colour; and such products will furnish, on calcination, a wide range of colour shades. When, as is the case with the finer qualities of ochre, the mineral contains only a very small proportion of impurities, there is no difficulty in bringing it, by simple grinding or levigation, into a condition in which it is at once fit for use as a pigment.

The Italian ochres have, for long ages, enjoyed a high reputation for their beauty of colour and permanence. This category includes, for example, the renowned Siena earth, Roman earth, Italian umber, and other ochre colours. This high renown is probably due less to the inherent properties of the mineral than to the circumstance that the art of painting attained a high state of development at an early period, and that the artists paid special attention to the use of bright and permanent colours for their work.

Although, at present, many deposits of ochre are known that are quite able to compete, on the score of beauty, with the best Italian products, the good name of these latter has nevertheless been maintained. It is true that the name of Italian ochre is often merely borrowed, for application to a product originating in some other country, varieties of terra di Siena, for instance, being put on the market that have actually been derived from deposits in Germany.

As a result of this custom, certain names, such as terra di Siena, umbra di Roma, have become generic terms, and their use denotes, not an intention to suggest that the earth colours in question really come from Siena or the vicinity of Rome, but that the properties of the article are equal to those of the old-established colours of Siena or Rome.

It would occupy too much space to go into an exhaustive description of all the native varieties of ochre, and would inevitably lead to a good deal of repetition. It will therefore be sufficient, for our purpose, to deal with only a few of them.

The best-known ochres are those of Rome and Siena, the latter being frequently called, in commerce, by its Italian name, terra di Siena.

Roman ochre forms yellowish-brown masses, of fairly fine texture and composed of ferric hydroxide and clay. They are put on the market both in the raw and calcined state. On calcination, the colour soon changes to red, and if carefully performed, the resulting colours have a very warm, fiery tone.

Closely approaching Roman earth is the English ochre, which is worked more particularly in Surrey, and is not infrequently sold as Roman. In many deposits this English ochre occurs in such a high state of purity that the best pieces are picked out and sold without being even crushed or ground. The pieces of lower quality are very carefully ground and levigated, for the purpose of being calcined for the production of different shades, and then furnish highly prized colours.

In point of chemical composition, the ochre family also includes terra di Siena, bole, umber and Cassel brown. These minerals, however, are not yellow like ochre, but brown, and will therefore be dealt with along with the brown earth colours.

ARTIFICIAL OCHRES

Products very similar, both in chemical composition and colour, to the native ochres can also be very simply and cheaply made by artificial means. Their preparation may be particularly recommended to colour-makers who desire to turn out a wider range of iron pigments, but are not in a position to obtain natural ochres at a low price.

In the manufacture of artificial ochre, an endeavour is made to imitate the natural processes which have led to the formation of ochre, and, of course, to avoid anything likely to hinder the production of a suitable colour earth, for example the presence of sand or a considerable admixture of extraneous minerals.

As already mentioned, the chief impurities in natural ochres are clay and sand, both of which can be easily excluded during the manufacture of artificial ochre, or their amount controlled in such a manner that paler or darker products can be obtained at will, and the tone varied, in any desired manner, by calcination, as in the case of the native article.

The raw material for artificial ochre is always a ferrous salt, which can be purchased in large quantities and at very low prices, namely green vitriol, which, in the pure state, consists of ferrous sulphate, FeSO{4} + 7H{2}O. This substance forms sea-green crystals, which are readily soluble in water and impart an objectionable inky flavour thereto. 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.

When a solution of green vitriol is left exposed to the air, basic ferric sulphate is also formed, which settles down to the bottom of the vessel as a rusty powder. If, however, a corresponding quantity of sulphuric acid be added to the solution at the outset, the resulting ferric sulphate remains in solution.

On treating the green vitriol solution with one of caustic potash, caustic soda or quick lime, the ferrous oxide is thrown down as the corresponding hydroxide, forming a voluminous greyish-green precipitate. This hydroxide still possesses a great affinity for oxygen, and when the precipitate is brought into contact with air, its colour rapidly changes to a rusty red, through the transformation of the ferrous hydroxide into ferric oxide. The ferrous hydroxide can also be precipitated by alkali carbonates, the deposits behaving in exactly the same manner as that thrown down by the caustic alkalis.

Various methods can be adopted in the preparation of artificial ochre, the selection depending on the properties desired in the finished product. To obtain an ochre with particularly good covering power, the method must be different from that employed to furnish a cheap product, in which low price is more important than covering power.

In the former case, the ferrous hydroxide is mixed with substances which, in themselves, possess fairly high covering power, such as chalk or white clay; in the second, gypsum, which is of low covering power, is used.

The preparation of the cheapest kinds of artificial ochre will be described first, followed by that of the higher grades which belong to the most valued artists’ colours.

For cheap artificial ochres, the ferrous hydroxide is thrown down by caustic lime from a solution of green vitriol. According as a lighter or darker shade is required, two to three parts of ferrous sulphate are dissolved in water, care being taken to select crystals of a pure green colour, since those that have a rusty look are only imperfectly soluble, because they contain basic ferric sulphate.

The solution will always be cloudy, owing to the partial precipitation of the hydroxide by the lime in the water; but this is immaterial. For the precipitation, a milk of lime is prepared by slaking one to two parts of quicklime (according to the quantity of ferrous sulphate to be treated) in water, and stirring this up in enough water to make a thin milk. Care must be taken to exclude any large particles of lime, since these would find their way into the finished product and make the colour uneven. On this account, the milk of lime should be carefully strained through a loosely woven cloth or fine sieve, into the precipitation vessel.

The ferrous sulphate solution is then poured in, the mixture being kept stirred, and an ugly, grey-green precipitate is produced, consisting of a mixture of ferrous hydroxide and calcium sulphate, the reaction being explained by the equation:--

FeSO{4} + Ca(OH){2} = Fe(OH){2} + CaSO{4}.

The larger the amount of ferrous sulphate solution added to the milk of lime, the darker the resulting ochre. As soon as all the ferrous sulphate is in, the stirring is suspended, and the liquid is left until quite clear. The water is drawn off through tapholes in the side of the vessel, care being taken not to disturb the fine precipitate, and fresh water is added, in which the deposit is stirred up and again left to settle down. This operation, which is once or twice repeated, is to wash the precipitate.

When this object has been sufficiently accomplished, the mass is shovelled out of the vessel and spread thinly on boards, where it is left until the desired shade of colour has been attained, the colour changing quickly on exposure to air, owing to the oxidation of the ferrous hydroxide into ferric hydroxide. To ascertain whether oxidation is complete, a large lump of the mass is broken across; and if it is of a uniform yellow-brown colour throughout, without being darker on the outside than in the middle, all the ferrous hydroxide will have been transformed into the ferric state. The product can now be dried at once, and when ground will be ready for sale.

To obtain different varieties from the product, it is carefully heated (in a finely powdered condition) in shallow pans; but the operation needs caution, or the water in the gypsum present will be expelled, giving rise to drawbacks that are manifested when the colour is used.

For instance, in mixing such a colour with water, the gypsum would again absorb water and cause the whole mass to set as a useless solid lump. Since gypsum parts with its water at a comparatively low temperature, it is better not to heat these cheap ochres at all, but to obtain the various shades by modifying the proportion of ferrous sulphate employed.

Another defect of the ochres prepared by this method resides in the excess of lime present, it being impracticable to measure out the quantity of lime used with such accuracy that only just enough is taken to precipitate the ferrous hydroxide, there being always a slight excess. This lime is transformed into calcium carbonate on the mass being exposed to the air, just as in the preparation of Vienna white; but as the saturation with carbon dioxide takes a considerable time, some of the lime remains in the caustic state and is liable to affect other colours that may be mixed with the ochre.

An artificial ochre uniting in itself all the qualities of the natural product, and also capable of being shaded by burning, can be prepared in the following manner. An accurately weighed quantity of pure crystallised ferrous sulphate is dissolved in a definite amount of water, and the solution is treated with successive small portions of crude nitric acid, until all the ferrous oxide has been changed into the ferric state. The change can be detected by a very decisive test. If a liquid containing ferric oxide in solution is brought into contact with a solution of red prussiate of potash (potassium ferricyanide), no precipitate is formed in the absence of ferrous oxide, but only a brown coloration; whereas, if ferrous oxide is present, a beautiful blue precipitate is formed at once, the colour of which is so intense that very small quantities of ferrous oxide can be detected by this means.

For the purpose now under consideration, the presence of small amounts of ferrous oxide in the solution is immaterial, because they are soon changed into ferric oxide on exposure to the air. It might, therefore, be asked, why take the trouble to oxidise the ferrous oxide by means of an agent involving expense, which could be saved by allowing the oxidation to take place in the air?

The advantage, however, of the direct employment of a solution of ferric oxide is that it gives at once a colour that can be dried straight away; whilst at the same time the colour undergoes no change in drying, whereas it does when ferrous oxide solution is used.

The method of producing ochres from this ferric solution varies according as the product is to be used without any further treatment than drying, or is to be modified by firing.

In the former event, caustic lime is again used as the precipitant, but in only just sufficient quantity to throw down all the ferric oxide in the solution. This amount can be calculated exactly, 36·84 parts by weight of pure burnt lime being required for every 100 parts of pure ferrous sulphate taken. The actual quantity, whether larger or smaller, will depend on the relative purity of the sulphate and lime; and this can readily be ascertained by a simple trial.

The lime is used in the form of milk of lime, as already described. If lime alone is employed, the precipitate will consist of pure ferric hydroxide and the calcium sulphate thrown down at the same time. The resulting colour, when dried, will be an intensely brown mass, which can be used in place of the very dark natural ochres.

In order to obviate entirely the disadvantages resulting from the presence of a large amount of caustic lime in the precipitate, fine levigated chalk or white clay is added in the preparation of the lighter shades of ochre, the addition being made as soon as the two ingredients have been brought into contact; and the mixture is thoroughly stirred, to ensure uniform admixture with the ferric hydroxide. The colour of the settled deposit will be lighter or darker in proportion to the amount of chalk or clay employed; and in this way the whole range of shades from pale yellow to bright brown can be obtained without the application of heat.

Ochre that has been made with chalk is unsuitable for toning by heat, because this treatment would causticise the lime, and the ochre could not be mixed with other colours, since these would be affected by that substance. On the other hand, when white clay is used in preparing the ochre, the latter can be more easily toned by firing, provided care be exercised in the process. The ochre must be dried completely in the air, and either spread out in thin layers on iron plates, for the burning process, or else put into a drum, of the kind already described, in which the mass is moved onward by a worm.

The clay remains unaltered in firing, but the gypsum parts with its water of crystallisation. In order to restore the latter, the ochre issuing from the drum is discharged direct into a vessel of water, in which it can be kept in constant motion by a stirrer. The water is soon warmed by the heat of the mass, and absorption by the gypsum proceeds at a rapid rate. When the whole charge has been fired and collected in the vessel of water, the stirrer is stopped and the precipitate dried, being then ready for use.

In certain circumstances, ochre can be made by other methods. In large towns, ammonium salts are sometimes obtainable at a moderate price, being manufactured in large quantities as a by-product in gasworks. For our purpose, crude gas liquor might be used, since it contains ammonia for the precipitation of the ferric hydroxide. In most cases, however, this gas liquor contains only very small quantities of ammonia, and, therefore, in a works of any size, very large vessels would be needed for the production of a comparatively small quantity of ochre. On this account, preference is given to crude carbonate of ammonia, which is also obtainable at low prices.

On bringing a solution of this salt into contact with one of ferric oxide, ferric hydroxide is precipitated, and the sulphate of ammonia resulting from the reaction remains in solution. By stirring white clay into the liquid at the same time, the ochre can be correspondingly lightened in shade.

The precipitates obtained in this way can be dried at once, and converted into any shade obtainable with natural ochre, from brown to red, by strong firing. The sulphate of ammonia still remaining in the air-dried product is completely volatilised by the heat, and the resulting ochres are even superior to the natural varieties in beauty and permanence.

OCHRES AS BY-PRODUCTS

In the manufacture of certain chemicals, substances of divergent composition are obtained which are sold under the name of ochre and are used as painters’ colours. Whereas ochre, properly so-called, consists of either ferric hydroxide or ferric oxide in association with clay, lime, etc., the products now under consideration are basic ferric salts composed of varying quantities of ferric oxide in combination with certain proportions of sulphuric acid.

These ochres are obtained as by-products in the manufacture of green vitriol from pyrites, and in alum manufacture; and, according to their origin, they are classed as vitriol ochre, so-called alum sludge, and pit ochre. All the basic ferric sulphates of which they are composed form fairly large crystals, and, therefore, in most cases, the covering power is small. On this account the products are of low grade and are put on the market at low prices, for which reason they are largely used in making cheap paints.

_Vitriol Ochre._--Commercial green vitriol is, for the most part, manufactured from native sulphides of iron. When many of these sulphides are piled in heaps and left to the action of the air, oxygen is gradually absorbed and green vitriol is formed which is dissolved out by rain and is collected in large clarifying tanks.

In the case of pyrites, however, the mineral must first be roasted in a current of air, since otherwise its conversion into green vitriol would only proceed in a very sluggish manner. In any event, the aqueous solution of ferrous sulphate has to be concentrated, by evaporation, to the point at which the green vitriol crystallises out.

Both in the clarifying-tanks and--still more so--in the evaporating-pans, a rusty-looking sediment forms at the bottom, consisting of basic ferric sulphate. This originates in the partial oxidation of the ferrous oxide (first formed) while the pyrites is exposed to the air, and since the quantity of sulphuric acid present is insufficient to saturate all the ferric oxide, basic salts are produced.

The yellow-brown sludge deposited in the pans during the concentration of crude green vitriol liquor, constitutes the product termed vitriol ochre, which contains varying amounts of ferric oxide, sulphuric acid and water, according to the quantity of ferric oxide resulting from the oxidation of the pyrites and the character of the latter, _e. g._:--

Ferric oxide 65–70%
Sulphuric acid 14–16%
Water 13–16%

Although the colour of these ochres is not particularly handsome, they can be transformed, by firing, into colours of fairly good quality. As this subject will be more thoroughly gone into when dealing with the preparation of the red iron pigments, the applicability of these ochres will only be casually referred to here. During the burning process, these ochres, of course, part with the whole of their contained water; and by protracted, high calcination, the whole of the sulphuric acid can also be expelled, so that finally nothing but pure ferric oxide is left.

_Alum Sludge._--Solutions of crude alum always contain a certain amount of ferric oxide which settles down at the bottom of the pans during concentration. This sludge, too, consists of basic ferric sulphate, but is inferior in covering power to vitriol ochre, the crystals being of coarser grain. On the other hand, the ochreous sediment from the alum concentrating-pans has the valuable property of being readily transformable into red-brown to pure red tones by burning. For this reason, particular attention has been devoted to this sludge in a number of alum works.

Since the products are only of value when burned, and the shades thereby obtained are always red, they will be dealt with more fully along with the red earth colours.

_Pit Ochre._--Springs containing small quantities of ferrous sulphate and other salts are met with in many iron mines, but, in most cases, the amounts are too small for their recovery by artificial concentration to be contemplated. If, however, the conditions allow of the springs being easily diverted, they may often be utilised for the preparation of low-grade ochre.

The chemical composition of these pit ochres varies considerably, and depends on the geological character of the locality. Water can only dissolve such minerals as occur in the form of fairly readily soluble compounds; and for this reason pit waters are always solutions of the metals which are found in the mine.

The variety of compounds that may be present in an ochre can be seen from the subjoined analyses of ochres deposited from pit waters at Rammelsberg. As elsewhere, two distinct classes of ochre are met with, having a conchoid and an earthy fracture respectively. The latter usually contain rather more ferric oxide, and, in particular, a higher content of foreign substances, the most important of which is quartz sand. In the Table, the ochres with conchoid fracture are marked A, and those with an earthy fracture, B.

A. B.
Ferric oxide 68·75 63·85
Zinc oxide 1·29 1·23
Copper oxide 0·50 0·88
Sulphuric acid 9·80 13·59
Water 15·52 18·45
Clay and Quartz 4·14 2·00

The preparation of the ochre is a simple matter, consisting in collecting the mass and sorting out the loose, earthy portions of a pure yellow colour from the denser and darker parts. The former are dealt with separately, usually by a simple process of levigation, for the sole purpose of getting rid of the earthy matter, quartz sand in particular.

The denser varieties require much more work, but yield a far superior product, which, by suitable treatment, can be converted into the finest grades of ochre. The first operation consists in a very careful crushing, and as the pieces are often very hard, they are treated in ordinary or stamp-mills, edge-runners being also employed with advantage.

The product reduced by any of these means is passed through a number of sieves, to separate the fine particles from the coarse; and the finest dust is burnt. This last treatment causes a considerable loss in weight, both the accompanying water and most of the sulphuric acid being volatilised. However, since, as already stated, all varieties of ochre can be obtained, the process is consequently very remunerative notwithstanding the loss in weight it involves.

_Yellow Earth._--From the particulars given in the general description of the earth colours, yellow earth may also be regarded, to some extent, as an ochre, but one containing a large proportion of foreign substances. It might, however, be more accurately termed a clay contaminated with a considerable amount of quartz sand and a certain proportion of ferric oxide. The method of preparation is on the same lines as for ochre, but burning is never practised, nor is the treatment so careful as for the better grades of ochre, the low price of the colour making this unremunerative.