Chapter II: The Raw Materials for Earth Colours

The minerals constituting the raw materials for the preparation of the earth colours occur under very divergent conditions in Nature. Some of them, such as chalk, form immense deposits, even whole mountains, whilst in other cases, _e. g._ the blue ferruginous earths, the occurrence is connected with certain local conditions, and many are found only in isolated deposits, as pockets or beds. This last is the case, for instance, with the handsome brown iron pigments; and indeed the names by which they are known indicate that they are only found in well-defined localities, or that they are met with of special quality there. The brown earth colour known to all painters as Terra di Siena, is found at many other places as well as near Siena, but the product from that city acquired aforetime a special reputation for beauty, and therefore all similar earths, provided they are equal to that from Siena, also bear the same name in commerce.

A number of raw materials for the preparation of earth colours are found, it is true, in many deposits, but their utilisation depends, in turn, on local conditions. For example, many copper mines contain, in addition to the other cupriferous minerals, those used, in the powdered state, as ultramarine or ultramarine green, and not infrequently lumps of mineral are found containing both blue and green together. However, it is only when these minerals occur in sufficient quantity to make the necessary sorting profitable that their manufacture into pigments can be regarded as practicable.

Before commencing to work a deposit it is essential to make sure whether the raw material, or pigmentary earth, is actually suitable for the manufacture of earth colour. Even the general character of the material is important, those of soft, earthy consistency being much easier to treat, and the cost of preparation smaller, than if the raw material be hard, tough and crystalline.

The extent and thickness of the deposit, and the ease with which it can be worked, also play an important, and even decisive part, since, other conditions being equal, it will not pay to erect a colour works unless the raw material is available in sufficient quantity and is cheap. Generally, the deposit is not homogeneous throughout, the mineral being purer in some places and more contaminated with gangue in others. The percentage of moisture also varies, and in short, a number of circumstances must be taken into consideration in forming a conclusion as to whether a deposit is workable or not.

In order to arrive at a reliable opinion on all these conditions, sampling is indispensable. If the samples are of uniform character, they can be mixed together to make an average sample. But if they differ considerably in appearance, general character, proportion of gangue, etc., it is preferable to examine them separately, more especially when the area which each represents is large.

The examination should extend, on the one hand, to the natural percentage of moisture, and, on the other, to the purity of the material. The water content is determined by thoroughly drying a weighed sample, bearing, however, in mind the fact that pigmentary earths of a clayey nature vary in water content according to the time of year, besides changing in accordance with the weather when the won material is stored in the open.

The purity can only be ascertained by an examination in which a sample of the soft, clayey material is crushed and passed through a narrow-mesh gauze sieve, the amount of the coarse particles--sand, small stones, etc.--remaining on the sieve being determined. A more accurate method, of course, is to separate the true pigmentary earth from the gangue by levigation. For this purpose, a weighed quantity of the crushed, air-dry sample is placed in a relatively narrow glass vessel and thoroughly mixed with water, the turbid supernatant liquid being poured off after a short interval. The residue is repeatedly treated in the same way, until no more fine particles remain in suspension, the residue then consisting of impurities, or gangue. Of course, the washings can be collected, the suspended matter allowed to settle, and finally weighed in an air-dry condition. By this means an approximate idea of the yield of earth colour can be obtained at the same time.

Raw materials which are not amorphous, soft and clayey must first be crushed, an operation facilitated by heating to redness and quenching in cold water. Oftentimes the heating causes a change of colour and improves the covering power--a point to which reference will be made later on.

In the following description of the various raw materials, the chemical composition of the pure minerals will be given, together with an enumeration of the most common impurities.

(A) WHITE RAW MATERIALS AND PIGMENTARY EARTHS

_Limestone_ (_Calcite_, _Limestone_, _Chalk_)

The number of materials furnishing white earth colours is comparatively large, and these colours are particularly important, because, not only are they extensively used by themselves, but they also serve as adjuncts to other colours and for the production of special shades. The chief raw material for the preparation of white earth colours is the mineral calcite in its numerous modifications.

Calcite, or calc spar, occurs very frequently in Nature, and is one of the most highly diversified minerals known. In its purest state it appears as “double spar” (calcite), in the form of water-white crystals, which are very remarkable for certain optical properties. White marble is also a very pure variety of calcite, in which the individual crystals are very small. The various coloured marbles owe their appearance to certain admixtures of extraneous substances, chiefly metallic oxides.

No sharp line of demarcation separates marble from ordinary limestone, the difference between them really consisting only in the degree of fineness of grain; and all limestones which grind and polish well may be classed as marble. As is the case with marble, there are also limestones of various colours, grey being, however, the most common. This grey limestone forms huge mountain masses which, in Europe, follow for example, the Alpine chain on its northern and southern edges.

A few other examples of calcite may be mentioned which occur in certain localities and, in part, are still in course of formation. To these belong the stalactites and stalagmites, which sometimes consist of extremely pure calcite. They are formed by the action of water, containing carbonic acid in solution, which trickles through cracks and cavities in limestone rock and dissolves out calcium carbonate from the adjacent stone. On prolonged exposure to the air such water gives off its free carbonic acid again; and as the calcium carbonate is insoluble in pure water, it separates out in crystalline form. The masses formed in this way usually resemble icicles in shape, and the finest examples are to be found in the well-known stalactite grottoes at Krain, whilst the grotto at Adelsberg is renowned for its beautiful stalactites. Occasionally, stalactites have an opaque yellow or brownish tinge, which they owe to the presence of iron oxide.

A formation similar in its origin to stalactites is the so-called calc sinter and calcareous tuff. The former often occurs in cavities as irregular masses which, in some places, enclose large quantities of fossil animal bones, in which case they form “bone breccia” (crag breccia). Calcareous tuff is deposited from numerous springs, occasionally in very large quantities, enveloping plants and sometimes forming thick deposits in which the structure of the plants can be clearly recognised.

In some places a more or less pure white, extremely friable variety of calcite is met with under the name “mountain milk” or “mountain chalk” (earthy calcite), which seems to be a decomposition product, and consists of a mixture of arragonite and chalk. Arragonite--which will be referred to later--is completely identical, chemically, with calcite--both being composed of calcium carbonate--the sole difference being their crystalline form.

The most important for the colour-maker, however, is the variety known as chalk. This is really a fossil product, _i. e._ it consists of the microscopic shells of marine animals united into solid masses. Despite the smallness of these animals, their epoch lasted long enough for their shells to form entire mountains which are encountered all over the world. A large part of the coast of England, the island of Rügen, and many other localities, consist entirely of chalk.

In many cases, chalk is found interspersed with nodular masses of flint, and in some places it also contains great quantities of the remains of other marine animals, such as sea urchins, the spines of which occur in such numbers in certain kinds of chalk as to unfit them entirely for use as a pigment.

The foregoing varieties of calc spar are the most important, and also occur in large quantities; but, to complete the tale, it is necessary to mention also a few others which, however, are only found in small amounts. To these belong, for example, anthracolite, a limestone stained quite black by coal; the oolithic limestones or roe stones, which are composed of granules resembling fish roe; muschelkalk, which is also of fossil character and is almost entirely composed of mussel shells cemented together with lime; the marls, which consist of calc spar mixed with varying quantities of clay and consequently often bear a great resemblance to loam in their properties. A few of these varieties find extensive employment for certain purposes, some marls for instance being used for making hydraulic lime, whilst all modifications of calc spar that are sufficiently pure can be burned for quick lime.

It has already been stated that the mineral arragonite is identical, chemically, with calc spar, since both consist of calcium carbonate, but differ in their crystalline habit. Thus, whereas the crystals of calc spar belong to the rhombohedral or hexagonal system, those of arragonite are always rhombic. This occurrence of one and the same substance in two different crystalline forms is known as dimorphism, and calcium carbonate is therefore dimorphous. Whether calcium carbonate assumes the form of calcite or arragonite depends entirely on physical causes. When the deposition of the carbonate takes place from a cold solution the shape of the crystals is always one belonging to the hexagonal or rhombohedral system; but when it is from hot solution, rhombic crystals are invariably formed, calc spar resulting in the former case and arragonite in the latter.

These different methods of formation which can be carried out in the laboratory by producing the requisite conditions, occur on the large scale in many parts of the world. Wherever a hot spring comes to the surface, containing considerable amounts of lime in solution, this separates out in the form of arragonite, which received its name from the circumstance that specially handsome crystals of this mineral are found in Arragon.

One of the best-known places where the formation of arragonite can be observed at the present time is Carlsbad in Bohemia. The hot springs there deposit a very large amount of lime, which is stained more or less yellow or red by the presence of varying quantities of iron oxide, and, under the name of “sprudelstein” is used for producing various works of art. When the hot springs bring up particles of sand, the lime substance incrusts these sand grains, forming globular masses resembling peas, and consequently named pisolite.

In chemical composition, calcite and arragonite consist of a combination of calcium oxide (lime) and carbonic acid, the formula being expressed by CaCO{3}. Calcium carbonate is insoluble in pure water, but dissolves somewhat freely in water charged with free carbonic acid. It is assumed that a compound is formed, which is known as calcium bi- (or acid) carbonate, is very unstable and can only exist in a state of solution. When a solution of calcium bicarbonate--which can be prepared by passing carbonic acid gas through water containing finely divided calcium carbonate in suspension--is exposed for some time to the air, it soon becomes cloudy, and a deposit of calcium carbonate settles down at the bottom of the vessel, because, in the air the dissolved calcium bicarbonate is decomposed into free carbonic acid gas and calcium carbonate, which latter, as has been mentioned, is quite insoluble in water. It has already been stated that this phenomenon goes on in Nature in the formation of stalactites, lime sinter and calcareous tuff.

Calcium carbonate is readily soluble in acids, the contained carbonic acid being liberated (as carbon dioxide) with effervescence. When such acids are employed for solution as form readily soluble salts with lime, such as hydrochloric, nitric, acetic, etc. acids, a perfectly clear solution is obtained; but if sulphuric acid is used, a white pulpy mass is formed, consisting of calcium sulphate, or gypsum, which, owing to its low solubility, separates out as small crystals. 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:--

CaCO{3} = CaO + CO{2}
Calcium carbonate = Quick lime + Carbon dioxide

If burnt lime be left exposed to the air for some time, it again gradually absorbs carbon dioxide and is reconverted into calcium carbonate. When burnt lime is sprinkled with water it takes up the latter avidly, becoming very hot and finally crumbling down to a very friable white powder, consisting of slaked or hydrated lime (calcium hydroxide, Ca(OH){2}). The considerable rise of temperature in quenching the lime is due to the chemical combination of the calcium oxide and water.

Both quick and slaked lime dissolve to a certain extent in water, and impart strongly alkaline properties thereto, lime being one of the strongest of bases. On exposure to the air, the solution of quick lime in water (lime-water) quickly forms an opalescent superficial film of calcium carbonate, and in a short time no more lime is present in solution, the whole having been transformed into calcium carbonate, which settles down to the bottom of the vessel as a very fine powder.

Limestone that consists entirely of calcium oxide and carbon dioxide is of rare occurrence in Nature, foreign substances being nearly always present. Since the nature of these admixtures is of the greatest importance to the colour-maker, owing to the considerable influence they exert on the suitability of the minerals for his purposes, it is necessary that these extraneous substances occurring in limestone should be more closely described.

Nearly all varieties of limestone contain certain proportions of ferrous and ferric oxides. The presence of ferrous oxide, when the relative amount is but small, cannot be detected by mere inspection; and even many limestones containing really appreciable quantities of ferrous oxide are pure white in colour so long as they are in large lumps. If, however, such a limestone be reduced to powder and exposed to the air for a short time, it gradually assumes a yellow tinge, the depth of which increases with the length of exposure.

The cause of this change is due to the fact that ferrous oxide has a great affinity for oxygen, by absorbing which it changes into ferric oxide. (Ferrous oxide consists of FeO, ferric oxide of Fe{2}O{3}.) Ferrous oxide and its compounds are of a pale green colour which is not very noticeable, whereas ferric oxide has a very powerful yellow colour, and consequently the limestone, when its superficial area has been greatly increased by reduction to powder, assumes the yellow tinge due to ferric oxide. A limestone exhibiting this property can evidently not be used for making white earth colours, but is, at best, only suitable for mixing with other colours.

Occasionally, limestone contains varying quantities of magnesia, and when this oxide is present in large amount, changes into another mineral known as dolomite. In many places this dolomite forms large masses of rock, which, however, is not employed for making colours, owing to the yellow shade imparted by the fairly large amount of ferric oxide present.

_Gypsum_ (_Alabaster_)

This mineral occurs native in many places, and is frequently worked for a number of purposes. Gypsum occurs in Nature in a great variety of forms. The purest kind is met with either as water-clear crystals, which cleave readily in two directions, or as transparent tabular masses (selenite) which also cleave easily. Micro-crystalline fine-grained gypsum is milk-white in colour, highly translucent and is largely used, under the name of alabaster, in sculpture. Owing to its low hardness, alabaster can be readily cut with a knife, and on this account is frequently shaped by planing or lathe-turning.

Gypsum is generally met with in dense masses, which may be of any colour, grey, blue and reddish shades being the most common, whilst pure white is rarer. The dark-coloured varieties can only be used for manurial purposes; but the white finds a twofold application as a pigment, and, in the calcined state, for making plaster casts.

In point of chemical composition, gypsum consists of sulphate of lime, or calcium sulphate (CaSO{4} + 2H{2}O). It is soluble in water, but only in such small quantity that over 400 parts of the latter are needed to dissolve one part of gypsum. On being heated to between 120° and 130° C., gypsum parts with its two molecules of combined water and becomes anhydrous calcium sulphate or burnt gypsum. When this latter is stirred with water to a pulp, it takes up the water again, with considerable evolution of heat, swelling up considerably and setting quickly to a solid mass.

The number of substances exhibiting this property being small, burnt gypsum is very frequently used for making casts of statuary, and for stucco work in building. Finely ground white gypsum can also be used as a pigment, but is inferior to calcium carbonate in covering power, and is therefore seldom employed for this purpose, though frequently added to other colours. The mineral known as muriacite or anhydrite consists of anhydrous calcium sulphate; and is therefore similar in composition to burnt gypsum; but it lacks the property of combining with water when brought into contact therewith.

_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, BaSO{4}. 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. Powdered native heavy spar, even when ground ever so fine, has not enough covering power, this property being peculiar to the artificial product.

When it is desired to mix other pigments with a white substance, to lighten the shade, permanent white can be specially recommended, since it is quite insensitive to atmospheric influences and has no chemical action on the colour, so that it can be used with even the most delicate colours without risk. In this way, not only can the colours be considerably cheapened, but over-dark colours can be shaded to the desired extent. Another advantage of such mixtures is that a smaller quantity of oil or varnish is required, barytes only needing about 8% of its own weight of vehicle to form a workable mixture, whilst other pigments take five times as much, or even more. In many cases the low covering power of barytes enables large quantities to be added, and this reacts favourably on the consumption of varnish.

Another barium mineral is witherite, or barium carbonate. This is not used direct as a pigment, but--in contrast to heavy spar--is readily soluble in hydrochloric acid, and therefore serves as raw material for the preparation of artificial barytes and other barium compounds, the first-named being obtained by treating a solution of barium chloride with sulphuric acid, insoluble barium sulphate being precipitated.

_Talc_, _Soapstone_, _Steatite_

Talc occurs in Nature either as a pure white mass, of greasy lustre, or occasionally as yellow, green or grey masses, all distinguished by a peculiar greasy appearance and a soapy feel. This appearance is common to all the minerals of the steatite group, and is the cause of their generic name, soapstone. Although the steatites have a very low degree of hardness--most of them can be scratched by the finger-nail--some difficulty is encountered in reducing them to fine powder. Calcination usually increases the hardness considerably, so that, in some cases, the calcined mineral gives off sparks when struck with a steel instrument.

Soapstone is composed of magnesium silicates, containing varying proportions of magnesia and silica, together with a small quantity of water, apparently in a state of chemical combination, a very high temperature, approaching white heat, being required to effect its complete expulsion, the residue then attaining the aforesaid high degree of hardness. The composition of talc can be expressed by the symbol H{2}Mg{2}(SiO{3}){4}, corresponding to 63·52% of silica, 31·72% of magnesia, and 4·76% of water. In some varieties of talc, a portion (1–5%) of the magnesia is replaced by ferrous oxide. Talc is quite unaffected by the action of dilute acids, boiling concentrated sulphuric acid being required to decompose it, with separation of silica.

Owing to its low specific gravity and chemical indifference, talc is suitable for lightening the shade of certain lake pigments. It can also be used as a pigment by itself, and also as a gloss on wall-paper, for mixing with paper pulp, and for various other purposes.

_Clay_

The mineral known as clay is, in all cases, a product of the decomposition of other minerals, mainly felspar. This substance is a double silicate of alumina and potash, K{2}O.Al{2}O{3}.(SiO{2}){6}. Pure kaolin is Al{2}O{3}(SiO{2}){2} + 2H{2}O, or 46·50% silica, 39·56% alumina, 13·9% water.

Clay may be supposed to have been formed by the conversion of felspar, under the action of air and water, into silicate of alumina, the silicate of potash being dissolved out. Being insoluble, the silicate of alumina would be transported by the water, in a very fine state of division, and finally deposited as a sediment, which in course of time became a solid mass. This, when again brought into contact with water, forms a very plastic pulp which, when dried and baked, forms a solid mass, brick, which is no longer affected by water. Perfectly pure clay forms a white mass, which, under the name of China clay or kaolin, is used for making porcelain, and is only occasionally met with in large quantities.

Pure kaolin is characterised by its great chemical indifference, being decomposed only by strong alkalis and sulphuric acid. At the high temperature of the pottery kiln, kaolin sinters to a very compact mass, but cannot be fused, except when small quantities are subjected to the intense heat of the oxyhydrogen flame, whereupon it fuses to a colourless glass of great hardness.

In an impure state, silicate of alumina occurs frequently in Nature, and then forms the minerals known under the generic names of clay, loam, marl, etc. These impure clays contain varying proportions of extraneous minerals which produce changes in the physical and chemical properties. They are grey, blue or yellow in colour, the grey and blue varieties mostly containing appreciable quantities of ferrous oxide, whilst the yellow kinds contain ferric oxide. When fired, all of them become yellow or red, the ferrous oxide being transformed into ferric oxide by the heat. Some fairly white clays are high in lime, which makes them fusible at high temperatures. In some very impure kinds, even the comparatively low heat of the brick-kiln is sufficient to cause partial fusion. For colour-making, the white clays, especially kaolin and pipeclay, form a highly important material, being procurable at very low prices and fairly easy to prepare.

The white clays are either used as pigments by themselves, or for mixing with other colours of low specific gravity.

(B) YELLOW EARTHS

The number of yellow earths is large, but most of them exhibit a certain similarity in chemical composition, the pigmentary principle in the majority being either ferric oxide or ferric hydroxide. The former is yellow, the latter brown, and the colour of the minerals resembles that of the preponderating iron compound.

_Brown Ironstone_

The mineral known as brown ironstone consists of ferric hydroxide, and usually forms compact masses, no decided crystals having, so far, been observed. The lumps have an irregular or earthy fracture, a hardness of 5–5·5, and a sp. gr. between 3·40 and 3·95. The colour ranges, in the different varieties, from yellowish (rusty) brown, through cinnamon to blackish-brown. The chemical composition of the pure lumps may be expressed by the symbol 2Fe{2}O{3} + 3H{2}O; but a little manganese oxide and silica is generally present even in the pure kinds.

The chief varieties of this mineral are:--

(_a_) Fibrous brown iron ore, or brown hematite, mostly forming reniform or stalactitic masses.

(_b_) Compact brown ironstone, usually in dense masses, and not infrequently also appearing in pseudo-morphs of other minerals.

(_c_) Ochreous brown ironstone. This variety is the most important to the colour-maker, for whose purposes it is preferably used. It nearly always forms very loose, earthy masses, yellow or brown in colour.

(_d_) Clay ironstone. This consists of a mixture of the above-mentioned varieties with variable proportions of other minerals, clay being the most common ingredient. Nodular iron ore, oölitic, bog and siliceous ore belong to this class, as also the minette ores that are found in great abundance in Alsace-Lorraine, Belgium and Luxemburg, and are classed with the oölitic brown ironstones.

In most cases, the varieties enumerated are found together, and are used for the production of iron. The ochre constituting the most interesting member to the colour-maker often occurs as deposits embedded in dense masses of brown ironstone, though in many places it is found by itself.

CHEMICAL COMPOSITION OF VARIOUS BROWN IRONSTONES

The following analyses of brown ironstone from different deposits will give an idea of the composition of these minerals.

_Ordinary Brown Ironstone_

+-------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
| | 1. | 2. | 3. | 4. | 5. | 6. | 7. | 8. | 9. | 10.|
+-------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
|Ferric oxide |76·76|73·75|77·54|78·50|78·42|48·25| -- | -- | -- | -- |
|Manganese | | | | | | | | | | |
| oxide |16·56|10·50| 2·70| 1·95| 1·30|24·73| -- | -- | -- | -- |
|Iron | -- | -- | -- | -- | -- | -- |33·9 |37·88|54·80|55·04|
|Manganese | -- | -- | -- | -- | -- | -- | 0·15| 0·17| 0·57| 0·20|
|Alumina | -- | -- | -- | -- | 1·13| 2·33|10·03| 0·88| 1·15| 2·50|
|Lime | 0·60| 2·75| 0·48| 5·08| 3·55| 2·85| 0·41| 0·32| 0·50| 0·34|
|Magnesia | 6·44| -- | 1·25| 4·50| 0·18| 0·90| 0·67| -- | 0·02| 0·38|
|Silica | -- | -- | 3·55| 0·85 5·48|11·35|28·29|33·38| 0·02| 0·38|
|SO{3} | -- | -- | -- | -- | -- | 0·09| -- | -- | -- | -- |
|P{2}O{5} | -- | -- | -- | -- | 0·09| 0·08| -- | -- | -- | -- |
|Sulphur | -- | -- | -- | -- | -- | -- | 0·33| 0·06| 0·04|Trace|
|Phosphorus | -- | -- | -- | -- | -- | -- | 0·04| 0·56| 0·02| 0·06|
|Loss on | | | | | | | | | | |
| incineration| 5·65|14·00|14·51| 9·12| 9·10| 9·80| 9·88| 7·77|10·55|10·71|
+-------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+

Deposits: (1) Hamm; (2) Schmalkalden; (3) Hüttenberg (Carynthia); (4) Styria; (5) and (9) Bilbao; (6) Algeria; (7) Schwelm (Westphalia); (8) Elbingerode (Harz); (10) Pennsylvania.

_Argillaceous Brown Ironstone_

+-------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
| |_a._ |_b._ |_c._ |_d._ |_e._ |_f._ |_g._ |_h._ |_i._ |_k._ |
+-------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
|Ferric oxide |80·76|19·4 |55·39|66·33|57·32|52·50|39·50|75·67| -- | -- |
|Iron | -- | -- | -- | -- | -- | -- | -- | -- |40·90|21·69|
|Manganese | | | | | | | | | | |
| oxide | -- | 8·2 | -- | 6·42| -- | 5·49| 6·12| 0·72| -- | -- |
|Manganese | -- | -- | -- | -- | -- | -- | -- | -- | -- | -- |
|Zinc oxide | 0·92| 1·6 | -- | -- | 0·47| -- | -- | -- | -- | -- |
|Alumina | 2·36|11·0 |12·80| 7·74| 1·68| 5·23| 9·89| 3·10| 4·95| 3·88|
|Lime | -- | 2·6 |Trace| 0·41| 0·13| 3·36|20·34| 5·01| 5·59|21·25|
|Magnesia | -- | 0·2 | -- | 0·37| -- | 0·36| -- | -- | 0·49| 0·30|
|Silica |4·58 |48·61|22·73|12·97|30·64| 8·64| 5·22| 8·70|16·63|14·71|
|P{2}O{5} | -- | -- | -- | -- | 0·32| 3·86| 2·19| 3·68| -- | -- |
|Phosphorus | -- | -- | -- | 0·02| -- | -- | -- | -- | 1·13| 0·48|
|SO{3} | -- | -- | -- | 0·03|Trace| -- | -- | -- | -- | -- |
|Sulphur | -- | -- | -- | -- | -- | -- | -- | -- | 0·10| 0·05|
|Loss on | | | | | | | | | | |
| incineration|12·71| 9·1 | 8·50|11·77|12·70|20·55|25·74| -- |16·04|28·70|
+-------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+

(_a_) Oölitic (pea) ore from Elligserbrick (Brunswick); (_b_) from Durlach (Baden); (_c_) and (_d_) Ore from Esslingen; (_e_) Oölitic ore from Siptingen (Baden); (_f_) from Adenstedt, nr. Pirna (argillaceous); (_g_) _Ibid._ (calcareous); (_h_) Minette from Esch; (_i_) Red minette from Dolvaux; (_k_) Brown minette from Redange.

_Limonite (Bog Iron Ore)_

+----------------+-----+-----+-----+-----+-----+-----+
| | 1. | 2. | 3. | 4. | 5. | 6. |
+----------------+-----+-----+-----+-----+-----+-----+
|Ferric oxide |61·00|67·46|65·66|67·59|70·05|62·20|
|Manganese oxide | 7·00| 3·19| 3·87| 1·45| 1·78| 7·60|
|P{2}O{5} | 2·00| 0·67| 1·13| 0·18| 0·34| 1·60|
|SO{3} -- | 3·07|Trace| 0·21|Trace|Trace| |
|Silica | 6·00| 7·00| 7·15| 7·89| 8·03|16·60|
|Alumina | -- | -- | 5·09| 4·18| 1·50| 2·20|
|Lime | -- | 0·90| 0·82| 0·47| 2·31| 1·60|
|Magnesia | -- | -- | 0·15| 0·23| 0·12| 3·73|
|Water and | | | | | | |
| organic acids |19·00|17·00| 6·22|17·81|15·87|19·90|
+----------------+-----+-----+-----+-----+-----+-----+

(1) Limonite from Lausitz; (2) Limonite from Auer, nr. Morizburg; (3 to 6) Swedish limonite.

_Ochre_

Ochre, or yellow Terra di Siena, forms earthy-looking masses, fawn, reddish-yellow to brownish-red in colour. Whilst not infrequent in Nature, ochre is only found in small quantities, as pockets, and not as extensive deposits. The discovery of a bed of good coloured ochre is, however, always a very valuable find, bright natural ochres being somewhat rare, and most kinds requiring special preparation before they can be used as painters’ colours. Owing to the comparative scarcity of good coloured ochres, they are often called after the place of origin, such as Thuringian, Italian (Siena), English, etc., ochre.

In nearly every case, ochre is a decomposition product of various ferruginous minerals, which has been transported by water, often in admixture with other minerals, and finally deposited in the places where it is now found. 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. This will be gone into more fully later.

_Yellow Earth_

Yellow earth is found in many places as compact masses, and less frequently as schistous deposits. It has a fine earthy fracture, and is mostly devoid of lustre, except for a faint shimmer on the surface of fracture; slightly greasy feel; and a tendency to crumble, in water, to a non-plastic powder. It contains silica, ferric oxide and water in varying proportions, and the yellow earths from different deposits always vary slightly in percentage composition. These differences are clearly shown in the following analyses of two varieties from the vicinity of Amberg (Bavaria):--

I. II.
Silica 33·23% 35·10%
Alumina 14·21 14·40
Magnesia 1·38 --
Ferric oxide 37·76 36·80
Water 13·24 13·60

When heated, the colour changes gradually to red, and the earth becomes extremely hard. There are several recognised commercial grades, the price of which varies mainly in accordance with the colour and fineness. The Amberg variety is specially esteemed, the Hungarian and Moravian kinds being less valuable.

The colour not being particularly good, this earth is never used for fine work, but is largely employed as a yellow wash for houses and as ordinary distemper. It may also be used as an oil paint.

Red Ochre is a less important, cheap variety of ochre, chiefly used in cheap paints and for low-priced wall-papers. It occurs in the deposits as clayey masses.

_Terra di Siena_

Terra di Siena is a very pure form of ferric hydroxide. When ground, the light to dark brown lumps furnish a pale to dark yellow powder, which can be transformed into a number of gradations by burning. In spite of its handsome colour, this pigment is deficient in covering power, in addition to which it darkens when mixed with varnish, and dries slowly.

(C) THE RED EARTHS

Apart from the small quantities of native vermilion handsome enough for direct use as painters’ colours when reduced to powder, the red earths, with practically no exception, consist of ferruginous minerals, and it is only within a recent period that red painters’ colours have been prepared from certain chemical waste products from manufacturing processes. In all cases, however, compounds of iron and oxygen constitute the bulk of the red earths. In addition to ferric oxide, which is the chief material used for making the important red colours, are compounds of ferric oxide and water, _i. e._ ferric hydroxides. The ferric oxide pigments are among the most important in the entire series of earth colours, being on the one hand very cheap, and on the other so handsome in colour that ferric oxide can be used for the finest paintings.

Ferric oxide can also be shaded very extensively by a fairly simple treatment, so as to furnish a whole range of very handsome shades.

In nature, ferric oxide occurs in numerous varieties of one and the same mineral, red iron ore, which is also known as hematite, blood stone, raddle, etc.

_Red Ironstone_

Red hematite occurs native as rhombohedral crystals, which mostly consist solely of ferric oxide, and may be considered as pure oxide for the purposes of the colour-maker. The difference between the several varieties is due, not to any chemical variation, but entirely to changes in physical structure. The varieties with a radial, fibrous structure are known as red hematite, the colour of which ranges from blood red to dark brown and is frequently accompanied by metallic lustre. The scaly modification of this mineral forms micaceous iron ore, and is usually a deep iron black. In the neighbourhood of volcanoes it is frequently found as particularly handsome crystals.

Iron cream (frosty hematite) is the name given to a beautiful cherry red variety, which easily rubs off, has a greasy feel and is composed of extremely fine scales.

The so-called raddle occurs in Nature as a readily pulverulent earthy mass of ferric oxide contaminated more or less with extraneous substances. On account of its abundance and low market price, it is largely used in painting.

Although mixed with numerous foreign substances, certain clay ironstones, oölitic ironstones and siliceous ironstones may be regarded as ferric oxide in the sense understood by the colour-maker, all these minerals having a deep red to deep brown colour and being capable of finding advantageous employment as pigments.

Ferric oxide is distinguished by two properties which render it specially valuable to the colour-maker. When combined with water, its colour is no longer red, but a handsome brown; and, on the other hand, when heated, the colour passes through brown into a permanent dark violet. By suitable treatment of such minerals as consist mainly of ferric hydroxide, mixtures can be obtained which contain the oxide and hydroxide in variable proportions and give a whole range of shades between brown and red.

The preparation of these colours is easy when very pure red ironstone is available. 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. 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.

The following analyses show the composition of a number of red ironstones, Nos. 1, 2 and 3 being hematite from Froment, or Wetzlar, No. 4 from Wetzlar, Nos. 5 and 6 hematite from Whitehaven, No. 7 from Thuringia, No. 8 from Bohemia, No. 9 from Spain, No. 10 from N. America, and No. 11 from England.

+-----+--------+-----------+---------+-------------+-----------+--------+
| | Ferric | Manganese | | | Alumina, | |
| No. | oxide. | oxide. | Silica. | P{2}O{5}. | lime and | Water. |
| | | | | | magnesia. | |
+-----+--------+-----------+---------+-------------+-----------+--------+
| 1 | 94·00 | Trace | 2·00 | -- | Trace | 2·00 |
| 2 | 80·95 | -- | 16·74 | 0·51 | 0·97 | 0·83 |
| 3 | 73·77 | -- | 23·16 | 0·45 | 1·41 | 1·21 |
| 4 | 92·45 | -- | 5·63 | 0·19 | 0·65 | 1·08 |
| 5 | 96·27 | -- | 4·20 | -- | 0·59 | 4·40 |
| 6 | 63·05 | 5·29 | 22·76 | -- | 0·49 | 4·40 |
| 7 | 85·00 | 1·601 | 3·304 | 0·457 | 8·795 | 0·633 |
+-----+--------+-----------+---------+-------------+-----------+--------+

+---+------+-------+-----+-----+------+-------+--------+-------+--------+
| | |Man- |Alu- | |Mag- | | |Phos- |Loss on |
|No.|Iron. |ganese.|mina.|Lime.|nesia.|Silica.|Sulphur.|phorus.|calcina-|
| | | | | | | | | |tion. |
+---+------+-------+-----+-----+------+-------+--------+-------+--------+
| 8| 33·64| 0·10 | 7·58| 8·10| 0·82 | 17·80 | Trace | 0·19 | 9·82 |
| 9| 31·38| 0·19 | 0·06|29·95| 0·35 | 0·87 | -- | 0·09 | 23·68 |
| 10| 62·54| 1·93 | 1·71| -- | 0·50 | 3·80 | 0·02 | 0·04 | 2·23 |
| 11| 62·91| Trace | 1·39| 0·70| 0·42 | 5·89 | 0·05 | 0·11 | -- |
+---+------+-------+-----+-----+------+-------+--------+-------+--------+

There are certain other minerals closely allied, both chemically and mineralogically, to red ironstone, namely, the brown hematites or ironstones used in the manufacture of iron. Brown hematite consists of ferric hydroxide, Fe{2}O{3}H{2}O, and occurs in a variety of forms in Nature, the most frequent being pea (oölitic) ore, which owes its name to the spherical shape of the grains. Some brown hematites are decomposition products of other minerals, and contain sulphur and phosphorus in addition to ferric hydroxide. Like the pure hydroxide, they are brown in colour, but differ therefrom considerably in their chemical behaviour when heated. This is particularly the case with the so-called bog ore, which is mostly found, as spongy yellow-brown to black masses, in swamps, and owes its origin to the decomposition of various ferruginous minerals. It varies greatly in chemical composition and occasionally contains up to about 50% of sand. The amount of ferric oxide in bog ore varies between 20 and 60%, and it also contains 7–30% of water, up to 4% of P{2}O{5}, small quantities of ferrous oxide and manganese hydroxide, together with, in most cases, mechanically admixed organic residues.

The phosphorus content makes bog iron a very inferior material for smelting, the resulting iron being of low quality. Nevertheless, it can sometimes be advantageously used in making earth colours, though the products cannot lay much claim to beauty of colour.

_Bole_

The native earth pigments known by this name form masses of the colour of leather to dark brown, with a conchoidal fracture and an earthy appearance. Bole chiefly consists of iron silicate combined with water, some varieties containing small quantities of alumina. The composition fluctuates very considerably, most varieties containing 41–42% of silica, 20–25% of alumina, and 24–25% of water, the remainder consisting of ferric oxide. Some kinds, such as Oravicza and Sinope bole, contain only 31–32% of silica and 17–21% of water.

Bole is used as a paint for walls, clapboards, etc., and is only mentioned here because of its relationship to the ferric oxide pigments.

_Alum Sludge_

Large quantities of clarification sludge are produced, in alum works, as the sediment from the red liquors. This sludge consists mainly of ferric oxide, with small quantities of other oxides and sulphuric acid (basic ferric sulphate), and would be an entirely worthless by-product except for the fact that it can be manufactured into pigments, some of them of great beauty.

All alum makers should treat this residue and convert it into pigments, which they could put on the market at a low rate, the cost of preparation being small. Since the material is chiefly composed of ferric oxide, the resulting colours are very similar to those obtained from iron ores; and all shades, from yellow-brown, through red, to the darkest brown, are represented.

_Mine Sludge_

The water frequently present in iron mines sometimes contains large quantities of sediment, which consist mainly of iron ochre and can be advantageously worked up into pigments. 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 difference 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.

(D) BLUE EARTHS

Only two minerals are known which are capable of direct use as blue pigments, viz. vivianite (native Prussian blue) and copper carbonate (azurite, ultramarine), and as neither of them is particularly handsome, they are only used for unimportant work. Lapis lazuli is no longer employed.

_Azurite, or Ultramarine_

This mineral, which is of frequent occurrence with malachite and other cupriferous minerals, forms small crystals of a beautiful deep azure blue consisting of cupric oxide in combination with carbon dioxide and water, expressed by the formula 2CuCO{3}, Cu(OH){2}, or Cu{3}(OH){2}(CO{3}){2}, and containing 69·19% of cupric oxide, 25·58% of CO{2} and 5·23% of water. The colour of the powdered mineral is much paler than that of the crystals. The pigment, which is used for cheap paints, is not particularly stable, and loses much of its beauty when applied to plaster.

_Vivianite_

This mineral occurs in many places as crystalline masses, but also forms earthy deposits, some of which, especially in certain bogs, attain considerable thickness. The colour is between indigo and blackish blue; and the freshly won mineral often has an unsightly whitish appearance, which, however, soon changes into the pure blue. The cause of this peculiarity is due to the fact that vivianite originally consisted of hydrated ferrous phosphate, which is white, this compound being transformed, under the influence of the air, into the blue ferric phosphate.

Vivianite contains ferric oxide, phosphoric acid and water, but in variable proportions. The original composition, expressed by Fe{2}(PO{4}){2} + 8H{2}O{2}, corresponds to 43·03% of ferrous oxide, 28·29% of P{2}O{5} and 28·68% of water; but, in the air, part of the ferrous phosphate is oxidised to basic ferric phosphate, so that the content of ferrous oxide may range from 9·75 to 42·71%, and that of ferric oxide between 1·12 and 38·20%. Vivianite is also sold as blue ochre, and is now seldom used as a painters’ colour, owing to the introduction of a large number of artificially prepared blues, which are superior to vivianite in colour and are cheaply made. However, it can still find application in localities where it is obtainable in quantity.

(E) GREEN EARTH PIGMENTS

The green earth pigments comprise green earth (Verona green) and malachite. Like the blue earths, they cannot lay any particular claim to beauty, but they are very cheap, and consequently are largely used where low price is the chief consideration.

_Green Earth_

In Nature, green occurs as an entirely non-crystalline earthy mass, which is probably a decomposition product of augite. It has a close, earthy fracture, a colour between seladon and olive green, and a slightly greasy appearance. In point of chemical composition it consists of silica, alumina, magnesia, sodium, potassium, ferrous oxide and water, the usual representative formula being ROS{1}O{2} H{2}O, in which RO symbolises a metallic oxide.

The colour is due to ferrous oxide; and if left exposed to the air for a long time, or subjected to powerful calcination, the great affinity of ferrous oxide for oxygen causes the colour to turn red and red-brown.

Green earth is found in many localities, _e. g._ Bohemia, Hungary, the Tyrol and Cyprus, the finest, however, occurring near Verona, on which account it is known as Veronese earth.

_Malachite_

The commercial pigment consists of powdered malachite, a mineral which usually occurs in compact masses of a handsome emerald green colour, though isolated lumps exhibit considerable variation in shade, some of them being dark green and others very pale. In chemical composition, malachite is closely allied to azurite, consisting of cupric oxide, carbon dioxide and water, and the difference is entirely one of percentage proportions. The formula is CuCo{3}, Cu(OH){2}, or Cu{2}(OH){2}CO{3}, corresponding to 71·90% of cupric oxide, 19·94% of carbon dioxide and 8·16% of water.

Powdered malachite (even the dark green varieties) is always rather light in colour, and for this reason is not much used. Furthermore, the mineral is rather hard (3·5), and is consequently difficult to grind; in addition to which the mineral is fairly expensive, on account of its employment as a source of copper, particularly fine pieces being also used as ornaments or for making works of art. Moreover, like all copper compounds, it is very sensitive to the action of sulphuretted hydrogen, and liable to discoloration in course of time.

(F) BROWN EARTH PIGMENTS

Numerous minerals are adapted for the manufacture of brown pigments. On the basis of chemical composition, they may be classed in two groups; those consisting of ferric hydroxide, and those in which the brown colour is due to organic substances.

The first group comprises the minerals which have already been mentioned in connection with the red earth pigments, bole and brown ochre (umber), Terra di Siena, Cologne earth and a number of other earths rich in ferric hydroxide belonging to this category. The second, or organic group, includes compounds that are very rich in carbon and are therefore of a very dark colour, the shades ranging from light brown to black, _e. g._ the true umbers and asphaltum.

_Umber_

As already mentioned, the term “umber” was formerly applied to brown varieties of ochre, whereas at present it is extended to certain masses of brown-coal character, often interspersed with iron ochre and sometimes containing manganese. Umber generally consists of fairly dense, earthy masses, which are dried and ground--after crushing and levigation, if necessary.

Valuable varieties are Cappagh brown and Caledonian brown, both with a reddish tinge.

It is thus evident that “umber” now implies two different kinds of materials, organic masses and iron-manganese compounds, which can also be used as oil paints. These umbers can also be extensively shaded by burning, the final colour being particularly influenced by the amount of manganese compounds present.

The carbonaceous umbers (Cassel brown, Carbon brown) are combustible, and mostly leave behind a merely small residue of ash. An important property of these umbers is their partial solubility in alkalis, a peculiarity which is utilised for the preparation of brown wood stains.

_Asphaltum_

Asphaltum forms very friable dark brown to black masses, which, in contact with a light, easily ignite and burn with a bright, but very smoky, flame, disengaging a peculiar, “bituminous” smell, and leaving only a very small quantity of ash.

Extensive deposits of asphaltum are found at the Dead Sea, the Pitch Lake on the island of Trinidad, in Dalmatia, and many other places, where, however, it is in an impure condition and frequently contains large quantities of sand. In many localities the rock is impregnated with asphaltum, which makes it dark brown to black in colour and gives rise to a bituminous odour when rubbed.

Peat beds sometimes contain pockets of a mass with a handsome brown colour and consisting of a mixture of humic acids and other organic substances which may be ranked with the humin bodies that are always formed when organic matter decomposes in presence of an insufficient supply of oxygen. These bodies are dark coloured, mostly deep brown, rich in carbon, and, to some extent, similar to brown coal or peat in chemical composition.

Their high carbon content renders these substances very inert towards chemical reagents, and therefore particularly adapted for the preparation of painters’ colours. Genuine Vandyke brown, which is the handsomest brown known, is an earth rich in humin compounds; and Cassel brown also belongs to this group.

(G) BLACK EARTH

The colour of these earths is entirely due to carbon, and pure carbon, a certain form of which occurs native, is itself used as a pigment. Actually, there are only two minerals that require to be mentioned in this connection: black schist and graphite.

_Black Schist_

In most cases this is a clay shale, so rich in carbon as to appear deep black. In commerce, this mineral is also erroneously called “black chalk”; but at present it is seldom used as a pigment or drawing-material, black chalks being produced far more cheaply than the expense of preparing the natural article.

Grey clay shales are used for making grey earth pigments (stone grey, and mineral grey).

_Graphite_

This mineral is found, in a very pure state, in many localities, celebrated deposits occurring in England, Siberia, Bohemia and Bavaria, whilst North American graphite has lately come into prominence.

Graphite is a modification of pure carbon, and is met with in the form of hexagonal (rhombohedral) crystals, usually occurring as hexagonal plates with a lustrous, iron-black colour. It rubs off easily, and readily burns away, leaving a very small amount of ash, when subjected to a very high temperature in presence of air.

The principal uses of graphite are as an anticorrosive paint for iron, and for making lead pencils.

As already mentioned, the term “earth colours” has been considerably broadened of late. Whereas, formerly, it was restricted to colours prepared exclusively from minerals by a simple treatment, limited to crushing, levigation or calcination, it now includes the pigments obtainable from large by-products of certain chemical processes. This latter class is especially important as affording an opportunity of utilising products formerly considered worthless and whose removal often entailed heavy expense.

By drawing on these materials the industry of the earth colours has greatly enlarged its scope. At present, many colours of this kind are on the market, and it is to the interest of many manufacturers to endeavour to utilise certain waste products in the same direction. The advantage of such a course hardly needs emphasising; but, to give only a single example, it may be mentioned that the manufacture of fuming sulphuric acid from green vitriol, by the old process, produces residues which were formerly looked upon as quite worthless, and sold at very low prices, but are now worked up, in a number of factories, into very handsome and durable pigments.