Read Ebook: The manufacture of earth colours by Bersch Josef Bersch Wilhelm Editor Salter Chas Charles Translator

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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.

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.

WHITE RAW MATERIALS AND PIGMENTARY EARTHS

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" , 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" . 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" , 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.

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 and carbonic acid, the formula being expressed by CaCO. 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- 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 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:--

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 ). 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 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 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.

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 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 . 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.

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.

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 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 HMg, corresponding to 63?52% of silica, 31?72% of magnesia, and 4?76% of water. In some varieties of talc, a portion 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.

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, KO.AlO.. Pure kaolin is AlO + 2HO, 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.

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.

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 brown, through cinnamon to blackish-brown. The chemical composition of the pure lumps may be expressed by the symbol 2FeO + 3HO; but a little manganese oxide and silica is generally present even in the pure kinds.

The chief varieties of this mineral are:--

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

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

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.

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, o?litic, 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 o?litic 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.

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