Chapter IV: White Earth Colours

The white earth colours are important for the purposes of the colour-maker, because not only are they used by themselves as paints, but also serve in the production of light shades of other colours.

The white colours containing clay or lime are the most abundant and important of all, and will therefore be described first. The lime colours comprise caustic lime, carbonate of lime (chalk or powdered limestone), gypsum and bone ash.

CAUSTIC LIME

Though this product is not used direct as a painters’ colour, it is employed in the preparation of compounds that are so used. It is made on a large scale for the preparation of mortar, and there is therefore no need for the colour-maker to manufacture it himself, since it can always be bought from a lime-burner. It must be borne in mind, however, that lime for the colour-maker’s purposes must possess certain properties, failing which it is of no use to him. What these properties are and how the product is made will now be briefly described.

When carbonate of lime, _i. e._ native limestone, is exposed to strong heat it parts with carbon dioxide and is transformed into burnt or caustic lime.

CaCO{3} = CaO + CO{2}
Carbonate of lime. Caustic lime. Carbon dioxide.

The limestone is burned either in kilns of very simple construction, or else in more complicated furnaces in which a continuous process is maintained. The ordinary limekiln, which can be found in many parts of the country, consists merely of four walls, with a door in the front one for the introduction of the fuel. Kilns of this kind are usually set up in the vicinity of the limestone deposits, and are abandoned when they get worn out.

The limestone is broken to lumps of fairly even size, about as large as a man’s head, and these are piled up in a domed heap in the kiln, sufficient space being left between the lumps for the passage of the flame. A fire is then lighted under the pile, pine wood being mostly used for this purpose on account of its high content of resin, which gives a very strong flame. The fire is kept up until the top of the pile has become white hot, and only a blue, smokeless flame is visible. The appearance of this denotes that the burning is ended, the fire being then allowed to die out and the lumps left until cool enough to be taken out of the kiln.

This operation is performed with great care, particular importance being attached to preserving the lumps as intact as possible and preventing the formation of dust, which is of little value. The lime made in this way is endowed with properties that render it valuable for the purposes of the colour-manufacturer; but, on the other hand, possesses certain disadvantages.

Owing to the use of wood as fuel, the caustic lime obtained in this way is usually a very pure white, because the burning is continued until the whole mass is glowing and the firewood has been completely consumed. If this is not the case, the burnt lime is greyish in colour, from the finely divided particles of carbon, which, of course, spoils the lime for colour-making. The defects existing in lime burned in the above type of kiln originate in the irregular character of the product. It will be evident that the limestone lumps that are nearest the fire will be far more strongly heated than those in the upper part of the dome; and when calcined lime is kept incandescent for a long time, it becomes so compact in texture that it quenches with great difficulty when brought in contact with water. This condition is known as “dead burnt,” and such lime is of little value.

The lumps at the top of the pile are least exposed to the heat, and very often still contain carbonate, as is shown by the effervescence produced on treatment with an acid. Such lime is imperfectly burnt, and the lumps frequently still exhibit the crystalline structure of limestone when broken. They quench rapidly, but when mixed with a little extra water, the mass is no longer of the buttery consistency typical of caustic lime, but contains gritty portions consisting of unaltered limestone.

Owing to the defects of dead burning on the one hand and insufficient calcining on the other, colour-makers now prefer lime that has been burned in continuous kilns, because, when properly made, such lime is very uniform in character, and is also cheaper than that burned with such an expensive fuel as wood. In consequence of the greater capacity of the continuous kiln, and the more uniform character of the product, the old-fashioned kilns are more and more falling into disuse.

The arrangement of the continuous kiln is very simple. The kiln consists of a fairly high shaft, open at the top, and provided at the bottom with a small hole for the removal of the burnt lime. A coal fire is lighted, and as soon as the kiln is heated up, alternate layers of limestone and sufficient coal for burning it are introduced. The burnt lime sinks to the bottom of the shaft and is pulled out, with iron hooks, from time to time.

Given the right proportions of coal and limestone, the lime made in these kilns is burnt to just the right degree, and is excellent for builders’ use. In many cases, however, it is less valuable to the colour-maker, and in some quite useless. For example, when the coal is not completely consumed, carbon, even though only a very small quantity, is deposited on the lime, and the burnt lime, instead of being a brilliant white, as it should be, is grey; and colour made therefrom is also greyish white and will spoil the shade of other colours with which it is mixed.

The chemical composition of the original limestone also has an influence on the character of the burnt lime. Limestone consisting entirely of carbon dioxide and lime is so rare that sufficient is never available for making burnt lime on a large scale. Even the purest limestone found native in large quantities--namely marble--is not pure carbonate of lime, but contains a certain proportion of extraneous substances. At the same time it is too expensive to use for technical purposes.

The ordinary impurities present in limestone are ferrous oxide, ferric oxide, magnesia and organic matter. The presence of ferrous oxide can usually be detected by the greenish tinge of the raw limestone, and the reddish cast of the burnt product. Ferric oxide is revealed by its reddish colour, in both the limestone and burnt lime.

Magnesia, which is present, for example, in dolomitic limestone, cannot be detected by the colour, either before or after burning, this oxide being itself perfectly white; but its presence is a drawback because if in large quantity it makes the lime very difficult to quench, and such lime is never of a fatty character.

Organic matter betrays itself by the colour, the lime being dark tinted, varying from grey to black. Black limestones usually contain carbon in an extremely fine state of division, and are quite useless to the colour-maker owing to the impossibility of completely burning off this contained carbon, which always imparts a greyish tinge to the burnt lime. The behaviour of limestones in this respect varies, however, considerably, and can only be ascertained with certainty by a trial burning. Many that are rather dark in colour will, nevertheless, burn perfectly white, whereas others, much lighter in shade, always give a product that is not quite pure in tone. This divergent behaviour seems to have some connection with the chemical composition of the organic matter in question. If it consists of coal, or substances analogous thereto, no really pure white lime can be obtained from a light grey limestone, it being impossible to burn off the finely divided carbon completely.

In addition to making a trial burning with a fairly large sample of material, the behaviour of a limestone towards hydrochloric acid will afford some information as to the nature of the grey colouring matter. If the limestone dissolves completely when suffused with the acid, the indications are favourable for its usefulness to the colour-maker. If, on the contrary, a black residue is left, the coloration is due to finely divided carbon, and there is then little prospect of the material furnishing a suitable product. In any event, a trial burning is the most reliable guide. In addition to carbon, the presence of any large proportion of ferric or ferrous oxide is objectionable, since, in either case, the product will be tinged red with ferric oxide, into which the ferrous oxide is transformed at calcination temperature.

In addition to comparing the colour of the product with a standard sample, the suitability of a burnt lime for colour-making can be tested by quenching. If a lump about the size of the fist be placed in a large porcelain basin and suffused with a small quantity of water, preferably poured in a thin stream, the lime, if properly burned, will continue to absorb the water for a considerable time, like a sponge, and will very soon give evidence of a brisk reaction by increasing in bulk and generating such an amount of heat as to cause the immediate evaporation of a few drops of water allowed to fall on the surface of the mass. Finally, the entire lump will crumble down to a very delicate, voluminous powder, consisting of slaked lime (calcium hydroxide).

This chemical reaction is expressed by the equation:--

CaO + H{2}O = Ca(OH){2}
Lime Water Calcium hydroxide.

When the amount of water added to burnt lime is no more than sufficient to effect its transformation into hydroxide, this latter, as already stated, forms a delicate white powder. The addition of more water results in the formation of a homogeneous pulp, of a peculiar fatty character. Since this fatty appearance is only possessed by pure lime, it is a criterion of high quality in burnt lime, and contrasts strongly with that of the less valued poor (or lean) lime.

Calcium hydroxide acts as an extremely powerful base, and therefore must not be mixed with colours that are sensitive to the action of strong bases. As a matter of fact, its direct use in painting is very small. Of course, a thin milk of lime is used for whitewashing walls, etc.; and if any colouring ingredients are added they must be such--_e. g._ ochres--as are not affected by the lime. Nevertheless, quick and slaked lime are very important in colour-making, as forming the originating material for the preparation of a number of colours.

When slaked lime is mixed with sufficient water to form a stiff pulp, and is left exposed to the air for some time, a change will be observed to take place, the mass solidifying gradually (commencing on the outside) and finally crumbling to a soft white powder. This change is due to chemical action, the lime having a great affinity for carbon dioxide, which it readily takes up from the atmosphere--a fact which explains the solidification mentioned. It would be erroneous to assume that the lime is again completely converted into calcium carbonate in this way; for, though such conversion does ultimately take place, it requires a very long time for completion.

The resulting compound is, actually, a double compound of calcium oxide and carbonate. Although this compound has fairly strong basic properties, they are, nevertheless, far weaker than those of caustic lime, being partly neutralised by the carbon dioxide absorbed. If the superficial area of the slaked lime be increased by spreading it out thinly, so as to offer greater opportunity for the action of carbon dioxide, the formation of the double compound in question will be greatly accelerated.

This double compound is prepared artificially in special works, and the resulting colours are put on the market under various names. They, too, must not be mixed with colours that are sensitive to alkali, and on this account they cannot be used in fine paints. If applied as a white priming to the walls of rooms, care must be taken to cover the coating with some substance that will protect the topping colour from the action of the lime. For this purpose, painters use a wash of milk, soap and water, etc.

An important property of lime is its behaviour towards casein, the substance forming the curd of milk. With this body it combines to form a mass which sets hard and is highly resistant, viz. calcium caseate, and is formed when limewash is stirred up with milk or freshly precipitated casein. Weatherproof distempers for outside use are prepared in this manner.

PEARL WHITE

The preparation frequently met with in commerce under this name is nothing more than a burnt lime of great purity. It is prepared in the coastal districts by burning oyster shells, the resulting burnt lime being easily transformed into a fine powder, the pure white colour of which is due to the absence of iron. It is used in the same way as pure burnt lime, and is mainly of interest in seaside towns where oyster shells are often accumulated. It may be pointed out that the name pearl white is often applied also to pure white grades of white lead.

VIENNA WHITE

This colour is prepared from any kind of burnt lime that is sufficiently pure; that is, free from ferric oxide. The method of preparation is simple, requiring no special apparatus, and can therefore be carried out wherever suitable lime is available.

Operations are commenced by carefully slaking well-burnt lime with water, a sufficient excess of which is added to produce a fairly thick pulp. To accelerate the absorption of carbon dioxide, the mass is exposed to the air in thin layers, by spreading it out on boards, so as to present a large surface to the air. As soon as the pulpy character has disappeared, the mass is detached from the boards, and is pressed and kneaded, with wooden paddles, into prismatic cakes which are left exposed to the air--being, of course, protected from the wet--until the absorption of carbon dioxide is complete--a condition that can be recognised by the earthy character of the product. The cakes are then dried, an operation entailing great care, since lightness is a sign of good quality, whereas a damp product is very heavy.

In forming the cakes they must not be touched by the bare hands, because the lime is so caustic that it would soon destroy the skin. The foregoing method of manufacture is capable of many improvements, which can be introduced without adding much to the cost of production.

If the lime is formed into large blocks, it will evidently take a long time for the mass to acquire, all through, the earthy character indicating combination with carbon dioxide. This drawback can be easily remedied by forming the mass into small cakes, which will become ripe, owing to their larger surface, much sooner than the bigger blocks.

A very good plan to adopt in moulding is to form the burnt lime into a stiff paste with water, preferably by adding enough water to make a viscous mass, and leaving this in a lime-pit for several weeks, the prolonged storage enabling the lime to acquire the already mentioned fatty character, and at the same time to become highly plastic. Lime treated in this way can be forced through a nozzle, forming a cylindrical rope, which can be cut by a knife into convenient lengths and left on boards for a few days until they have become firm enough to stand up without breaking. Cylinders made in this manner, with a length of about four inches and a diameter of two inches, will absorb carbon dioxide very quickly.

The absorption can be still further accelerated by setting up the cylinders in an atmosphere highly charged with the gas, for instance in the vicinity of a manure pit, as they will then avidly take up the carbon dioxide abundantly liberated from the rotting manure. Similar acceleration will take place if the boards carrying the cylinders are placed in a stable, or in a room where wash for making spirits is fermenting, because large quantities of carbon dioxide are liberated in both places.

Working the caustic mass by hand is accompanied by so many inconveniences that it seems highly desirable to employ some mechanical moulding device which will render contact with the wet lime entirely superfluous. It may be pointed out that such a device can also be advantageously used for moulding all earth colours in paste or pulp form, and in particular for shaping ferric oxide colours into rods or small cylinders.

Such a machine (Fig. 26) is composed of a rectangular box with semi-cylindrical bottom, a detachable shaft carrying a sheet-metal worm being arranged in the box so that the worm is in contact with the rounded bottom and is continued into the cylindrical extension of the box. This extension terminates in a hollow cone, to the mouth of which nozzles of varying aperture (square, rectangular or round) can be attached. A knife, operated by hand or mechanical means, enables the extruded soft mass to be cut into convenient lengths, which drop on to a series of easy running rollers in front of the nozzle, and are thereby delivered to an endless-belt conveyor from which they can be transferred to the drying-boards.

When the box has been charged with the lime pulp and the worm is rotated, the latter forces the soft mass into the cone and extrudes it through the nozzle, so that, as long as there is any material in the box, it is discharged as a continuous rope, of square, rectangular or cylindrical section, on to the guide-rollers, where it can be cut off into lengths by the knife.

A fundamental condition for the preparation of a good Vienna white is the employment of pure raw material, which must be free from ferric oxide or earthy impurities, and fully burned. An excellent material for this purpose is calcined mussel shells, which furnish a loose, and at the same time very pure, lime, and are very largely used for lime-burning in places such as Holland, where they are available in large quantities.

Vienna white is not much used as a paint colour, owing to its powerful alkaline properties which have a destructive effect on many colours. It is, however, largely employed as a polishing agent, for which purpose it is powdered and is put on the market--mostly in bottles--as Vienna lime. Its very handsome white colour and low price render it particularly suitable for coarse painting, for example as a prime coating for painted interior walls. To guard against the danger of the painted decoration being destroyed by the alkaline nature of the white, it is advisable to coat the dried ground with alum solution, the alumina of which combines with the lime to form an insoluble compound to which organic colours adhere well. The sulphuric acid also enters into combination with the lime, the resulting gypsum having no effect on the paints subsequently applied.

CHALK

The name chalk is used for a number of commercial substances which differ considerably in both the mineralogical and chemical sense. French chalk, for instance, is a mineral belonging to the steatite group and, apart from its name, has nothing in common with true chalk, except the white colour, and even this differs altogether from that of chalk properly so called. It is therefore necessary, in the interests of proper nomenclature, to differentiate the various kinds of chalk, commencing with the mineral known by that name to the chemist and mineralogist.

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

Such progress has been made that the zoological status of the animals which inhabited the shells--many thousands of which are present in a lump of chalk--has been identified; and it is known that these animals were of marine type. Fig. 27 shows the appearance of the animal remains in Meudon chalk when highly magnified, the upper half being viewed by transmitted light and the lower by reflected light.

Notwithstanding the extremely minute dimensions of the chalk animalculæ, their remains form rocks of great thickness in all parts of the world. In Europe we find, for example, extensive chalk formations in England, whose Latin name Albion was bestowed on account of the white chalk cliffs occupying long stretches of the coast. The hills of Champagne consist almost entirely of chalk; and Rügen, together with many other islands, is nearly all chalk cliffs.

It is only in very rare cases, however, that chalk occurs in sufficient purity to be immediately suitable for use as a pigment or writing-material. For the most part it contains other minerals, or large fossils, from which it has to be separated by mechanical treatment. Nodular flints are often met with in chalk, and many deposits contain such large numbers of the petrified shells of the sea urchin that the chalk really cannot be used as a pigment at all, by reason of the high cost of purification. The only places where chalk can be advantageously worked for the preparation of pigment is where the mineral is in a high state of purity, and also contains only very few sandy particles. Such chalk deposits are worked on a mining scale, and, as a rule, in the state in which the chalk comes from the quarry; it is in the form of a soft mass, easily scratched with the finger-nail and of fairly high density, owing to the considerable quantity of water with which it is ordinarily impregnated.

In order to convert this crude chalk into a product that can be used as a pigment, it is first left to dry until the lumps can be easily broken, and then crushed into small pieces, from which all the extraneous minerals, which occur as large lumps, are sorted out and removed. This picking process is important, especially when the chalk contains flints, because these latter are very hard and would injure the millstones in the subsequent grinding.

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

Experience has shown that this method of prolonged storage in contact with water greatly improves the colour. The only explanation of this fact is that the chalk still contains a very small amount of organic matter, which gradually decomposes in presence of water. The evidence in favour of this is the peculiar smell given off during storage.

Even with the most careful grinding, chalk cannot be transformed into such a fine powder that is directly fit for all purposes; and the only way to obtain the requisite fineness is by levigation. Owing to the large quantities that are usually handled in this process, the milky liquid coming from the mill is mostly run into large brick tanks, where it is left to settle until all the chalk has deposited and the supernatant water is perfectly clear. Tapping-off being usually impracticable, the water is generally drawn off by careful syphoning, so as not to disturb the fine sludge at the bottom of the tank.

The deposit in the settling-tanks is shovelled into wooden boxes, perforated at the sides to enable the water to drain away, the chalk being prevented from escaping by lining the boxes with linen cloths. The pulp soon loses its liquid character and shrinks considerably, the boxes being then filled up with more sludge, and so on until the contents have ceased to shrink. When the mass is so far dry that it will no longer run when lifted, the boxes are covered with boards and inverted, discharging the contents on to the boards, on which the mass is left to become quite dry. Filter-presses are also used.

Large prismatic masses of chalk never dry so uniformly as to prevent the formation of cracks, and if the chalk is to be sold in this form the cracks are plastered up with thick pulp; this operation, however, being superfluous when the chalk is to be sold as powder.

In order to obtain a more compact product and accelerate the drying of the moulded lumps, some makers use presses, in which the fairly dry chalk is subjected to progressive heavy pressure.

Owing to the fineness of the component particles of chalk, they adhere so firmly together, without any bind, that a fair amount of force is necessary to break down a piece of perfectly dry levigated chalk. Sometimes, however, chalk exhibits the unpleasant property of losing its cohesion almost completely when dry, and in such cases it can only be shaped into prisms with great trouble. This peculiarity is specially accentuated when the chalk contains magnesia; and in order to mould chalk of this kind into blocks, a binding agent, such as ordinary glue, must be added to the water used in grinding, care being taken not to use too much, or the chalk will become too hard, when dry, for certain purposes, _e. g._ as drawing or writing chalk.

For some purposes, chalk is sold in powder form, and very high purity is not then essential, an admixture of magnesia or clay being harmless. Gilders, for instance, use large quantities of chalk for priming picture frames, and stir the chalk up with a certain amount of bind (mostly size), to give the particles the desired cohesion.

The chief requirement exacted of a good quality chalk is a handsome white colour; and this depends entirely on the quality of the raw material, not on the method of preparation. It is known that a substance quite devoid of colour will furnish a perfectly white powder, because the colourless particles reflect the light in all directions without breaking it up into its constituent yellow, red and blue rays. Chalk, too, is in reality a colourless substance, and reflects light with greater uniformity in proportion as the fineness of the particles increases. Consequently, when one has a chalk that is not perfectly white, it can, nevertheless, be made to furnish a very handsome product by bestowing great care on grinding and levigation. Properly prepared chalk should be as fine as the finest flour.

When the colour of the best grades of chalk are compared with what may be termed pure white--such as that of white lead, zinc white, permanent white--a skilled eye will always detect a greyish or yellowish tinge in the former, even if obtained from the whitest Carrara marble.

The grey tinge is due to the presence of organic matter, which cannot be eliminated by any known means, but which can be shown to exist by the fact that when such chalk is heated to incandescence in the air for a short time, the resulting burnt lime is pure white, the organic matter having been burned off. A yellow tinge is caused by minute traces of ferric oxide, which--as also ferrous oxide--almost invariably accompanies calcium carbonate; and limestone free from determinable quantities of these oxides is of rare occurrence. Ferrous oxide does not reveal its presence in limestone unless in large proportion, its pale green colour being of low tinctorial power, whereas ferric oxide, which is a very strong colouring agent, can be more readily detected.

To those who are engaged in the manufacture of white earth colours, however, it is quite immaterial whether a limestone or chalk contains ferrous oxide, because that oxide quickly changes into ferric oxide in the finely divided product, and a chalk which was originally pure white will become decidedly yellow in a short time.

Fortunately, such a yellow-tinged product can be rendered perfectly white by simple means and at small cost, all that is necessary being to add a suitable quantity of a blue colouring matter. When this has been done, the chalk will seem pure white to even the most skilled eye.

This result of adding a blue pigment is based on the well-known physical fact that certain kinds of coloured light produce white light when combined, the colours that give this effect being termed “complementary.” A pure blue is complementary to a yellow with a reddish cast--_e. g._ ferric oxide--and therefore a chalk that is tinged yellow by a small quantity of ferric oxide can be changed into a seemingly pure white substance by the addition of a blue pigment.

The only pigments of use in this connection to the colour-maker are such as have very intensive colouring power and at the same time are low enough in price. Such substances are ultramarine, smalt and coal-tar dyes. Smalt is the best because its colour is unalterable. In point of chemical composition, this substance is a very hard glass coloured blue by cobaltous oxide. For improving the colour of chalk or any other white, the smalt must be in an extreme state of fine division, and levigated to an impalpable powder. Ultramarine can be used for the same purpose, but is not so permanent.

To ascertain the correct proportion of blue pigment, it is advisable to make a systematic experiment, which is easily performed. Exactly 90 parts of the chalk in question are triturated with 10 parts of blue pigment in a mortar until the entire mass has become a perfectly uniform pale blue powder, which contains 10% of the blue ingredient.

Several samples, each representing one hundred parts of the white pigment to be corrected are carefully weighed out, 1 part of the blue powder being added to the first sample, 2 parts to the second, 3 to the third, and so on, and the mixtures are compared with a standard white substance, such as best white lead or zinc white, to see which most nearly approaches the standard colour. It is then easy to calculate how much of the blue requires to be added to 100 or 1000 lb. of the material to be corrected.

The correction can be effected in several ways; for instance, by grinding the blue pigment directly with the bulk, by adding it at the levigation stage, or mixing it with the dry, finished product. The first two methods are attended with certain drawbacks which render it difficult to obtain a perfectly uniform product, owing to the specific gravity of the blue pigments being higher than that of the whites. Consequently, when the two are mixed in presence of water--as is always the case in grinding and levigation--the heavier blue pigment settles down more quickly, and several strata can be clearly distinguished in the sediment. The upper layers will still have a decided yellow tinge--the proportion of blue being too small for proper correction--whilst the next in order will be pure white--accurately corrected--and those at the very bottom will be decidedly blue, because they contain the largest proportion of the blue substance.

The most satisfactory results are obtained by dry mixing; and this can be successfully practised when the colour-maker has a cheap source of power (such as water power) available. Where, however, costly power plant has to be provided, only the finest grades of white pigments can be improved in this way, the expense of labour being too high for cheap materials.

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

PRECIPITATED CHALK

Many chemical processes furnish soluble salts of lime that constitute a by-product of little value. These salts, however, can be advantageously utilised for the preparation of an artificial chalk which is preferable to the native article in many respects. For instance, where large quantities of calcium chloride solution are available, and soda can be purchased at a sufficiently cheap rate, they can be converted into artificial chalk, because these two substances react on each other, forming, on the one hand, calcium carbonate, which is precipitated as a very delicate, insoluble powder, and on the other, sodium chloride, or common salt, which remains in solution, according to the equation:--

CaCl{2} + Na{2}CO{3} = CaCO{3} + NaCl.

If, however, these solutions were mixed together in a crude state, the resulting product would be of only low value as a pigment, being of a yellow tinge and never pure white. This is due to the fact that the impure lime salts, being waste products from chemical works, frequently contain fairly large amounts of ferric oxide, and the soda also is often so high in that impurity that the colour of the precipitated chalk is considerably impaired.

Fortunately, there is no difficulty in eliminating this ferric oxide by chemical means, and obtaining a product of superior colour to the best native chalk. This is effected by treating the perfectly neutral lime-salt solution with calcium carbonate, which causes the precipitation of the iron, a corresponding amount of lime passing into solution.

In order to eliminate the ferric oxide from the lime-salt solution so completely that not even the most delicate chemical test known will be able to reveal any trace remaining, the solution is placed in a vat and stirred up with finely powdered chalk. If the solution contains any free acid, effervescence, due to the liberation of carbon dioxide, will take place; and in such event the addition of chalk is continued until the free acid is all neutralised, and the added chalk sinks to the bottom undissolved. The chalk should be in slight excess, so that a decided sediment is visible at the bottom of the liquid when at rest.

This deposit is stirred up again at intervals with the liquid for several days. When ferric oxide is present, the colour of the deposit will gradually change to a yellowish brown, through the precipitation of ferric hydroxide by the chalk; and in this way the final traces of iron can be removed.

The liquid is then carefully drawn off, without disturbing the sediment, and the soda solution is run in so long as a precipitate of calcium carbonate continues to form. The completion of the reaction can be ascertained by pouring a small quantity of the liquid into a tall, narrow glass, leaving it to clarify, adding a little more soda solution and observing whether any further precipitate is produced. On the other hand, it may be that an excess of soda has already been added in the precipitating tank; and this can be determined by testing a sample with turmeric paper--blotting-paper soaked in a solution of the colouring-matter of turmeric root--which is turned brown by alkaline reagents. Even in very dilute solution, soda will give this colour change, and the test is therefore very accurate. The complete precipitation of the lime in the solution can be ascertained by passing a small quantity through blotting-paper and treating it with a little acid potassium oxalate solution, which, if lime be present, will at once produce a strong crystalline precipitate of calcium oxalate, which is only very sparingly soluble in water. If the oxalate gives merely a slight turbidity, the residual amount of lime is so small that the process may be regarded as complete.

Since carbonate of soda is usually much dearer than the lime-salt liquor, it is preferable to leave a small quantity of the lime unprecipitated. Given sufficient care in effecting the precipitation, and especially when fairly strong solutions are used, a brilliant white precipitate of calcium carbonate is obtained, which is in such a finely divided state that the minute constituent crystals can only be detected under a high magnifying power.

This precipitated chalk being already in an extremely fine condition needs no further preparation, and, when washed, is ready for immediate use, forming a handsome pigment with excellent covering power.

When precipitation is ended, the deposit is allowed to settle down, and the clear supernatant liquid is carefully drawn off so as not to disturb the delicate sediment, which is then stirred up thoroughly with clean water, left to subside, washed again, and then spread out to dry on cloths which are suspended by the four sides. The surplus water drains away and the residue gradually assumes the consistency of paste, in which condition it can easily be moulded to any desired shape. If left long enough to dry completely, it forms a very delicate powder, furnishing a pigment of excellent quality.

If this precipitated chalk be moulded into prisms for sale, the blocks are laid on one of their broad sides until firm enough to turn over on to one of the narrow faces, slabs of gypsum being used as the supporting material, in order to ensure uniform drying. The gypsum absorbs water with avidity and thus dries the prisms evenly.

A defect of these prisms is their great fragility; but their strength may be improved by mixing a little very weak solution of dextrin to the mass after the last washing-water has been completely removed. In drying, the dextrin binds the material of the prisms sufficiently to keep them from breaking except under the influence of a fair degree of force.

CALCAREOUS MARL

As already mentioned, calcium carbonate rarely occurs in a perfectly pure condition in Nature; and chalk, also, is frequently contaminated by other minerals. A variety of limestone occurring as extensive deposits in many places is that in which calcium carbonate is associated with clay. Sometimes the clay predominates, and the mineral is then known as marl, being really a clay contaminated with chalk. If, on the other hand, the chalk forms the chief constituent, the mineral is termed calcareous marl.

Calcareous marls are used in much the same way as limestone, some modification, however, being necessitated by the presence of the clay. Although limestone containing a certain amount of clay can be burned in the kiln, it yields an inferior lime that is of little use to the builder owing to its low binding power. Marl of a certain composition finds an important application in the manufacture of hydraulic lime or cement.

The only kind of marl suitable for pigment is that containing clay with very little colour; and this is of somewhat rare occurrence, because most marls contain sufficient ferric oxide to give them a yellow shade. Marl that is fairly free from ferric oxide, however, can very well be used as pigment; and many white pigments sold as “chalk” are really finely ground marl. In accordance with the general practice, in the colour industry, of giving colours a great variety of names, and suppressing the real names, which, so far as the artificially prepared colours are concerned, should bear some reference to their chemical composition, numerous white earth colours bear fancy names, though really consisting of chalk, lime (generally marl), or white clay.

In France, where both chalk and clay are of frequent occurrence--the soil of Champagne, for instance, being all chalky--the manufacture of the white earth colours is extensively practised, and a large number are put on the market, usually named after the place of origin, and consisting of either calcium carbonate or marl.

The trade names of the white earth colours include Cologne chalk, Bologna chalk, Briançon chalk, Champagne chalk, Blanc de Bougival, Blanc de Meudon, Spanish white, Blanc d’Orleans, Blanc de Troyes, etc. All are either more or less pure chalk, marl, or a fairly white clay, pipeclay--which is also used for making clay pipes and for removing grease spots.

GYPSUM

The mineral known as gypsum, or alabaster, consists of calcium sulphate, or sulphate of lime, its composition being expressed by CaSO{4} + 2H{2}O. In gypsum the crystalline structure is just discernible, whilst other varieties, such as the so-called “marine glass,” occur in considerable quantities as large, perfectly transparent masses. “Russian glass” consists of large, transparent lumps possessing the specific property of gypsum, viz. that of cleaving in two directions, in a high degree. Alabaster is composed of finely granular masses, which are either quite white, or else yellowish, or traversed by grey veins. This variety of gypsum is very abundant in central Italy, and the best blocks are employed for the production of works of art.

Ordinary gypsum, which frequently occurs in the vicinity of dolomitic limestones, is found in a great variety of colours, bluish-grey, yellowish or reddish tints being the most common. Pure white lumps, which are plentiful in some deposits, can be used as white pigment, the method of preparation being simple, viz. merely reducing the mass to powder. This is easily effected, the specific hardness of gypsum being only 2; and in many cases it is soft enough to scratch with the finger-nail.

If the original gypsum is white, the powder forms a dazzling white flour which, notwithstanding, is of comparatively little value as a pigment, on account of its low covering power. For this reason, powdered gypsum is chiefly used for making plaster of Paris (calcined gypsum) for plaster casts and stucco. Gypsum may also be employed to advantage for lightening various colours, since it is inert towards even the most delicate.

KAOLIN, PIPECLAY

Large areas of the earth’s surface are covered with clay, which often attains a considerable thickness. Nevertheless, the kind of clay that is suitable for use as pigment is comparatively scarce. The principal requirement for this purpose is a pure white colour, but by far the great majority of clays are either yellow or of a shade between blue and grey (for example the clay of the Vienna basin).

The character of clay is just as varied as its colour. In some places, large deposits of extremely fine clay are found, the material, when mixed with water, forming a highly plastic mass which, when dried and subjected to slight pressure, furnishes a very soft powder. On the other hand, some clays are so interspersed with large quantities of sand, large stones and the debris of mussels, that they cannot be used until they have been put through very careful mechanical treatment.

This great divergence in the physical character of clays is due to their method of formation. Clay originated in the weathering of felspar, which chiefly consists of a double salt, a compound of the silicates of alumina and potash. Under the influence of air and water, this compound is decomposed, the potassium silicate passing into solution, whilst the aluminium silicate, being insoluble in water, is carried away by that medium. When the water can no longer carry the particles of aluminium silicate in suspension--for example when it reaches a sea or lake--the silicate settles down to the bottom, and a deposit of clay is formed.

If the original felspar was very pure, and in particular very low in iron, the resulting clay will be of a handsome white colour. An example of this is afforded by kaolin, or porcelain earth, which is preferably used for making china. If, however, the felspar contained a considerable proportion of ferric oxide, the resulting clay is yellow; and if stones or mussel shells became incorporated with the clay prior to deposition, these bodies will be found as inclusions in the deposit, and such clay will require much troublesome preparation--grinding and levigation--before it is fit for use.

For the purposes of the colour-maker, the most suitable clay is one that is pure white, free from inclusions, and does not change colour when exposed, in a finely divided state, to the action of the air. Many clays that were originally white gradually assume a yellow tinge on prolonged exposure to air and moisture, because the clay contained ferrous oxide, which changes, in the air, to the stronger pigment, ferric oxide.

Many kinds of clay merely require a simple levigation to fit them for use as pigment. The lumps of freshly dug clay are placed in large tanks, etc., filled with water and stirred up continuously in order that, instead of forming a plastic mass which is very difficult to distribute in water, the particles detached from the lumps may pass at once into suspension. This turbid water is then transferred to another tank, etc., where the minute particles of clay are allowed to settle down, and the water becomes quite clear.

Where this work is carried on on a large scale, it is advisable to put the freshly won clay into large pits close to the clay deposit, and to leave it there, covered with water, during the winter season. The freezing of the water breaks down the larger lumps of clay, by the resulting expansion, and this facilitates the subsequent levigation, the cohesion between the particles being destroyed.

If the clay contains larger proportions of lime or magnesia, a little experience will enable their presence to be detected at once by the way the clay behaves on being placed in contact with water. Pure clay quickly forms a fatty and extremely plastic paste, and sticks closely to the tongue when applied in the dry state. On the other hand, clay containing much lime or magnesia is far less plastic when mixed with water, and the dry clay hardly adheres to the tongue at all.

These latter clays are classed as poor or lean, in contrast to the fat, plastic kinds. For certain purposes for which clay is used as pigment, these admixtures are not harmful; whereas others, especially quartz sand and mica, not infrequently present in white clays, constitute a serious drawback.

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

Quartz sand can be eliminated from clay without any special difficulty, quartz being of higher specific gravity and therefore settling down quickly, leaving the delicate particles of clay in suspension in the liquid. The scales of mica are harder to get rid of, their tabular form retarding deposition from the suspending liquid; and on this account, several washings are often required to separate them completely.

In all cases where clay is to be used as a white distemper, the presence or absence of lime is immaterial; but where it is to be employed for removing grease, lime is a drawback. This is also sometimes the case when the clay is wanted for the purposes of the colour manufacturer. The author has found, by experience, that perfectly pure, white clay forms a good paint, in a vehicle of oil or varnish--a purpose to which it has, so far, been seldom applied, if at all. Such paint is of good covering power, and possesses the valuable property of remaining quite unaffected by atmospheric influences.

If, however, the clay contains even but a small quantity of lime, it cannot possibly be used as an oil or varnish paint, for though the freshly made paint has a very good appearance, its character soon changes, turning viscous and suffering a considerable diminution of covering power. Thinning with turps or boiled oil results in the formation of small lumps, so that it is quite impossible to obtain a uniform coating on even a small surface.

This behaviour is apparently due to the presence of the lime, the explanation being that the fatty acids always present in the oils and varnishes used for the paint combine with the lime to form compounds which, from the standpoint of the chemist, must be regarded as soaps. The small lumps already mentioned really consist of lime soap, and the formation of these colourless compounds accounts for the lessened covering power.

Given a fine white clay, otherwise capable of forming a valuable pigment, it is sometimes possible, by simple means, to eliminate accompanying lime, provided the amount of the latter is not too great, and also provided that very cheap hydrochloric or acetic acid is available. The acid need not be pure, and the impure but very strong pyroligneous acid, which is very cheap on account of its empyreumatic smell, may be used.

To eliminate lime from the clay, the still moist levigated mass is introduced, in small quantities, into a vat containing the requisite quantity (see later) of hydrochloric or acetic acid, the addition being continued until the liquid gives only a faintly acid reaction with blue litmus paper. When the clay is run in, effervescence is produced by the liberation of the carbon dioxide displaced by the stronger acid employed.

The amount of lime present in a clay may be determined by very simple means. A small sample of the clay is dried by artificial heat, until of constant weight, and exactly 100 parts by weight of the dry mass are placed in a glass and suffused with hydrochloric acid, sufficient of the latter being used to make the liquid still strongly acid after effervescence has ceased.

The contents of the glass are transferred to a filter, and washed with pure water so long as the washings continue to redden blue litmus paper. The residue is then dried until of constant weight, and the difference between the initial and final weights will give the percentage of substances soluble in hydrochloric acid.

After performing this simple test on a clay, it is easy to calculate the quantity of acid needed to extract all the soluble constituents from a given weight of the material. All that is necessary is to measure the volume of acid required to extract a small quantity of the clay completely. Thus, if one pint of the acid at disposal is sufficient to treat one pound of the clay, the amount needed for a given quantity of clay is a simple matter of calculation.

Since, on account of the cost of pure hydrochloric acid, crude acid will always be used, it will be necessary to remember that this crude acid always contains ferric oxide in solution--this being the cause of its yellow colour. If the amount of acid taken is barely sufficient to combine the whole of the lime, leaving the latter slightly in excess, the ferric oxide--which would otherwise tinge the clay yellow--will be precipitated.

If, on the other hand, the acid is in excess, the clay is obtained free from all constituents soluble in the acid. The purified clay must then be freed from the calcium chloride, formed by dissolving the lime, by a thorough washing, since the clay would otherwise always remain moist on account of the hygroscopic properties of the chloride in question. Moreover, any small residuum of free acid would constitute a drawback on the clay being mixed with other colours.

Calcium chloride is very soluble in water, and therefore can be completely removed from the clay by washing. The purified clay is left to settle down as completely as possible, and after drawing the liquid off from the sediment, the latter is suffused with pure water and left to settle once more. As a rule, two such washings will cleanse the clay of calcium chloride and free acid sufficiently to render the product suitable for any purpose.

When large quantities of clay have to be treated in this manner, considerable amounts of calcium chloride solution will be obtained, which can be advantageously utilised for the production of precipitated chalk, all that is necessary being to collect the liquor in a large tank and treat it with a small quantity of slaked lime, to transform the surplus free acid into calcium chloride and precipitate the ferric oxide present in solution. At the end of a few days the liquor in the tank will consist of a very pure solution of calcium chloride which will furnish an excellent precipitated chalk when treated in the manner already described under that heading.

BARYTES, OR HEAVY SPAR

This mineral--chemically, barium sulphate, BaSO{4}--occurs native, as extensive deposits, in many places--England, Bohemia, Saxony, Styria, etc. It sometimes forms handsome tabular crystals, but more frequently compact masses, which may be pure white, grey yellow, etc., in colour, and are distinguished by high specific gravity (usually 4·3–4·7), to which the mineral owes its name. This high density also limits the application of the mineral, and it cannot be used as a pigment, in the true sense of the term, being only suitable as an adjunct to artificially prepared colours.

The employment of barytes in the colour industry is often regarded as adulteration, which, however, it is not when the case is considered from the right point of view. For instance, the only preparation which can properly be termed white lead consists of basic lead carbonate. This, when pure, is a rather expensive pigment, whereas, for certain purposes, the consumer requires a product that can be obtained at a low price. In order to satisfy this demand, the only course open to the colour-maker is to mix the white lead with a cheap white substance, which enables him to turn out different grades of white lead, which, although low in price, are far inferior to the pure article in covering power. Pure white lead being itself a very heavy substance, the only bodies suitable as adjuncts are such as are also of high specific gravity; and of all the cheap pigments known, heavy spar is the only one endowed with this property. Consequently, this substance is extensively used in making the cheaper grades of white lead and the pale kinds of chrome yellow.

The only cases in which the addition of heavy spar to a colour can be regarded as an intentional fraud on the consumer is when he is sold, as pure white lead, chrome yellow, etc., a product really composed of a mixture of such colour and barytes. Moreover, the presence of barytes in white lead can be easily detected by a simple examination, pure white lead readily dissolving, with considerable effervescence, in strong nitric or acetic acid, whereas barytes is insoluble in all acids, and therefore remains, as a heavy white powder, at the bottom of the vessel. In this way both the presence and amount of barytes contained in a sample of white lead or chrome yellow can easily be ascertained.

The preparation of barytes for the purposes of the colour-maker is entirely a mechanical operation. The barytes, which though fairly hard is easily reduced, is crushed with stamps, ground in a mill and finally levigated, it being impossible to obtain a sufficiently fine powder even by repeated grinding.

Native barytes must not be confounded with the artificial barium sulphate sold as permanent white or blanc fixe, which is an extremely finely divided barium sulphate obtained by precipitating a solution of a barium salt with sulphuric acid or a soluble sulphate, and is a painters’ colour that is highly prized for certain purposes. Both the native sulphate and the artificial variety have the property of remaining completely unaltered by exposure to air, and they can therefore be mixed with any kind of pigment without fear of the colour deteriorating.

As a rule, barytes is first roughly crushed in edge-runner mills or stamps, and then ground to the extreme degree of fineness obtainable in ordinary mills. Even with the greatest care, however, it is impossible by this means to obtain sufficient fineness of division for mixing with fine colours, the only way in which this can be accomplished being by levigation.

Given a fairly pure white barytes to begin with, levigation furnishes a handsome white pigment that can be mixed with colours of any kind; but when used by itself in association with oil or varnish, its covering power is very low and the colour never perfectly white. Native barytes is therefore unsuitable, as such, for paint.

Varieties that are not pure white are sometimes corrected with ultramarine, added in the grinding-mill. If the yellow tinge is due to iron compounds, this can often be remedied by treating the finely ground material with hydrochloric acid, which dissolves them out, this treatment being followed by a thorough washing with pure water.

As already mentioned, white lead is most frequently mixed with barytes, this being usually added when the white lead is being ground, by feeding the two materials to the mill and grinding them together.

The crudeness of mechanical methods of reduction is clearly exemplified by comparing the most carefully ground and levigated barytes with that obtained by artificial means. The permanent white largely used in the production of wall-paper, and quite unalterable in air, is, chemically speaking, identical with native barytes, viz. barium sulphate. The two also seem to be identical in crystalline habit, as is usual in the case of one and the same mineral, whether native or prepared by artificial means. Artificial barytes is obtained by treating a soluble salt of barium with sulphuric acid, or a solution of sodium sulphate (Glauber salt), so long as a precipitate continues to form.

This precipitate is barium sulphate, which subsides completely on account of its extreme insolubility, this being greater than that of any other salt known. The rapid rate of deposition results in the formation of extremely small crystals, which, being colourless and reflecting the light completely, appear to be perfectly white. Even when permanent white is applied in very thin layers to any surface, its covering power is very considerable, by reason of the extremely fine subdivision of the material.

This behaviour of artificial barytes in comparison with that of the natural product, affords an important hint in connection with the preparation of earth colours, namely, that in order to obtain products of specially good quality, the endeavour should be to reduce the raw materials to the finest condition possible. This result is accomplished most securely by bestowing the greatest care on grinding and levigation; and it is therefore highly important that the manufacturer should select, from the various apparatus used in reducing the materials, those that are best adapted for the purpose.

CARBONATE OF MAGNESIA

Although carbonate of magnesia is seldom used alone as a pigment, it can be advantageously employed as such when circumstances permit. It is met with not infrequently, in Nature, in a crystalline form, as magnesite or bitter spar, the latter name arising from the fact that the soluble salts of magnesia have a bitter taste. Still more frequently, magnesia occurs in association with calcium carbonate, in the mineral dolomite, which contains up to 20% of magnesia.

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

This product can also be prepared artificially, by treating a dissolved magnesium salt with a solution of carbonate of soda, the result being the formation of a pure white precipitate, which is very brilliant when dry, and is characterised by unusually low specific gravity. In some places, conditions are such that this preparation can be made on a large scale at very low cost. For instance, there is a spring at Bilin, in Bohemia, the water of which contains large quantities of alkali carbonates in solution; whilst in the vicinity of Saidschütz is a spring fairly rich in magnesia salts. The waters from these two springs are concentrated by evaporation, and mixed in large tanks; and when a sufficient deposit of the resulting basic carbonate of magnesia has accumulated, it is taken out of the tanks, placed on linen filters and washed with water. The residue is dried slowly, without the employment of a high temperature, and then forms a white powder, which is very light and can be used for a number of purposes, chiefly medicinal, though it is also well adapted as a material for paint.

For this latter purpose it is, however, far too expensive; but since the conditions obtaining at Bilin are certain to occur elsewhere, we have included carbonate of magnesia among the earth colours.

On account of its specific lightness, carbonate of magnesia is specially adapted for making pale shades of certain delicate lake colours, which, if toned with even perfectly pure chalk, would undergo alteration in course of time. Carmine, for instance, can be graded, by the addition of carbonate of magnesia, into every possible variety of shades between the pure red of carmine itself and the palest pink; and the resulting colours are quite permanent whether mixed with gum solution or any other vehicle.

TALC

Although this mineral is not used as a pigment by itself, it must be mentioned here because it is not infrequently employed for mixing with other colours, and is also used in the wall-paper industry. It also serves to distribute certain pigments in a state of fine division, the “rouge végétal” of the perfumer, for example, usually consisting of talc and a small quantity of very fine carmine.

In commerce the name talc is sometimes applied to two separate minerals, true talc and steatite or soapstone. The former is rarely met with native as well-defined crystals, mostly occurring as scaly masses in primitive rocks. Thin pieces exhibit a certain degree of flexibility. The hardness of this mineral is so small that it can be scratched with the finger-nail; and its sp. gr. is 2·9–2·8. Talc is easily scraped, and the powder remains sticking to the knife, a property which renders the substance difficult to reduce to powder, because it balls together and takes a very long time to convert into a fine flour. The process is facilitated by calcining the talc and quenching it in cold water, this treatment increasing the hardness and at the same time making it more brittle, and thus more easy to pulverise.

A characteristic feature of all the talc minerals is their peculiar greasy appearance and feel. The colour varies, white pieces alone being of any use to the colour manufacturer. The yellow- or green-tinged varieties owe their shade to the presence of ferric and ferrous oxides. In chemical composition, talc consists of a combination of magnesium silicate with hydrated silica, the supposed formula being: 4MgO . SiO{2} + H{2}O . SiO{2}, and the percentage composition: silica, 62·6%; magnesia, 32·9%; water, 4·9%.

STEATITE OR SOAPSTONE

Steatite so closely resembles talc in most of its properties, that the two minerals were long regarded as identical. Whereas, however, talc is scarcely acted upon at all by the strongest acids, steatite is completely decomposed by prolonged boiling therewith, although both minerals have exactly the same composition.

As a pigment, steatite is far more important than talc, and, as French chalk, is largely used for drawing or writing. To prepare it for this purpose pure white steatite requires no preliminary treatment, beyond cutting the large lumps up into quadrangular prisms, which are mounted in wood, like lead pencil, and used for writing on the blackboard. The powder produced in the cutting process is made up into pastel crayons. With this object, the powder is mixed with a sufficient quantity of some mineral pigment to produce a mass of the desired shade, and is kneaded to a stiff paste with water containing an adhesive such as gum, glue or tragacanth mucilage. The mass is shaped into prisms, which, when dry, are cut into pencils and mounted in wood. Steatite being like talc, without action on even the most delicate colours, can be used as a diluent in the preparation of light shades.