The manufacture of earth colours

by Josef Bersch

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.

CaCO3	=	CaO	+	CO2
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	+	H2O	=	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.