The manufacture of earth colours

by Josef Bersch

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.