The manufacture of earth colours

by Josef Bersch

(G) Black Earth

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

Black Schist

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

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

Graphite

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

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

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

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

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

CHAPTER III
THE PREPARATION OF THE COLOUR EARTHS

The preparation of the raw materials for the purpose of making earth colours is a very important matter, because many minerals or pigmentary earths merely require mechanical treatment to render them at once fit for use. The mechanical preparation differs considerably, in accordance with the raw material under treatment, substances that are found native in a finely powdered condition only needing, for the most part, to be levigated.

It rarely happens, however, that the raw material occurs in condition for use direct, an example of this kind being afforded by the finest clays or ochres. Whilst these are found in a state of extremely fine powder, they nearly always contain certain quantities of sandy ingredients or even large lumps of foreign minerals, and therefore require levigating. Sometimes they need crushing as well, the small particles cohering so strongly that mere treatment with water (levigation) is unable to separate them. Mechanical force is therefore necessary, a passage through grooved rollers being generally sufficient to crush the lumps; but in some cases stamps have to be used.

When solid materials have to be treated, mechanical appliances must always be used, their selection depending on the materials in question. Thus, gypsum, for example, can be crushed with ordinary rolls or mill stones, its degree of hardness being so very low (2) that it can be scratched with the finger-nail.

If, however, the material to be reduced is limestone, which belongs to the third degree of the scale of hardness (can only be scratched with an iron nail), or heavy spar (hardness 3–3·5), very powerful stamps or edge-runners must be employed to break it down into small lumps, which can then be further reduced, without any special difficulty, in an ordinary mill.

It is thus evident that a great variety of mechanical appliances are used in the manufacture of earth colours. Before going into their construction it is necessary to point out that, whatever the mechanical treatment employed, a considerable expenditure of mechanical force is entailed; and more power is needed when mixtures have to be prepared. It is therefore essential, in planning a factory for making earth colours on a large scale, to make provision for ample motive power.

This power may be supplied by a steam engine; but it must not be forgotten that the prime cost and running expenses of such an engine are considerable, and form an important item in view of the low value of most earth colours. Consequently, it is highly important to be able to generate motive power as cheaply as possible.

Now, the cheapest and most uniform source of power is water; and therefore, wherever the conditions allow of the erection of the colour works near a stream or river, which can supply the power to run the various machinery, the most favourable circumstances will have been secured, the power being obtained at minimum cost, whilst the upkeep of the motor cannot be very great. If there is sufficient head for the water to be run through a trough over the top of the levigation tanks, the conditions will be ideally favourable.

Wind power costs nothing, once the motor has been installed; but unfortunately, one is dependent on the weather, and sometimes there is not enough wind, for days together, to drive the sails at all, and therefore all the operations have to be stopped, including levigation, the water for which has to be raised by a windmill pump.

In districts where the winters are severe, water power may also fail and work have to be stopped; and consequently, even when water power is the prime source of energy, a steam engine must be installed as a stand-by, being, of course, only used when the main source of power gives out or proves insufficient.

The machines employed for preparing the raw materials in the manufacture of earth colours may be divided into the following groups:—

Machines operating entirely by pressure: crushers; machines acting by impact: stamps; those acting by impact and pressure: vertical mills (edge-runners), ball mills, centrifugal mills; and, finally, machines with a frictional action: grinding mills. Then there are the levigating machines, which do not reduce the material but separate the coarser particles from the finer. The construction of the foregoing machines is a matter for the machinery manufacturer rather than the maker of earth colours; but as the business of the latter is dependent on them, a short description is considered necessary. The selection depends, on the one hand, on the nature of the materials to be treated, and, on the other, on the size of the works, since a manufacturer who has to deal with large quantities of a given raw material will require different machines from those used on a small scale. The sole purpose of the following description is to indicate to the colour maker the way in which the reduction of the raw material can be accomplished.