The manufacture of earth colours

by Josef Bersch

Mixing and Improving

It is very important that the maker of earth colours should always be able to turn out his products uniform in shade, and since the raw materials are liable to vary in character, and the composition of the earths from one and the same deposit is not invariable, the desired shade has to be obtained by mixing. For this purpose, standard samples must be prepared, for comparison in matching.

Fig. 24.

Mixing is a highly important operation, on the proper performance of which oftentimes depends the sale of certain colours and the reputation of the maker. It may be effected in various ways, such as shovelling the ingredients together or by combining the work with grinding in edge-runner mills, ball mills, etc. Another method is the mixing barrel shown in [Fig. 24], a strong cask mounted on an axial shaft driven by a motor, etc. The barrel is filled about two-thirds full of the materials to be mixed, and, after closing the feed door, is slowly rotated, since, if run at excessive speed, the contents are merely projected against the sides of the barrel by centrifugal force, and it can then be turned for hours without result. The mixing effect can be considerably increased by mounting the barrel so that the shaft is offset from the longitudinal axis of the barrel by an angle of about 30°, the contents being then moved from side to side at each revolution and thus more intimately intermixed by the twofold motion.

Fig. 25.

In addition to such home-made appliances, there are mixing-machines of the type illustrated in [Fig. 25], the body of which is fitted with a distributing worm at the top, and a pair of rollers at the bottom. Below the rollers, which are covered by plates that can be adjusted at a convenient angle, is a worm conveyor delivering into an elevator, outside the machine casing, which connects the two worms. One or more discharging-doors, according to the size of the machine, are provided under the worm conveyor at the end next the elevator. The feed hopper can be arranged on the elevator or on top of the machine, according to local conditions.

In working this machine, the elevator and distributing worm are started and the full charge is fed into the hopper. When it has all passed through the distributor and is lodged on the sloping plates and bottom rollers, the latter and the worm conveyor are set in motion, the material being then carried through by the rotation of the rollers and dropping on to the conveyor, which delivers it to the elevator, to be returned to the distributor. In this way the charge is kept in continuous circulation, and the finely divided particles are repeatedly intermingled, a uniform mixture being obtained. The effect is heightened by the grinding action of the rollers as the material passes between them.

The serial order of the various ingredients, their physical condition (granular or powder), and their density, are all immaterial, the mixing being effected so intimately that when, for example, a colour is shaded with aniline dyes, the ingredients are so completely blended in less than an hour that even the smallest sample then taken will perfectly represent the bulk.

These machines are made in various sizes, are entirely automatic, both in charging, discharging and mixing, and are quite dust-proof, the consumption of power being also small. If necessary, they can be combined with a crusher or sifter feeding direct into the hopper.

A simple means of ascertaining whether the mixing is completed, and one that can also be employed for judging the character of ground materials, consists in placing a sample of the product on a sheet of white paper and spreading it out, under gentle pressure, with a steel or horn spatula. No irregularities, streaks, spots or granules should then be discernible either by the unaided eye or under a magnifier.

Improving, which consists in staining earth colours with other (usually organic) colouring agents, to improve the shade, is an operation which is generally resorted to only in case of need, because it means extra expense, and is of no value unless light-proof colours are used. No permanent effect can be obtained by merely mixing-in coal-tar dyes at random. In addition to certain organic dyestuffs, artificially prepared mineral colours and colour lakes—artificial preparations of an organic dyestuff with an inorganic substratum—are also used for improving.

Another way of improving earth colours is by precipitating certain coal-tar dyes on them, in presence of a fixing agent. Of course the dyes used must not only be fast to light, but also inert towards the substratum and to any other ingredient, such as lime, that is subsequently added to the earth colours.

The following dyestuffs (Höchst) are suitable for direct precipitation on siliceous colours (green earths, clay, ochres, etc.).

Auramine, conc. O, I, II; new phosphine extra; chrysoidine A cryst., B cryst., C extra; Vesuvine (all marks); cachou brown D, G; dark brown M, MB; safranine G, GS conc., B conc.; rhodamine O extra, B, B extra; fuchsine (all marks); fuchsine acetate; cerise G, R; grenadine O, R, RR; maroon O extra; new fuchsine O, P; methylene violet (all marks); peacock blue P; Victoria blue B, R; thionine blue GO; methylene blue (all marks); malachite green (all marks); brilliant green (all marks); coal black O, I, II.