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.
Fig. 26.
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.