Vienna White
This colour is prepared from any kind of burnt lime that is sufficiently pure; that is, free from ferric oxide. The method of preparation is simple, requiring no special apparatus, and can therefore be carried out wherever suitable lime is available.
Operations are commenced by carefully slaking well-burnt lime with water, a sufficient excess of which is added to produce a fairly thick pulp. To accelerate the absorption of carbon dioxide, the mass is exposed to the air in thin layers, by spreading it out on boards, so as to present a large surface to the air. As soon as the pulpy character has disappeared, the mass is detached from the boards, and is pressed and kneaded, with wooden paddles, into prismatic cakes which are left exposed to the air—being, of course, protected from the wet—until the absorption of carbon dioxide is complete—a condition that can be recognised by the earthy character of the product. The cakes are then dried, an operation entailing great care, since lightness is a sign of good quality, whereas a damp product is very heavy.
In forming the cakes they must not be touched by the bare hands, because the lime is so caustic that it would soon destroy the skin. The foregoing method of manufacture is capable of many improvements, which can be introduced without adding much to the cost of production.
If the lime is formed into large blocks, it will evidently take a long time for the mass to acquire, all through, the earthy character indicating combination with carbon dioxide. This drawback can be easily remedied by forming the mass into small cakes, which will become ripe, owing to their larger surface, much sooner than the bigger blocks.
A very good plan to adopt in moulding is to form the burnt lime into a stiff paste with water, preferably by adding enough water to make a viscous mass, and leaving this in a lime-pit for several weeks, the prolonged storage enabling the lime to acquire the already mentioned fatty character, and at the same time to become highly plastic. Lime treated in this way can be forced through a nozzle, forming a cylindrical rope, which can be cut by a knife into convenient lengths and left on boards for a few days until they have become firm enough to stand up without breaking. Cylinders made in this manner, with a length of about four inches and a diameter of two inches, will absorb carbon dioxide very quickly.
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.