The most important material of natural origin is, of course, wood. It is used for the vats employed in the manufacture of colours, for agitators, scaffolding, and, above all, for the construction of the sheds themselves. In recent years the place of wooden buildings has begun to be taken by reinforced concrete, but it remains to be seen how this lasts. Wood is surprisingly resistant to all chemicals, as it is only attacked on the surface, and this damaged layer serves to protect the material beneath.

In the first place comes American pitch-pine, together with larch and pinewood. Beech cannot be used owing to the large cracks, but oak vats are often found, which are expensive but very resistant. Other woods, with the exception of ash, are not used owing to their high price.

Vats are made of capacities of 20,000 litres; the stirrers are made of ash and are fixed to the stirring gear by means of wrought iron collars (see also under cast iron). Vats of these dimensions are seldom placed on a platform, as shown on Plate VII, but are usually stood directly on the ground. The pressure boiler is either sunk in the ground or else is stood by its side, and the liquid sucked out by vacuum.

Disintegrator for Colours (scale 1 : 20).

Fig. 42. - A. Charging hopper. B. Vibrating device for admitting charge, fitted with magnetic separator. C. Disintegrator.

D. Motor. E. Dust conduit, F, Dust chest. G. Dust filters.

Plate XVII.

If a tub is to be evacuated it must be strengthened internally with a cross-beam; further, to prevent it from flying to pieces on applying a pressure of 2-3 atmos. it must be strengthened by means of strong iron bars. In addition to the syphon tube, a small air tube is also fitted so that the suspensions of the precipitated dye may be kept stirred up by means of compressed air. If this precaution is not observed it may happen that a large portion of the colour remains at the bottom of the tub. All iron bands must be carefully painted with red lead, and often the whole vat is covered with it. If the liquid in the tub is to be heated to boiling it must be covered in to prevent the steam from escaping and as a precaution against accidents; a proper steam waste-pipe is also necessary, as shown clearly on Plate VII. Steam flues are provided with an air or steam tube with which a powerful draught can be created.

The chambers and frames of filter-presses are made of wood, and where alkaline liquids are to be dealt with larch, or, better, oak, is used in place of the resinous pitch-pine. For filter-press taps small pear-wood faucets are made use of.

Leather is used for driving bands, for the leather collars of hydraulic presses and other less important purposes.

Rubber is the most important of the artificial organic materials. It is used in many forms, such as tubing, as hard rubber for covering centrifuges, ladles, and taps. The rubber coatings of centrifuges last very well, but are rarely used in dye chemistry. Gallic acid is "whizzed" in them, but copper baskets, and even baskets coated with lead by Schoop's process, can often be used.

Manufactured organic substances are represented by filter cloths, which are made from cotton, jute, hemp, and wool. Filter-press cloths are usually made of cotton, wool being rarely used. Strongly acid precipitates are pressed in camel-hair cloths; for some time cloths made from Chinese pig-tails were in use, which exceed all others for durability. The so-called nitro-filters are much used as filter-cloths, but not in filter-presses, as they have only a very moderate degree of mechanical strength. These are always prepared from a special type of filter-cloth, and since the cotton shrinks on nitration the warp and woof must be of equal strength. Acid-resistant filters can be made only in the following manner: the dry, crude cotton filter is lightly stretched on an aluminium frame, and is dipped into 85-88 % nitric acid at 15-20o; it is then left in 66° Be. sulphuric acid for 20 minutes, after which it is thoroughly washed. Such filters can stand the action even of 60 % sulphuric acid at 100°, but they are at once destroyed by acid solutions of ferrous salts.