Pigments, Paint and Painting: A practical book for practical men

by George Terry

To warm the interior of this chamber, which is constructed of any material that will resist the action of the vapours and gases, and which is not too absorbent, Prof. Gardner makes the bottom of the chamber of such a shape as to form a heating vessel to hold water or steam, this water or steam, or both, being kept at the required temperature by means of a steam coil. Matters are so arranged that the contents of this coil are protected from any excess of pressure, and consequently the temperature seldom or never exceeds about 212° F., unless for any special reason it is desired to raise it to a higher point. Sometimes the sides are constructed similarly to the bottom.

The materials of which these chambers are constructed must be capable of resisting the heated and acidified vapours within them. Cast or wrought iron faced with glass, slate, tiles, pewter, or glazed bricks; tinned copper, tinned brass, or pewter; timber, whether green or after carbonising by heat or sulphuric acid; all are more or less suitable. Means of observing the progress of the conversion, and means of lifting the contents in or out, must also be provided, as well as thermometers to indicate the temperature prevailing inside.

The gases and vapours which effect the conversion of the blue lead into white lead are generated outside the chamber just described, and are conducted into it by means of pipes, being first raised to such a temperature in excess of that which should exist inside the converting chamber as to allow for the cooling effect arising from the friction and loss of heat in passing through the various pipes and distributors attached to the converting chamber.

Owing to this extra heating, the gases and vapours are expanded, diffused, and commingled, and do not rob the interior of the converting chamber of any heat on entering it, so that the heat inside the converting chamber is kept constant, and can operate to further expand and diffuse the gases and vapours in contact with the blue lead inside the chamber. In practice it is found that the temperature of the converting chamber cannot be suitably controlled if any portion of the gases or vapours be generated within that chamber.

The blue lead is arranged in the converting chamber in trays, or on shelves or frames, so as to allow it to be completely surrounded and attacked by the vapours or gases, and thereby be converted into white lead, at the same time preventing the formation of direct currents or eddies. Framed supports resembling a dinner waggon serve well for holding the lead, and may easily be arranged to lift bodily in and out of the chamber with their burden of blue or white lead. The surface on which the blue lead is directly supported is made of graphitic carbon, hard coke, platinum, or carbonised or platinised material, such as is used for plates in electric batteries, or of other material standing in a similar electrical relationship to lead, or capable of generating with it an electric current.

In applying this development of electrical energy to the ordinary “stack,” as for instance the Dutch process, the pots containing the acetous liquid and blue lead are made of, or lined with, such electrical carbon, or contain a portion of it in suspension, by which the same effect is realised. Advantage may also be derived from furnishing a supplementary supply of carbonic acid to the stack beyond that due to the decomposing dung, &c.; as well as from injecting a current of air or oxygen at suitable temperatures, and from the admission of steam in a coil throughout the stack.

The generation or production of the acetic or nitric acid vapours is attained by heat in a vessel so arranged that its contents are kept at a certain temperature for vaporising, by having the boiling acid solution at about 1·003 sp. gr., procured by mixing water with vinegar or acetic or nitric acids.

The supply pipes conveying the air or oxygen and the carbonic acid, or either of them, can easily be made to emit their contents close to the boiling acid solution, whereby the vapours arising from the latter are mixed and intermingled with them, and all pass together through the pipes and distributors into the converting chamber or stack.

In preparing the carbonic acid it is necessary to observe certain precautions, especially that it shall be pure and that no carbonic oxide shall gain admission to the chamber. If the ordinary chalk and carbon method be employed, the furnace must be provided at the uptake with a series of air inlets, so as to ensure that the gases passing from the furnace to the delivery tube shall be most thoroughly oxidised.

The purification of the carbonic acid may be effected in the usual way by passing it through a vessel containing water; or, if requisite, it may be cleansed, expanded, and heated in one operation by passing it through or over hot water, or over a hot aqueous solution of carbonate of soda, all of which, however, incur considerable cost in plant and manipulation.