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

by George Terry

In order to obtain the necessary draught, the conduits h are open to the outside atmosphere, and introduce a supply of air just below the mouths of the retorts a, so that it impinges against the current of escaping zinc vapour. After passing through the settling chambers f, the superfluous air finds an outlet at i into a sufficiently capacious flue k, which communicates with the chimney stack d. Thus the draught created by the fuel consumed in the reverberatory furnaces is made to assist the current through the settling chambers.

These settling chambers f, are constructed of wood, and are usually about three in number, intercommunicating of course. The zinc oxide enters the first compartment through an aperture in the top of the side to which the discharge end of the iron flue e is attached. After traversing the first chamber, the stream of air and such oxide as has not yet settled passes into the second chamber through the orifice l, near the bottom of the partition dividing the two chambers. To reach the next compartment, the stream has to ascend again, the aperture being at the top of the partition, and this alternation is carried on to the end of the series, thus checking the through draught and facilitating the settlement of the zinc oxide. The floors of all the chambers are made funnel-shaped, with a door at the lowest point, so that the discharge of their contents may be as automatic as possible. The flue k contains screens hung at intervals for the purpose of hindering, as far as possible, the escape of minute particles of zinc oxide into the chimney, and thence into the outer air, whereby they would be lost.

A description of the process as conducted in Belgium, says that ingot zinc is placed in a series of retorts within one furnace, and the oxide is formed in an exhaust chimney, and then passes through a long series of passages and condensing chambers, in which are ranged tanks of sheet iron or cloth to collect deposits. At a certain hour of the day it is collected into casks, and after being tested as to quality it is ready for delivery. According to the purity of the metal various qualities are produced. The best is called “blanc de neige,” or snow white, and is of a very superior quality; No. 1 white is the most used, the ores for this quality being selected and purified by remelting; No. 2 white is the common variety. In the process of manufacture there is more or less waste material, imperfectly oxidised, deposited in the retorts and passages. This residuum is carefully ground, washed and dried, and is employed in painting in the place of lead.

In the American method of making zinc white they use the ore direct. This is cheaper than the Liège method, but its product is of inferior quality to that produced by sublimation. There are but two works in Belgium for making zinc white, the Vieille Montagne Company (at the Valentine Cocq works), which produce yearly 3000 tons by sublimation; the other is at Ougree, near Liège, where the American method is employed, but at present it is idle.

There are many other modifications in detail in different works. One may be noticed here as it is claimed for it that the pigment is gifted with greater covering power or body, the limited degree of which is the only drawback to zinc oxide whites. The plan consists in this, that the oxide is allowed to collect in the condensing chambers till it is of such a depth that a man entering stands waist deep in the pigment. The latter is gathered in pieces of sacking, which are drawn together and squeezed up tightly, so that the oxide, when newly prepared, is pressed into hard dense masses.

(2) Sulphide.—Prof. Phipson, in a paper read before the International Health Congress, at Paris, remarked that for several years efforts had been made to discover some white substance to replace white lead for painting buildings, ships, &c. He himself had devoted several months to this important subject, but without success. There has been found, it is true, in oxide of zinc a substance less poisonous than lead, and serving very well as a white pigment in oil painting; but its production is very expensive, and its mechanical properties as a colour in oil are not pronounced enough to allow it to compete in commerce with white lead. Such is not the case, however, with an invention of Mr. Thomas Griffiths, of Liverpool, who has succeeded in obtaining a very interesting product. This new preparation, which is being manufactured at the present time on a pretty extensive scale, has for its base sulphide of zinc (or an oxysulphide of that metal), the properties of which as an oil colour are of the most remarkable character. It is prepared by precipitating one of the salts of zinc by a soluble sulphide, and washing and drying the precipitate. The latter is then calcined at a red heat, with some precautionary measures, then taken from the furnace, and, while still warm, thrown into cold water. It is afterwards levigated and dried. The result is a white pigment, very fine, and of great beauty. Regarded from a hygienic point of view, Griffiths’ new white is infinitely superior to white lead, as it also is in its practical bearing; it possesses no injurious qualities; its manufacture and use do not affect the health of workmen; its durability in climates of the most diverse kinds is, so to speak, illimitable; it is altered neither by gaseous emanations nor by dampness; and its price is comparatively low. The most remarkable thing about this new white is that it covers as well as white lead, while it withstands the effects of all kinds of weather, so that its use is not only deprived of all danger to health, but it is much more economical than white lead. Prof. Phipson stated to the Congress that he regarded this new chemical preparation as being among the most ingenious and useful products that have been discovered in our time.

A later method, introduced by Griffiths and Cawley, consists in making an artificial sulphide of zinc by bringing the vapours of zinc and sulphur into intimate contact.

In carrying out this process, sulphur is melted in a jacket pan heated preferably by high pressure steam. The melting vessel is connected with a cast-iron still by means of a jacketed pipe, and the connection is regulated by means of a valve in the bottom of the melting pan. The latter should be at such a height above the still that the pressure due to the column of sulphur in the conduit pipe may be greater than the tension of the sulphur vapour in the still, so that when the valve is opened, the sulphur in the melting pans may descend into the still. The still is kept during the process at a temperature of incipient redness, so that when the sulphur reaches it, the sulphur is immediately vaporised and the resulting current of vapour passes to the chamber described below.

Metallic zinc is melted in a retort or crucible, heated preferably by means of a furnace on the Siemens or a similar principle, and raised to such a temperature that it begins to volatilise freely. When this takes place, the resulting zinc vapour is met by a current of sulphur vapour obtained as above described, and in excess of that required to form with the zinc sulphide of zinc. The reaction takes place according to the chemical equation Zn + S = ZnS.

The sulphide of zinc is in the form of white extremely light powder, which is carried along by the current of sulphur into the collecting chamber, such as is used for the manufacture of oxide of zinc. This allows of the separation of the different constituents of the products into different qualities; those parts that are carried the farthest are the whitest and best generally. Those portions nearest to the part of the apparatus where combination takes place may sometimes contain metallic zinc, if the sulphur supply has not been carefully attended to, but this may be separated from the sulphide by levigation.