Chapter VII: Whites (4)

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.

The collecting apparatus should be kept at a temperature slightly superior to that of the boiling point of sulphur, in order that sulphur, which is necessarily in excess, may not be deposited with the sulphide of zinc, but may pass on in the vaporous form to a suitable condenser, where, after condensation, it may be collected and used again.

Before the sulphur vapour reaches the condenser, the last traces of sulphide carried along with it are collected by the interposition of metallic screens or sieves, placed between the sulphur condenser and the apparatus.

In carrying out the process, care must be taken to keep the collecting apparatus as cool as possible consistently with the fulfilment of the conditions above mentioned, viz. that no sulphur be condensed therein. In practice, this object can be effected with little difficulty. Impurities in the zinc and sulphur are of little consequence provided they are not volatile, and not of such a nature that they would detract from the whiteness of the sulphide of zinc formed.

The accompanying diagram, Fig. 29, is given as an example of a plant that may be employed with good results.

_a_ is the sulphur melting pan with its steam jacket _b_, and steam pipe _c_; _d_ is a cast-iron still, arranged within a gas furnace _e_; _f_ is a crucible for melting and volatilising zinc, the said crucible being contained in a gas furnace _g_; _h_ is an automatic apparatus for freeing the mouth of the zinc vessel from deposited sulphide of zinc; _i_ is the collecting apparatus; and _k_, the condensing apparatus.

(3) _Mixtures._--There are a number of compound pigments commonly known as zinc whites, which only deserve the name in so far as they contain a proportion, greater or smaller, of some zinc salt. At the same time it must be admitted that some of these combinations possess very good qualities, and that the foreign ingredients largely correct the weak points of the zinc compounds.

Freeman’s.--This pigment, when ground with oil in the customary way, forms a paint equal in body and covering power to the best white lead, while it is superior in colour, permanence, and density, and is free from odour and noxious qualities.

It is produced by grinding together “zinc white” (either oxide or sulphide), lead sulphate and barium sulphate, in certain proportions, in the dry state, in an edge-runner mill. By thus grinding the several pigments together, their particles become intimately incorporated and undergo changes in character. The barium sulphate not only cheapens the product by reason of its low cost, but also imparts a distinct feature in rendering the paint more free working. The proportions generally adopted, calculated by weight, are 5 parts lead sulphate, 2 of zinc white, and 1 of barium sulphate. The duration of the grinding will necessarily vary in accordance with various governing conditions, but it should be continued until the mixture has a density of about 200 lb. per cubic foot.

Orr’s.--The pigment known as Orr’s enamel or Charlton white, is a compound of oxide and sulphide of zinc and sulphate of strontia, or of baryta. It is prepared in two ways.

(_a_) Barytes is calcined for some hours at white heat with charcoal, and the calcined mass is lixiviated with water to wash out the barium sulphide; to one-half of this solution is added zinc chloride, which produces a precipitate of zinc sulphide, leaving barium chloride in solution. To this mixture of zinc sulphide and barium chloride is added the remaining half of the barium sulphide and some zinc sulphate, the result of which is a double precipitate of zinc sulphide and barium sulphate. This is water-washed, filter-pressed, dried, calcined at red heat, thrown immediately into cold water, ground very fine, and finally dried.

(_b_) The second process closely resembles the first, but celestine takes the place of the barytes.

Either form of Orr’s enamel is a good useful white pigment, very permanent, mixing well, of excellent covering power, and pure in colour.

_Characters._--Zinc oxide, being an expensive pigment, is liable to adulteration; fortunately, all such adulterations are easily detected, and their nature ascertained by a few simple tests. Zinc oxide, if pure, should dissolve entirely without effervescence in nitric acid; any residue would indicate adulteration with barytes or china clay; the former may be distinguished by its weight and the yellowish green colour it imparts to the Bunsen flame, the latter is lighter and gives no colour to the Bunsen flame. Boiled with strong sulphuric acid, barytes is not acted on, while china clay is. If, after cooling, the mass be diluted with water, and ammonia be added to the liquor, if barytes is present no precipitate will be obtained, while if china clay is present a white precipitate is produced.

If the zinc oxide dissolves with effervescence, white lead or whiting may be present; the solution should give no precipitate of black sulphide of lead on passing sulphuretted hydrogen through it. On neutralising the solution in nitric acid with sufficient ammonia, and adding ammonia sulphide to precipitate all the zinc (the precipitate should be white, any other colour would show some impurity), filtering off and adding a little oxalate of ammonia, no white precipitate of calcium oxalate should be obtained; such a precipitate would show presence of whiting or gypsum.

The white pigments having as a base the sulphide of zinc, also contain barytes, oxide of zinc, sulphate of strontium, &c. They can be distinguished by evolving sulphuretted hydrogen gas, recognisable by its odour, on treatment with an acid. They are not entirely soluble in acids, the residue being mostly barytes, but may also be sulphate of strontium; it is immaterial whether the two be distinguished or not.