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.
Fig. 29.—Apparatus for Making Zinc Sulphide.
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.