White zinc as a pigment is next in importance to white lead. It is made by strongly heating metallic zinc in fireclay retorts in a reverberatory furnace. The heat vaporizes the metal, which vapor is brought into contact with air heated to 300 degrees Fahrenheit. Oxidation results. The oxide is a very loose flocculent material and is carried by the hot air into condensing chambers where it is deposited ready for use.

Zinc white is often prepared directly from its ores. The roasted ores are pulverized and heated in a furnace on a bed of coal, and when fully ignited are submitted to a current of air from beneath the grate. The vapors formed are kept strongly heated along with a current of air and led into condensing chambers. Zinc made direct from the ores varies in whiteness, but is, for the most part, a good commercial product, the off grades produced being sold for other purposes where color is not an object.

It is also produced by the action of lime water on a solution of zinc chloride. Zinc white takes much more oil than white lead to make a suitable paint and, hence, will cover more surface, but is more transparent and, therefore, requires more coats to produce the same dense covering given by white lead. It is practically a neutral pigment in the sense that there is little chemical action between it and linseed oil, and zinc has the great advantage of not being darkened or discolored by sulphur gases or sulphides as is lead, and is less injurious to workmen, being non-poisonous. It dries with linseed oil to a hard glossy surface, and is less likely to blister when exposed to the sun and withstands changes of temperature better. In hot climates it is very suitable to use. Its chief fault is a tendency after a time to crack under certain conditions when used alone, and its poor covering (masking) qualities. When zinc is used, instead of lead, more dryer is needed as it does not assist the oil in this function.

Oxide of zinc has the property of combining with the gum resins, and this property is taken advantage of in the manufacture of enamels in which, for this reason, and because of its whiteness and the sharper, clearer tints it gives with colored pigments, makes it particularly useful.

A combination of zinc and lead in the proportion of 75 per cent. lead to 25 per cent. zinc, gives a paint of great durability, good body and covering power, and less likely to be acted upon by external influences than pure white lead.

Such has been the experience of many who have carefully tested this combination. In passing, it might be stated that zinc oxides produced by the French process direct from pure metallic zinc or spelter, particularly the imported article, is superior in whiteness. The terms green seal and red seal French zinc were originally used to designate first and second grade oxides, but at the present time the designation means little or nothing and in purchasing oxide of zinc, one must depend upon the reputation of the grinder or distributor as a guarantee of the quality represented.

The advantages and drawbacks of both white lead and zinc oxide have been plainly stated. It is simply a matter of judgment as to using the one or the other or a combination of the two according to the requirements of the paint to be used.

Tests

The purity of zinc oxide is easily determined. If pure, it will dissolve completely without effervescence in boiling dilute nitric or hydrochloric acid. If it does not completely dissolve, barytes is likely present. If it effervesces while dissolving, it contains whiteing, white lead, or zinc carbonate, which is an inferior pigment.

If, on heating in a tube, the sample turns yellow and gives off vapor, white lead is present, as pure oxide of zinc undergoes no change when heated.