through the grate. The zinc content of the ore is thereby reduced to metal, volatilized, and then burned in the excess of air in the upper part of the furnace and in the flues. The process is applicable to all the oxidized ores of zinc (oxide, carbonates, and silicates) or to the roasted sulphide, provided the gangue (waste rock) is not so fusible as to sinter together and form a mass that is impervious to the blast. Even fusible ores can be used if infusible material is added to keep the bed porous.

Ore. While the process can be worked on most ores, not all ores are sufficiently pure to yield a product with a good enough color to make it available as pigment. In general any impurity that will form a colored oxide or sulphide is deleterious. Cadmium is one of the commonest. Being very volatile, it always enters the fume. Its dark brown oxide and bright yellow sulphide both have strong tinctorial powers. A fraction of 1 per cent of this element in these forms seriously affects the color of the pigment. It is usually possible to convert cadmium in the pigment to the white sulphate or carbonate, and analyses of the 12 leading brands of zinc oxide, French process, lead free, and leaded grades on the market show 0.02 to 0.35 per cent cadmium (or up to 0.45 per cent, if figured as sulphide), but they are of satisfactory color.1

Lead, which is of almost universal occurrence in zinc ores, also injures the color, but to a much less degree (in spite of the fact that its sulphide and oxides are black or colored) than cadmium, because a basic sulphate, which is white, is generally formed. Since zinc oxide containing over 0.2 per cent lead can not be used in rubber making, practically lead-free ores must be selected for making the higher grades of zinc oxide. Sulphur may be found in the fume in the form of sulphide, sulphate, or sulphurous anhydrides. The first form is the most objectionable as to injuring the color, but the last-mentioned compound is believed by most, though not all, paint manufacturers to have a bad effect on the grinding properties of the pigment. Sulphates, if soluble, are exceedingly undesirable, since, especially in outside paints, they are liable to be leached out from the coat, leaving discolored spots. (The desirability of a low sulphur content of the furnace charge is evident.) Chlorides in appreciable quantity are rarely evolved from the charge, but should they be present in appreciable amounts in the fume they are open to the same objection as sulphates.

Fuel. The main requirement of the fuel is that it must not produce a black smoke. In the West, where anthracite is expensive, semibituminous coal, low in volatile matter, is sometimes substituted for the harder coal, but is not so good. Coke does not work well because it produces a locally intense heat, which may cause sintering and make the charge impervious to the blast. It is, however, used occasionally as a part of the charge fuel, although never alone.

Limestone. Lime flux is often added to siliceous ores to aid in their reduction, but it generally does more harm than good since it tends to make the residue too fusible.

Both ore and coal as usually received at the oxide plant are fine enough to obviate the necessity of further crushing; the ore is concentrated at or near the mine and is therefore fine enough, while the

IL. E. Wemple, Discussion, Trans. A. I. M. E. LVII (1918), p. 694.

smaller sizes of coal, being suitable and generally cheaper, are almost invariably used. If fluxes (limestone, etc.) are employed, a small crusher must be installed to break them down to approximately the same size as the rest of the charge.

A good example of zinc pigment works is that of the New Jersey Zinc Co., at Palmerton, N. J. This works has been described in detail,1 and the description with cuts of the furnaces and plans of the works are available in the auxiliary file.

At this plant the ores consist almost wholly of concentrates from the company's mines at Franklin furnace. They are divided into three classes as received. The average analyses of these ores illustrate the difference in composition.

The franklinite is worked by itself. The residue is rich in manganese and is reduced to spiegeleisen in a small blast furnace. The spiegeleisen is sold to makers of Bessemer steel.

The quality of the fume and the amount recovered depend on the selection of ore, the ratio of ore to coal, the ratio of both (ore and coal) to the grate surface, and, most important of all, the correct proportioning of blast and exhaust to the charge worked. Considerable care and experience are necessary to make a high-grade oxide. The recovery, expressed in percentages of the total zinc in the charge recovered as zinc in the fume, is never very high, as some zinc is inevitably left in the residue. The zinc recovery from franklinite averages about 86 per cent and that from the "half and half" and fines, which are worked together in the same charge, is about 78 per cent.

PRODUCTION.

There are no published production statistics covering the manufacture of lead-free or leaded zine oxides. In the census reports the productions of the various paint industries can not be satisfactorily separated. The manufacture of white lead or zinc white is the principal business of some concerns; some are engaged wholly in paint mixing, while other establishments make both crude pigments and mixed paints.

In addition to the products listed above, there were manufactured by smelters direct from the ore 106,791 tons of pigments, valued at $9,978,710, in 1914, as compared with 87,525 tons, valued at $7,963,332, in 1909. Since zinc oxide and leaded zinc oxide are practically the only pigments manufactured direct from the ore by a smelting process (the production of high lead fume is probably only a comparatively small fraction of such pigments) these figures can be assumed, without much exaggeration, to represent the combined productions of these two commodities by the American process. The above figures show an increase of 22 per cent in the amount of product

1 Geo. C. Stone, Oxide of zinc, Trans. A. I. M. E. LVII, pp. 682-695.

and an increase of 25.3 per cent in its value for the five-year period. The growth of the industry has been as rapid since 1914.

There are no data at hand covering the production of French process oxides. This is a comparatively new industry in the United States and has seen the most of its development since the outbreak of the war.

The growth of the zinc-pigment industry is illustrated by the following table, which shows the zinc content of zinc pigments made in the United States (from both foreign and domestic ore):

Primary zinc content of zinc pigments made in the United States, 1908–1915.1

The only zinc pigments exported in any quantity from the United States are American process oxides. In 1909 these exports amounted to about 27,000,000 pounds, or approximately 25 per cent of the domestic production. The maximum exportation was in 1915, when over 37,580,000 pounds of zinc oxide, valued at $1,808,970, were exported. In 1916 and 1917 the amount exported was practically the same as in the prewar period, running a little more than 31,000,000 pounds annually, but the enhanced price increased their value to fully three times that of the annual exports before the war.

The heaviest exports of zinc oxide have always been to England, and before the war the only important importers of American zines were European countries. Even Germany, the chief producer of French process oxide, imported over 2,000,000 pounds of the cheaper American material to be used in the making of lower-grade paints. A steadily increasing amount has been shipped to Canada, and during the war period the amount increased over 100 per cent. An important effect of the cutting off of supplies from the Central Powers and Belgium has been the diversion of a large part of the exports that formerly went to European countries to South America. While the amount of exports has not increased appreciably, this transfer of their destination may be important. The mixed-paint business has grown so rapidly that it has left little margin of American process oxides to be exported as crude pigment, particularly in the face of lessened importations.

FOREIGN PRODUCTION.

Data as regards foreign production are lacking. The United States is the only important producer of American process oxide. Prior to 1914 Germany was doubtless the leading producer of zinc oxide by the French process and was followed in close order by Belgium, Holland, and France. Very little American process oxide has been produced in Europe, due to the expense of the anthracite coal, and a

general lack of ore of the necessary purity. Practically the only source of the product was the Dutch imports of willemite (high-grade zinc ore) from New Jersey. About 20,000 tons of this ore were annually shipped from the United States to the Netherlands and were all used in the making of pigment.

There is very little leaded zinc oxide manufactured outside of the United States. Recently a small output has developed in Italy.

IMPORTS.

Since October 4, 1913, the classification of imports of zinc pigments adopted by the Department of Commerce has been as follows: Zinc oxide containing less than 5 per cent of lead (zinc oxide largely, but also includes some of the higher grades of leaded zincs), (1) dry and (2) ground in oil or water (mixed paints); lithopone; zinc sulphide.

Prior to the above date the distinction between zinc oxide and leaded zinc with large amounts of lead was not made. Importations of the latter class of pigments have never been more than a negligibe amount.

Tlhere was a large and steady importation of zinc pigments before the outbreak of the European war. These consisted of French process oxides and lithopone. About five million pounds of high-grade zinc oxide (dry) was imported annually from Germany, Belgium, and France. At that time no zinc oxide was made from spelter in the United States and it is doubtful if it would have been profitable to make it from American spelter, as the metal sold at a much higher price in the United States than in Europe. The imports of highgrade mixed paints was only a fraction of those of the cruder pigment and averaged less than 400,000 pounds annually.

By the wording of the 1913 act, zinc oxide containing up to 5 per cent of lead was classified with the highest grade or lead-free oxide; whereas leaded zincs were not included in this classification previously. This broadening of the classification had slight effect, however, on the importation because the domestic demand for leaded oxide even the higher grades has always been supplied by domestic manufacturers. Since French process oxide was not manufactured in the United States prior to 1915, the increase of nearly 20 per cent that followed the reduction of the duty on the dry pigment is due to the fact that it could be obtained at a lower price and could therefore compete on more nearly equal terms with American process pig

ment.

The tariff reduction in the case of zinc oxide ground or mixed with oil or water (mixed paint) served temporarily to check the steady decline in these imports that had been in progress since 1912. The imports in 1914 were practically equal to those of the preceding year.

The lowered duties on zinc pigments resulted in a marked reduction in the amount of revenue collected by the Government. This was most marked in the dry oxide and lithopone import duties. Although the amount of the former increased almost 20 per cent and the latter (including "zinc sulphide") nearly doubled, the reduction in revenue was from $61,500 to $41,200 and from $64,500 to $46,9001 or 33 per cent and 27 per cent, respectively.

1 Calculated to a basis of 12 months.

The outbreak of the great war cut off the European supplies of all zinc products and naturally resulted in a rapid falling off of the imports of all kinds of zinc pigments.

PRICES.

The price of all zinc pigments is normally dependent on the price for spelter, and the zinc contents of even the lower grades of American process leaded zinc sell at a slightly higher price than their value in terms of spelter. The only exception to this rule was in the first few months following the outbreak of the European war. Spelter prices then soared to above 30 cents a pound. French process oxide, being produced directly from the metal, promptly reflected the advanced prices and was hard to get because of the cutting off of German shipments and the general stringency of spelter. The price of American process oxide, however, was held back within moderate limits by the producers in order not to unduly hamper the paint industry which had become largely dependent on this product. When spelter was selling at from 20 to 30 cents a pound, the same amount of zinc was sold as oxide at a price less than half that of the metal.

Quotations for the various grades of oxide are listed on page 165 of this survey.

The average price received by makers of lithopone in 1917 was approximately $116.06 a short ton, or 5.8 cents a pound, as compared with quoted wholesale prices, which ranged from 6 to 93 cents a pound, being generally 6 to 7 cents a pound throughout the year.'

COMPETITIVE CONDITIONS AND TARIFF.

The duties fixed by the act of 1913 were so low as to permit of fairly active competition between domestic and foreign supplies of all zinc pigments. Even the importation of French process oxide, which at that time was not being manufactured in the United States, was increased because its lowered price placed it in a position to compete on more nearly equal terms with the American process oxides. While the imports of all zinc pigments were increased, the revenue derived from the duties collected were cut one-third on imports of dry zinc oxide and nearly as much on lithopone.

The competitive status of the domestic manufacturers of American process zinc oxide is excellent, and it is not likely that any foreign oxide of this character would be imported even if it were placed on the free list.

The domestic manufacture of French process oxide is a result of war conditions. Its survival is more closely linked with the spelter industry than is that of any of the other zinc base pigments, all of which require compensatory duties to cover the duty on spelter, which may, in the future as in the past, determine the differential between the foreign and the domestic quotations on the metal.

Zinc oxide ground in oil (paste) is not now an important item, as American pigment grinders are equipped to meet foreign competition

James M. Hill, Mineral Resources. U. S. Geol. Survey, Barytes, etc., 1917. Advance publication. (Vol. II, p. 290.)