METALLIC brown, natural indian reds, Venetian reds, purple oxides, ochres, siennas and umbers come under this head, and are various combinations, consisting mainly of iron oxide and silica or clay found in nature.

Metallic brown, an iron oxide much used, is prepared after being mined by simple roasting and grinding. It contains from 50 to 75 per cent. of oxide of iron, the balance being clay and silica.

In the case of the ochres, raw siennas and raw umbres, the earth is simply dried, washed, ground and floated; whereas, burnt sienna and burnt umber is produced by first roasting the raw product to the desired shade, or depth of color, and subjecting it to the further treatment above described. Other red oxides are similarly treated. These natural earth pigments are very stable and permanent and should be preferred wherever possible, for tinting, or as bases where lead or zinc is not indicated.

Some of the strongest and best toned ochres are produced in England, but those most generally used in this country are imported from various parts of Europe. France, notably, sends us the so-called Rochelle ochres. These ochres vary in quality according to the locality from which they come, and the care given them in their preparation for the market. The difference between yellow ochre and the various red ochres, or red oxides is a chemical one. The color in every case is due to the iron they contain. The difference being that, in the case of the yellows, the iron oxide exists in combination with water, hence, these ochres are called hydrated iron oxides; while the red ochres are anhydrous, contain little or no water. If yellow ochre be roasted, therefore, it becomes red or dark brown, as the moisture is driven off. This is the case also in the formation of burnt siennas and burnt umbers. The various tones of yellow ochre depend upon the varying amount of clay or silica present, and the greater or less percentage of combined water they contain.

Yellow ochre has been used for centuries in painting and decorating. It is, to all intents, a permanent pigment, and has no appreciable effect on other pigments, except, perhaps, a few of the very sensitive lakes, which latter are too fugitive to be used. It is seldom adulterated because of its price, and the fact that there are vast quantities of cheap ochre obtainable. Its yellowness is sometimes artificially improved with turmeric or other vegetable yellows, or by the admixture of chrome yellow, notably in producing so-called golden ochre. By pouring ammonia water mixed with alcohol over the suspected sample, such adulterations can usually be detected. If pure, the liquid will not discolor, otherwise, it will be stained. Ochres ground in oil are largely adulterated with barytes to save linseed oil, as pure ochres are light and very absorbent.

Italy has famous sienna deposits of beautiful tone and texture, and the umbers come from Turkey and southern Europe.

We produce some very fair ochres and other natural pigments in this country, but, as a rule, they lack strength and brilliancy, and because of their careless preparation, are not widely used except for paints of a crude nature or in the arts where the paint forming qualities are not the factor. In truth, the average paint grinder prefers, when he has cheap goods to produce, rather to adulterate the imported pigments with barytes or some such substance, than to grind the native pigments straight; because he saves oil, and produces a better appearing and sometimes stronger color for tinting purposes, than had he used the native article pure. These earth colors, for the most part, take a great deal of oil in grinding, hence the use of barytes as an adulterant. When we consider that ochres take from 20 to 30 per cent. and barytes only 7 per cent, of oil to make a suitable paste, we can understand the great saving.

Venetian red, as originally used, was a natural ferric oxide or oxide of iron, free from water and varied much in shade, depending upon the amount of iron oxide and other substances such as clay or silica, which it contained. At present, the better grades of Venetian red in point of strength, and tone, are produced by roasting copperas, which is sulphate of iron. Strong heat converts it into the oxide forms. It is cheaply produced, as copperas is a by-product from the large iron and steel works and the supply is very great. The production of copperas reds forms a convenient outlet for the copperas thus produced. Of course, you seldom get these oxides pure, being mixed with clay, silica or chalk, to improve their qualities, as paint forming materials and to cheapen the product, as in the case of low grade Venetian reds artificially produced. Strength, as a rule, is not a requisite in pigments like metallic browns and Venetian reds, which are used only secondarily as tinting colors, but principally alone with oil and suitable drying material as paint bases. Even tinting Venetian red is not likely to be absolutely pure as it is more easily ground and better preserved in paste form when mixed or reduced with clay, chalk or barytes.

Copperas reds are permanent only when free from soluble salts and sulphates. "When containing these substances or traces of acid are still present from the copperas, an injurious action is manifest on some of the vegetable pigments when used in conjunction, and when used alone on iron, corrosion or rust will result.

The indian reds are, for the most part, of natural origin, being another variety of ferric oxide, or red ochre of great purity and having a slightly purplish hue. Indian reds are imported and also to some extent produced in this country.

Metallic browns are similar to indian reds in composition, but may contain less iron and less care is taken in their production.