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CHAPTER VII.

BROWN PIGMENTS.

THIS group of pigments is a small one-umber, vandyke-brown, sepia, manganese brown, Cappagh brown, and one or two others of little importance complete the list. Most of these browns are natural pigments.

UMBER.

Probably the most important of the brown pigments is umber. It is an earthy pigment closely resembling the ochres and siennas in its composition and properties; in fact, these three pigments form a natural group of yellow to brown colours having the ochres at one end of the scale and the umbers at the other, while between the two extremes it is possible to find all or nearly all the intermediate tints or shades.

Umber is found native in many places; as in Derbyshire, Devonshire, Cornwall, Wales, &c., in this country; in France; in Italy; and in many localities in America. The finest umber comes from Cyprus.

It is found in veins and layers of varying thickness in rocks of all geological ages, and from which it has, in some cases, been derived by decomposition. At Ashburton, near Dartmoor, the umber is found in a layer of from 20 to 30 feet in thickness, overlying the bed rock, which is a dolomitic limestone containing some manganese and iron and from which it has been formed; above the umber there is an overburden of soil.

Umber varies somewhat in hue from a reddish-brown to a violet-brown, the former hue being characteristic of the Derbyshire umbers, while Turkey umber (which comes from Cyprus) has a warm violet-brown hue. This umber owes its trade name to the fact that it was imported through Constantinople, and its real source was at one time not properly known.

Umber is sold in three forms-raw lump umber, raw powdered umber, and burnt umber.

Raw lump umber is the pigment just as it is obtained from the

mines. The method of mining will vary somewhat according to the varied conditions under which the umber is found. At Ashburton it is mined by taking off the overburden of soil, digging square pits until the bed rock is reached, and lining the pits with timber as the material is removed. When the bottom is reached these timbers are removed and fresh pits sunk. A similar method is probably in use for mining other deposits of umber.

Powdered raw umber is the lump umber ground and levigated in the same manner as ochres are treated.

Burnt umbers are the raw umbers calcined at a red heat in a furnace, by which treatment the colour becomes darker and warmer; the change which occurs is similar to that which ensues when ochres and siennas are calcined (see pp. 105, 136, 141).

COMPOSITION AND PROPERTIES OF UMBERS.— Umbers have, as has already been stated, a great resemblance to the ochres and siennas in their composition; but they contain more manganese, which probably accounts for their darker colour. The following analyses of various colours will serve to show the average composition of these pigments:

No. 1 is an analysis of Cyprus umber; this sample had a dark, warm brown tint.

No. 2 is an analysis of crude Derbyshire ochre; this sample was of a soft character and of a reddish-brown tint.

No. 3 is an analysis of a sample of umber, probably of English origin. This sample had a yellowish-brown tint. The above analyses were made by the author.

No. 4 is an analysis made by J. J. Beringer (quoted by Frecheville*) of the umber from Ashburton, already referred to; and the next shows the composition of the dolomitic limestone from which it is derived.

The decomposition has probably been brought about by the dissolving action of carbonated water on the calcium and magnesium carbonates, so as to leave the iron, manganese, and silica behind to form the umber; the amount of limestone which must have been disintegrated to form deposits of umber, 20 to 30 feet in thickness, must have been enormous, as but little of the calcium and magnesium remains behind in the umber. The umber found at Veryan (near Truro), Milton Abbot, and other places in Cornwall is undoubtedly formed from limestone rocks in a similar manner.

Umbers are pigments of a warm brown colour, varying in hue from yellowish to violet-brown. By calcining, the colour is rendered darker and warmer. As pigments, they work well in both oil and water, and they can be mixed with all other pigments without any change occurring. They are perfectly permanent, being unaffected by all the ordinary conditions to which pigments are exposed. Umbers, therefore, meet with extensive use among all classes of painters.

Umbers are not readily attacked by acids, but prolonged digestion with strong hydrochloric acid dissolves the larger proportion of the umber, forming a brownish-yellow solution containing iron, alumina, manganese, and lime; the silica and the barium sulphate remain undissolved. The metals may be tested for by the usual analytical methods. Caustic soda has no action on umbers.

ASSAY AND ANALYSIS OF UMBERS.-Umbers may be assayed for colour or hue, colouring power, covering power, and similar properties by the usual methods.

A chemical analysis of umbers is rarely required, as they are rarely, if ever, adulterated, except possibly a dearer umber by a * Trans. Roy. Geolog. Soc., Cornwall, xvii.,

p. 217.

cheaper one; but this kind of adulteration would be very difficult to detect. In case an analysis is required the method detailed under oxide reds (p. 108) is applicable here.

VANDYKE-BROWN.

Next to umber the most important brown pigment is Vandykebrown, so named after the great painter, who was particularly partial to the use of browns in his pictures. No record remains as to the origin of the particular brown which he used, but, it was presumably a natural brown found, perhaps, in the vicinity of the artist's residence and, probably, more or less organic in its origin. The Vandyke-browns now sold are, however, all of artificial production from a variety of sources. In many works on pigments it is stated that this pigment is prepared by calcining ochres and copperas; it is extremely doubtful whether these browns were ever made in this way, as neither ochres nor copperas yield Vandyke-browns when calcined.

Vandyke-browns are made in several ways:

1. From natural deposits of a brown colour occurring much in the same manner as the ochres and umbers, but differing from them in being derived largely from organic sources, such as peaty matter mixed with more or less earthy matter. Sometimes these pigments are named after the localities in which they are found, as, for instance, Cassel earth.

For use as pigments these natural products simply require to be ground as fine as possible.

2. From cork cuttings and waste, bark and twigs of trees, and other organic matter of vegetable origin, by calcining slightly in a closed vessel.

These Vandyke-browns have a warm brown colour of a reddish hue; they mix very well with oil and water, and can be used for all kinds of painting.

An analysis of such a brown made by the author showed it to have the following composition :

3. Most of the common Vandyke-browns are made by mixing together lamp-black, vegetable black, or other black pigment with

red oxide and a little yellow ochre; the proportions used vary according to the quality and shade of the oxide used, and whether ochre is also used. Vandyke-browns thus made, and containing from 36 to 50 per cent. of black (chiefly lamp-black) form the great bulk of these pigments as used by the house-painter. This variety of Vandyke-brown is a permanent pigment, and works well in oil, if care be taken in regard to the quality of the black used; if it has any fault, it is that of being a bad drier. In water it does not mix quite so readily as the other varieties of Vandyke-brown, although, when mixed, it works well.

Vandyke-brown is sold in the form of small angular pieces, powder, and paste ground with either oil or water according to the use it is to be put to.

PROPERTIES OF VANDYKE-BROWN. - Vandykebrown is a perfectly permanent pigment and withstands any amount of exposure to light and air. It works well in either oil or water and with any kind of vehicle. It can be mixed with all other pigments without any alteration whatever.

SEPIA.

Sepia is a brown pigment of slightly varying hue, much used by artists, especially for monochrome work. It is obtained from various species of cephalopodous animals, such as Sepia officinalis, Sepia loligo, &c. These animals have a peculiar gland which secretes a blackish-brown liquor that collects in what is called the ink-bag. This liquor is secreted for the purpose of defence; when an enemy approaches, some of the contents of the bag is discharged, and this, owing to its strong colouring powers, colours the water for some distance around, and under the cover of the opacity thus produced the sepia makes its escape. The animals are caught and the ink-bag carefully taken out and dried; it then forms the pigment known as sepia. The commercial article is in the form of small pear-shaped pieces of a blackish-brown colour, to which fragments of the sac or skin of the bag usually remain attached. For purposes of use as a pigment for artists it is necessary to remove the sac or bag; this is done by boiling the crude sepia with a solution of soda, which dissolves the colour but not the bag; the liquor is then filtered, and to the filtrate acid is added to precipitate the pigment; the precipitate is collected, washed, and dried.

Sepia is a blackish-brown pigment of very fine texture, mixing well with both oil and water. It is somewhat transparent, but its colouring power is very great, and it is capable of being so