Pagina-afbeeldingen
PDF
ePub

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.

[merged small][merged small][ocr errors][merged small]

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. 115) 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 :Organic matter and water,

70:289 per cent. Calcium carbonate, CaCO3,

3.490 Oxide of iron and alumina,

1.615 Alkaline carbonates and alkaline salts, 24.606

[ocr errors]

100.000

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

used as a water-colour as to show a great variety of tints and shades ; it is this property which makes it of value for monochrome work to artists; no other pigment is capable of being used in this manner with so much facility.

Sepia is a compound of calcium and magnesium carbonates, with an organic colouring principle; Prout has analysed it, and gives its composition as Melanin or black pigment,

78.00 per cent. Calcium carbonate, CaCO3,

10.40 Magnesium carbonate, Mg C 03, .

7.00 Alkaline sulphates and chlorides,

2.16 Organic mucus,

0.84

98.40

The black pigment may be isolated by boiling the bag, first in water, which takes out the alkaline salts; then in hydrochloric acid, which takes out the calcium and magnesium carbonates; then, after washing with water, filtering, and drying, the pigment is ready for use.

Sepia is a fairly permanent pigment, being but little affected by exposure to light and air. It is not altered by admixture with any other pigment.

CAPPAGH BROWN.

This pigment is of a reddish-brown hue, and much resembles raw umber both in appearance and composition. It is found at the Cappagh Mines of Lord Audley, which are situated about 10 miles from the town of Skibbereen, in the county of Cork.

Prof. A. H. Church gives * the following analysis of Cappagh brown :Water, given off at 100° C.,

18:7 per cent. Water, given off at a red heat,

11.6 Ferric oxide, Fe, 03,

34.4 Manganese dioxide, "Mn 02, :

27.2 Alumina, Al, 03,

2.6 Lime, Ca 0,

1:1 Magnesia, MgO,

trace. Silica, Si 02,

46 Phosphoric acid, P, 03,

0:4 There were traces of organic matter, but not enough to show that it had been derived from bog earth or peaty matter. It is

* Church, Chemistry of Paints and Painting, p. 206.

possible that part at least of the manganese, if not the whole of it, existed in the pigment in the form of the red oxide, Mn, 04.

When heated above 100° C. it acquires a rich red colour not unlike that of burnt sienna.

Cappagh brown works well in oil- or water-colour, and is a permanent pigment. Its qualities as an oil colour are much improved by a preliminary drying at a temperature not exceeding 75° to 80° C. It has been much used by artists since its introduction.

MANGANESE BROWN.- This brown is an oxide of manganese. It is prepared artificially from the waste still-liquors of the chlorine manufacturer, by precipitating these with sodium carbonate, collecting the precipitate, and calcining in a furnace to a low red heat, until samples taken out and allowed to cool show that the pigment has acquired the desired shade. It is a good and permanent pigment, but it has such excessively-strong drying properties as to make it very unusable as a pigment; hence it has gone out of use. The manufacture of manganesebrown was patented in 1871 by Rowan.

COLOGNE EARTH, CASSEL EARTH, RUBENS BROWN, and other browns are pigments of natural origin of varied composition; some partake of the character of brown lignite of a soft character, others more nearly resemble the umbers in their composition. Their value as pigments is very variable, and, as their composition cannot be depended upon as being constant, it is advisable to avoid the use of these pigments for all artistic painting.

BONE-BROWN is made by gently calcining bones until they acquire a brown colour; it resembles bone-black in composition, but contains some undecomposed animal matter; it is not much used. PRUSSIAN BROWN.—This pigment is rarely met with

It was prepared by gently calcining Prussian blue, and hence is a mixture of ferric oxide and carbon ; necessarily the pigment was a costly one, and it has no advantage over such pigments as umber or Vandyke-brown.

BISTRE.—This pigment is prepared from the soot of wood, especially from that of beech wood, which gives the finest quality. The soot is collected and washed with hot water until the latter does not extract any more soluble matter from it; sometimes the soot is subjected to a preliminary grinding before the washing. The bistre is dried, and is then ready for use. Bistre is not used as an oil-colour. It has a fine warm brown colour of a yellowish hue. Its permanence depends very much

now.

« VorigeDoorgaan »