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Oxford, of which the following section is given in “Ure's
1. Summit of hill, highly ferruginous grit, 6 feet,
1 4. Yellow sand,
6 5. Cream-coloured loam,
4 6. Ochre,
Oxford ochre contains
6.887 per cent.
The layer of ferruginous concretions is, probably, the source of the colouring matter of this ochre, while the clay which underlies the layer of ochre is the source of the base of the pigment.
2. Welsh Crude Ochre.—The exact locality from whence this sample was derived is not known; it has a fairly good colour and covering power. It contains
2.000 per cent. 12.500
1:315 29•725 33:315 20•705
Some of the iron oxide, 0.765 per cent., exists in a soluble form,, probably as sulphate, for there is 0.555 per cent. of soluble sulphur trioxide. This, and the fact that there is copper sulphide present, indicate that this ochre has been formed by the decomposition of a cupreous pyrites, which supposition is further strengthened by the fact that small pieces of pyrites may be picked out of the crude ochre. This ochre requires well levigating to get rid of the pyrites as this body would introduce
an element of change which may exercise an injurious influence upon the permanent properties of the ochre.
3. Irish Crude Ochre.—This sample contains
9:050 per cent.
The insoluble matter consists of silica and some gritty matter.
Of these constituents a portion (0.33 per cent. of sulphur trioxide, 0.191 of ferric oxide, 0·118 of calcium oxide, and 0.228 of copper oxide) was soluble in water in the form of sulphates, the iron being in the ferrous form; this points to the fact that this ochre was formed by the oxidation of the cupreous pyrites existing, disseminated through a siliceous mineral matrix which was broken up by the oxidation, and part of which went to form the base of the ochre, while much of it exists in the form of gritty angular pieces which must be separated before the ochre is fit to use as a pigment.
4. Derbyshire Crude Ochre.—This is of a reddish colour and is found incrusting masses of pyritous minerals and barytes, from which the ochre is separated by levigation. This ochre, like the others just noticed, has been formed by the oxidation of pyrites, small fragments of which, in an unchanged condition, are disseminated through the crude ochre. This sample contained more iron than any other examined by the author, as the following analysis shows :
5. Derbyshire Prepared Ochre. This sample is of a reddish
shade, not so dark as the last. The covering power of this ochre is good. Water, combined,
6.100 per cent. Barium sulphate, Ba S 04,
20.946 Silica, SiO2,
4.530 Calcium sulphate, Ca s104,
2.516 Calcium carbonate, Ca C 03,
21.755 Alumina, Al2O3,
10.655 Ferric oxide, Fe, 03,
33.498 Magnesia, MgO,
6. Cornwall Prepared Ochre.- This ochre is of a pale brownish-yellow shade and has not much covering power, It contains
1:40 per cent.
7. French Prepared Ochre.—The exact locality from which this was obtained is not known to the author, but it is usually of a bright brownish-yellow colour with a good covering power. Water, hygroscopic,
1.80 per cent. Water, combined,
9.20 Silica, SiO2,
54:00 Alumina, Al, 03,
13:75 Ferric oxide, Fe, 03,
20*73 Calcium oxide, Ca 0,
8. South Australian Ochres.—The following analyses of South Australian ochres are quoted in the Journ. Socy. Chem. Ind., 1889, p. 313 :
Per cent. Per cent. Per cent.
1.82 1.92 0.21
6:48 7.60 4.00 Silica, SiO2,
41.20 56.60 65.20 Ferric oxide, Fe, 03,
38.40 11.68 5.76 Alumina, Al, 03,
12:56 19.12 21.74
9. American Ochre_Eastern State.—A sample of American ochre from one of the Eastern States examined by the author had the following composition Water, hygroscopic,
1.30 per cent. Water, combined,
3.70 Ferric oxide, Fe, 03,
11.03 Alumina, Al, 03, .
4.06 Silica, SiO2, .
4:47 Calcium carbonate, Ca C 03,
trace. Barium sulphate, Ba S 04,
99.95 The amount of barium sulphate in this ochre is rather large, but is present as a natural constituent of the ochre, which was of good colour and very fine in quality.
Most American ochres are ochres in name only, for they differ markedly from the typical English and French ochres; they usually contain much more iron, which gives them a darker tint and a redder hue. The following is a typical analysis of an American ochre :Iron oxide and alumina,
59:15 per cent. Water, hygroscopic,
4:10 Water, combined,
2.80 Insoluble, chiefly silica,
32.50 10. Siennas.—The following are some analyses of raw siennas, which are of Italian origin—the first and second from the neighbourhood of Rome; the locality from whence the third came is unknown. The Roman siennas are found in hollows on hill sides, which hollows are now filled up with deposits of sienna; but, at one time, were the site of small ponds into which flowed streams highly charged with iron and manganese, from deposits of those materials situated above the ponds. The rocks of the district consist chiefly of trachyte and granite, charged with ferriferous and manganiferous minerals, which are the source of the colouring matter of the sienna.
It may be assumed that the shade of the siennas varies with the amount of manganese it contains, as is shown by the analyses given above.
11. American Sienna.—The American siennas which have come so far under the notice of the author differ from the true Italian siennas in being somewhat more opaque, and rather redder in tone; they, nevertheless, make good pigments. The following is an analysis of a sample of American raw sienna :
7.00 per cent.
This sample is notable on account of the large proportion of iron oxide it contains.
The commercial value of an ochre depends upon its colour and body; those which excel in these points naturally commanding the best prices. The following is approximately the order of the various ochres as regards price Oxford, 100; French, 33; Derbyshire and Welsh, 25; Irish and Devon, 20.
The siennas are sold in two forms, raw and burnt; the first has already been dealt with and the latter will now be described. Burnt sienna is prepared by calcining the raw sienna at a moderate red heat until it has acquired the desired shade. The tint of the burnt sienna depends not only upon the temperature used and the length of time it is exposed to heat, but, also, upon the shade of the raw sienna used. Burnt sienna is a pigment of a reddish-orange shade, very similar to that of the coal-tar colour known as Bismarck-brown. It is very transparent, and is, therefore, mostly used as a glazing or tinting colour by painters and artists. It is sold in the form of small pieces, and of a paste ground up with water or oil. The former variety is very difficult to grind.
The composition of burnt sienna naturally resembles that of raw siennas, only that the heat has driven off most, if not all, the water the latter contains. The following analysis will serve to show the composition of burnt sienna :