(B) Yellow Earths

The number of yellow earths is large, but most of them exhibit a certain similarity in chemical composition, the pigmentary principle in the majority being either ferric oxide or ferric hydroxide. The former is yellow, the latter brown, and the colour of the minerals resembles that of the preponderating iron compound.

Brown Ironstone

The mineral known as brown ironstone consists of ferric hydroxide, and usually forms compact masses, no decided crystals having, so far, been observed. The lumps have an irregular or earthy fracture, a hardness of 5–5·5, and a sp. gr. between 3·40 and 3·95. The colour ranges, in the different varieties, from yellowish (rusty) brown, through cinnamon to blackish-brown. The chemical composition of the pure lumps may be expressed by the symbol 2Fe₂O₃ + 3H₂O; but a little manganese oxide and silica is generally present even in the pure kinds.

The chief varieties of this mineral are:—

(a) Fibrous brown iron ore, or brown hematite, mostly forming reniform or stalactitic masses.

(b) Compact brown ironstone, usually in dense masses, and not infrequently also appearing in pseudo-morphs of other minerals.

(c) Ochreous brown ironstone. This variety is the most important to the colour-maker, for whose purposes it is preferably used. It nearly always forms very loose, earthy masses, yellow or brown in colour.

(d) Clay ironstone. This consists of a mixture of the above-mentioned varieties with variable proportions of other minerals, clay being the most common ingredient. Nodular iron ore, oölitic, bog and siliceous ore belong to this class, as also the minette ores that are found in great abundance in Alsace-Lorraine, Belgium and Luxemburg, and are classed with the oölitic brown ironstones.

In most cases, the varieties enumerated are found together, and are used for the production of iron. The ochre constituting the most interesting member to the colour-maker often occurs as deposits embedded in dense masses of brown ironstone, though in many places it is found by itself.

Chemical Composition of Various Brown Ironstones

The following analyses of brown ironstone from different deposits will give an idea of the composition of these minerals.

Ordinary Brown Ironstone

	1.	2.	3.	4.	5.	6.	7.	8.	9.	10.
Ferric oxide	76·76	73·75	77·54	78·50	78·42	48·25	—	—	—	—
Manganese oxide	16·56	10·50	2·70	1·95	1·30	24·73	—	—	—	—
Iron	—	—	—	—	—	—	33·9	37·88	54·80	55·04
Manganese	—	—	—	—	—	—	0·15	0·17	0·57	0·20
Alumina	—	—	—	—	1·13	2·33	10·03	0·88	1·15	2·50
Lime	0·60	2·75	0·48	5·08	3·55	2·85	0·41	0·32	0·50	0·34
Magnesia	6·44	—	1·25	4·50	0·18	0·90	0·67	—	0·02	0·38
Silica	—	—	3·55	0·85	5·48	11·35	28·29	33·38	0·02	0·38
SO₃	—	—	—	—	—	0·09	—	—	—	—
P₂O₅	—	—	—	—	0·09	0·08	—	—	—	—
Sulphur	—	—	—	—	—	—	0·33	0·06	0·04	Trace
Phosphorus	—	—	—	—	—	—	0·04	0·56	0·02	0·06
Loss on incineration	5·65	14·00	14·51	9·12	9·10	9·80	9·88	7·77	10·55	10·71

Deposits: (1) Hamm; (2) Schmalkalden; (3) Hüttenberg (Carynthia); (4) Styria; (5) and (9) Bilbao; (6) Algeria; (7) Schwelm (Westphalia); (8) Elbingerode (Harz); (10) Pennsylvania.

Argillaceous Brown Ironstone

	a.	b.	c.	d.	e.	f.	g.	h.	i.	k.
Ferric oxide	80·76	19·4	55·39	66·33	57·32	52·50	39·50	75·67	—	—
Iron	—	—	—	—	—	—	—	—	40·90	21·69
Manganese oxide	—	8·2	—	6·42	—	5·49	6·12	0·72	—	—
Manganese	—	—	—	—	—	—	—	—	—	—
Zinc oxide	0·92	1·6	—	—	0·47	—	—	—	—	—
Alumina	2·36	11·0	12·80	7·74	1·68	5·23	9·89	3·10	4·95	3·88
Lime	—	2·6	Trace	0·41	0·13	3·36	20·34	5·01	5·59	21·25
Magnesia	—	0·2	—	0·37	—	0·36	—	—	0·49	0·30
Silica	4·58	48·61	22·73	12·97	30·64	8·64	5·22	8·70	16·63	14·71
P₂O₅	—	—	—	—	0·32	3·86	2·19	3·68	—	—
Phosphorus	—	—	—	0·02	—	—	—	—	1·13	0·48
SO₃	—	—	—	0·03	Trace	—	—	—	—	—
Sulphur	—	—	—	—	—	—	—	—	0·10	0·05
Loss on incineration	12·71	9·1	8·50	11·77	12·70	20·55	25·74	—	16·04	28·70

(a) Oölitic (pea) ore from Elligserbrick (Brunswick); (b) from Durlach (Baden); (c) and (d) Ore from Esslingen; (e) Oölitic ore from Siptingen (Baden); (f) from Adenstedt, nr. Pirna (argillaceous); (g) Ibid. (calcareous); (h) Minette from Esch; (i) Red minette from Dolvaux; (k) Brown minette from Redange.

Limonite (Bog Iron Ore)

	1.	2.	3.	4.	5.	6.
Ferric oxide	61·00	67·46	65·66	67·59	70·05	62·20
Manganese oxide	7·00	3·19	3·87	1·45	1·78	7·60
P₂O₅	2·00	0·67	1·13	0·18	0·34	1·60
SO₃ —	—	3·07	Trace	0·21	Trace	Trace
Silica	6·00	7·00	7·15	7·89	8·03	16·60
Alumina	—	—	5·09	4·18	1·50	2·20
Lime	—	0·90	0·82	0·47	2·31	1·60
Magnesia	—	—	0·15	0·23	0·12	3·73
Water and organic acids	19·00	17·00	6·22	17·81	15·87	19·90

(1) Limonite from Lausitz; (2) Limonite from Auer, nr. Morizburg; (3 to 6) Swedish limonite.

Ochre

Ochre, or yellow Terra di Siena, forms earthy-looking masses, fawn, reddish-yellow to brownish-red in colour. Whilst not infrequent in Nature, ochre is only found in small quantities, as pockets, and not as extensive deposits. The discovery of a bed of good coloured ochre is, however, always a very valuable find, bright natural ochres being somewhat rare, and most kinds requiring special preparation before they can be used as painters’ colours. Owing to the comparative scarcity of good coloured ochres, they are often called after the place of origin, such as Thuringian, Italian (Siena), English, etc., ochre.

In nearly every case, ochre is a decomposition product of various ferruginous minerals, which has been transported by water, often in admixture with other minerals, and finally deposited in the places where it is now found. Most ochres consist of varying mixtures of clay, ferric hydroxide and lime; and, as a rule, the higher the proportion of ferric hydroxide, the deeper the colour. Thus, for example, the ferric hydroxide may amount, in the dark grades, to 25% of the entire mass, whilst in the lighter kinds it may be as low as 3%. It is very rare that ochre is put on the market in its native condition, being mostly subjected to chemical treatment enabling a definite shade of colour to be obtained. This will be gone into more fully later.

Yellow Earth

Yellow earth is found in many places as compact masses, and less frequently as schistous deposits. It has a fine earthy fracture, and is mostly devoid of lustre, except for a faint shimmer on the surface of fracture; slightly greasy feel; and a tendency to crumble, in water, to a non-plastic powder. It contains silica, ferric oxide and water in varying proportions, and the yellow earths from different deposits always vary slightly in percentage composition. These differences are clearly shown in the following analyses of two varieties from the vicinity of Amberg (Bavaria):—

	I.	II.
Silica	33·23%	35·10%
Alumina	14·21	14·40
Magnesia	1·38	—
Ferric oxide	37·76	36·80
Water	13·24	13·60

When heated, the colour changes gradually to red, and the earth becomes extremely hard. There are several recognised commercial grades, the price of which varies mainly in accordance with the colour and fineness. The Amberg variety is specially esteemed, the Hungarian and Moravian kinds being less valuable.

The colour not being particularly good, this earth is never used for fine work, but is largely employed as a yellow wash for houses and as ordinary distemper. It may also be used as an oil paint.

Red Ochre is a less important, cheap variety of ochre, chiefly used in cheap paints and for low-priced wall-papers. It occurs in the deposits as clayey masses.

Terra di Siena

Terra di Siena is a very pure form of ferric hydroxide. When ground, the light to dark brown lumps furnish a pale to dark yellow powder, which can be transformed into a number of gradations by burning. In spite of its handsome colour, this pigment is deficient in covering power, in addition to which it darkens when mixed with varnish, and dries slowly.