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.