Chapter VIII: Green Earth Colours

Although the number of green-coloured minerals is large, but few of them are suitable for painters’ colours, because they occur so rarely in Nature that their employment for this purpose is out of the question, more especially since a very large number of green pigments can be obtained by artificial means. The most important of the earth colours in this category are Celadon green, or green earth, and malachite green--the latter, however, less so, because the substance of which it is composed can be prepared artificially.

GREEN EARTH, OR CELADON GREEN

This mineral is of a peculiar green colour, and the name “Celadon green” has been universally adopted in the nomenclature of colour shades. Green earth occurs native in many places, being the decomposition product of an extensively distributed mineral, augite, crystals of which are found in many of the deposits. The green earth of Monte Valdo, on Lake Garda (Upper Italy) has been used for a very long time as a pigment. It is chiefly prepared in Verona for distribution in commerce, and from this circumstance has acquired the name “Verona green,” or “Verona earth.” The earth is also found in Cyprus and Bohemia, where it frequently occurs as the decomposition product of basaltic tuff. However, whether obtained from Monte Valdo or elsewhere, the product is always placed on the market as Verona earth.

Native green earth is always tough, mostly occurring in amygdalous lumps, but occasionally in the crystalline form of augite. It has a fine-grained fracture, a hardness between 1 and 2, and a sp. gr. between 2·8 and 2·9. The colour is not always quite uniform, pure lumps having the characteristic Celadon green appearance, whilst impure lumps are olive green to blackish green. In chemical composition it is chiefly ferrous silicate, and this compound must be regarded as the actual pigmentary principle of green earth. In addition, it contains varying quantities of other compounds which influence the depth of shade of the product.

Verona earth chiefly consists of ferrous oxide in combination with silica; alumina, magnesia, potash, soda and water being also present. Analysis shows it to contain: ferrous oxide, 21%; silica, 51%; magnesia, 6%; potash, 6%; soda, 2%; and water, 7%.

The green earths from Gösen, Atschau and Männelsdorf, near Kaaden (Bohemia) and the Giant’s Causeway (Ireland) have the following composition:

+---------------+----------+----------+
| | Kaaden. | Giant’s |
| | | Causeway.|
+---------------+----------+----------+
|Silica | 41 | 56·4 |
|Alumina | 3 | 2·1 |
|Ferrous oxide | 23 | 5·1 |
|Ferric oxide | -- | 14·1 |
|Lime | 8 | -- |
|Magnesia | 2 | 5·9 |
|Potash | 3 | 8·8 |
|Carbon dioxide | 19 | -- |
|Water | -- | 6·8% |
+---------------+----------+----------+

On account of the large quantity of mechanically associated water, freshly dug green earth is greasy in character, like wet clay. In partial drying, most of this water evaporates, the mass becoming earthy and adherent to the tongue. Sometimes the colour is an ugly brownish-green, owing to the presence of a considerable amount of ferric oxide formed as the result of changes set up by exposing the mineral to the air. Ferrous oxide is a very unstable compound, having an energetic tendency to combine with more oxygen and thus undergo transformation into ferric oxide; so that when green earth is left in the air for a long time, a considerable proportion of its ferrous oxide is oxidised to ferric oxide, the mass thereby assuming the brown tone in question.

Such an unsightly product can, however, be converted, by simple treatment, into one of very bright and handsome appearance; and it is this possibility that first enabled green earth to attain importance as a painters’ colour. Formerly it was only used as a material for common work, being added to whitewash or employed for indoor paints.

When the crude green earth is treated with very diluted hydrochloric acid, the compound of ferrous oxide and silica is left intact, but most of the extraneous admixtures are removed. Ferric oxide, in particular, passes into solution, and the calcium carbonate largely present in some kinds of green earth is also dissolved. After prolonged contact with the crude earth, the acid liquor takes on a brownish coloration from the dissolved ferric oxide. Since the presence of iron salts has no influence on the purification of the green earth, the most impure, highly ferruginous hydrochloric acid can be used, and the liquor can afterwards be employed in the preparation of artificial ochre by leaving it in prolonged contact with any strongly ferruginous mineral, such as brown ironstone, which neutralises the surplus acid. This liquor is then precipitated by lime, alkali, etc., the resulting deposit consisting mainly of ferric hydroxide, the further treatment of which is conducted exactly as described in dealing with the preparation of artificial ochre.

The treatment of the crude earth is best carried on in the same vessels as are to be used in the subsequent levigation process. After the acid liquor has been drawn off, the earth is brought into contact with water, stirred up well, and the water run off, by opening tapholes in the side of the vessel, into settling-tanks, where it is left until all the suspended matter has completely subsided.

The colour of green earth can also be toned by the addition of yellow ochre, thus producing a range of greens with a yellowish tinge. These lighter shades, however, are seldom met with in commerce, the trade judging the quality of green earth more particularly on the depth of colour.

Green earth is a valuable pigment for all kinds of painting, on account of its extreme permanence. It may be applied directly over lime without suffering any change, whereas most of the cheap green colours are destroyed in like circumstances. This behaviour renders green earth specially valuable in fresco work, although it is also largely used as an oil colour.

Augite is of frequent occurrence in volcanic districts; and in such localities, deposits of green earth are certain to be found. The test for the suitability of a green earth consists mainly in treatment with dilute hydrochloric acid. If the mineral assumes a handsome green tone, it will generally form a useful pigment. The test may be supplemented by applying the colour to a fresh coating of whitewash, under which conditions it should remain unaltered.

ARTIFICIAL GREEN EARTH (GREEN OCHRE)

A product sometimes put on the market as green earth or green ochre has nothing beyond its name in common with green earth properly so called, except a certain similarity in colour. This pigment is prepared by mixing yellow ochre to a thin pulp with water and adding about 2% (of the weight of ochre) of hydrochloric acid. After a few days, a solution of 2 parts of yellow prussiate of potash is added, and if the liquor still gives a precipitate when tested with ferrous sulphate, this last-named salt is added so long as such a precipitate continues to form.

The deposit is washed, and dried in the ordinary way. When the right proportions have been taken, a pigment is obtained that coincides fairly in point of tone with true Verona earth. It is, however, inferior in point of permanence, the Berlin blue present being somewhat unstable and decomposing very quickly when brought into contact with lime. The reaction taking place in the production of so-called “artificial Celadon green” is that the hydrochloric acid used dissolves ferric oxide from the ochre, the addition of the yellow prussiate of potash then forming a blue precipitate of Berlin (Paris) blue which, in conjunction with the yellow of the ochre, gives a green-coloured mixture.

MALACHITE GREEN

Although the pigment sold under this name is nearly always an artificial product, it cannot be omitted from a work dealing with the earth colours, because, in former times, it was prepared exclusively from the mineral malachite. Owing to the fact that artificial malachite green is one of the cheapest of colours, the troublesome work involved in the mechanical preparation of the native pigment has been almost entirely abandoned, and the malachite itself is now utilised to greater advantage as a source of copper.

Malachite green (or mountain green) is found in nearly every case where copper ores exist, and is still--though very rarely indeed--prepared, in a few places, from the mineral, the dark-coloured lumps being picked out because the lighter-coloured ones would furnish much too pale a colour.

The treatment of malachite for the preparation of pigment presents certain difficulties owing to the comparative hardness (3·5–4) of the mineral, which is also rather heavy (sp. gr. 3·6–4·0). On the large scale, the selected mineral is first put through a stamping-mill, and then ground, very hard stones being required for this purpose. The fine product from this (usually wet) process is levigated and dried.

The pit water of some copper mines contains certain quantities of blue vitriol (copper sulphate) in solution; and such pit water is generally treated for the recovery of a very pure form of copper, the so-called cementation copper. The liquor might also be worked up into malachite green, by collecting it in large tanks and precipitating the dissolved copper oxide with milk of lime, the bluish-green deposit separating in association with gypsum being transformed into a light malachite green by washing and drying. A darker green, free from gypsum, could be prepared by using a solution of carbonate of soda as precipitant.

Neither the native nor the artificial malachite green is particularly handsome in colour; and both possess, in addition, the unpleasant property of gradually going off colour in the air, all the copper compounds being quite as sensitive to sulphuretted hydrogen as those of lead, and finally turning quite black under the influence of that gas.