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.