Field's Chromatography Part 17

Another Prussian green, with a base of cobalt, is obtained by precipitating the nitrate of that metal with yellow prussiate of potash.

According to the mode adopted, and the degree of heat, either a light or dark green results; but this also is inferior in colour, and presents no advantage as to permanence.

188. SAP GREEN,

_Verde Vessie_, or _Iris Green_, is a vegetal pigment prepared from the juice of the berries of the buckthorn, the green leaves of the woad, the blue flowers of the iris, &c. It is usually preserved in bladders, and is thence sometimes called _Bladder Green_. When good, it is of a dark colour and glossy fracture, extremely transparent, and a fine natural yellowish green. This gummy juice, insp.i.s.sated and formed into a cake, is occasionally employed in flower painting. It is, however, a very imperfect pigment, disposed to attract the moisture of the atmosphere, and to mildew; while, having little durability in water and less in oil, it is not eligible in the one and is totally useless in the other.

Similar pigments, obtained from coffee-berries, and named Venetian and Emerald Greens, are of a colder colour, equally defective and fugitive, and now obsolete.

189. TERRE VERTE,

or _Green Earth_, is a sober bluish green with a grey cast. It is a species of ochre, containing silica, oxide of iron, magnesia, potash, and water. Not bright and of little power, it is a very durable pigment, being unaffected by strong light or impure air, and combining with other colours without injury. It has not much body, is semi-transparent, and dries well in oil. Veins of brownish or reddish ochre are often found mixed with terre verte, to the detriment of its colour; and there are varieties of this pigment with copper for their colouring matter, which, although generally brighter, are inferior in other respects, and not true terre vertes. Verona Green and Verdetto or Holy Green, are ferruginous native pigments of a warmer hue. These are met with in the Mendip Hills, France, Italy, and the island of Cyprus, and have been used as pigments from the earliest times. Rubens has availed himself much of terre verte, not in his landscapes merely, but likewise in the carnation tints in his figures of a dead Christ. It is evident that much of the glazing is done with this colour: it is, in fact, most useful in glazing; because, having only a thin substance, it can be rendered pale by a small portion of white; although in the end it becomes darker by a concentration of its molecules. Merimee states that in the greater part of Alexander Veronese's works--in his Death of Cleopatra, in the Louvre, for instance--there are some demi-tints which are too green, and which it is certain were not so originally. Terre verte, therefore, must be employed with caution; and it would be well to ascertain beforehand whether a mineral colour will in time become darker than when first laid on the picture, by putting a drop of oil on the powder in its natural state. If the tone this gives to it be more intense than that which it acquires by being ground up, it may fairly be a.s.sumed that it will attain to the same degree of strength whenever, having completely dried, its molecules shall have re-united as closely as it is possible. Umber and terra di Sienna are of this cla.s.s.

In combination with Indian red and Naples yellow, terre verte forms a series of mild russet greens, of much use in middle distance.

190. _Chrome a.r.s.eniate_

is an agreeable apple-green colour, prepared from a.r.s.eniate of potash and salts of chromic oxide. It is durable, but possesses no advantages over the chrome oxides, and is of course poisonous.

191. _Cobalt Green_,

Rinman's Green, Vert de Zinc or Zinc Green. True cobalt green is made by igniting a very large quant.i.ty of carbonate of zinc with a very small quant.i.ty of carbonate of cobalt. To give a green tint to an enormous proportion of the former, an inappreciable amount of the latter will suffice. Some samples which were a.n.a.lysed, consisted almost entirely of zinc, there being only two or three per cent. of cobalt present. This green presents an example of a pigment being chemically good and artistically bad, or at least indifferent. It is a moderately bright green, apt to vary in hue according to the mode of manufacture, permanent both alone and compounded, but so sadly deficient in body and power, as to have become almost obsolete. With other physical defects, and a colour inferior to the chrome oxides, cobalt green has never been a favourite with artists, though justly eulogised by chemists.

192. _Copper Borate_

is obtained by precipitating sulphate of copper with borax, was.h.i.+ng the residue with cold water, and, after drying, igniting it, fusion being carefully avoided. In this manner, a pretty yellowish green is produced, which upon longer ignition a.s.sumes a dark green shade: the ma.s.s is levigated for use. The compound has the objection of being gla.s.sy, and possessing little body, but is preferable to verdigris as to permanence.

193. _Copper Chrome_

may be prepared by several methods, but the colour is in no case so fine as Scheele's or Schweinfurt green, nor is it as stable.

194. _Copper Stannate_,

or Tin-Copper Green, equals in colour any of the copper greens free from a.r.s.enic. The cheapest way of making it is to heat 59 parts of tin in a Hessian crucible with 100 parts nitrate of soda, and dissolve the ma.s.s when cold in a caustic alkali. To the clear solution, diluted with water, a cold solution of sulphate of copper is added: a reddish-yellow precipitate falls, which on being washed and dried, becomes a beautiful green. On the palette it would be superfluous, but for common purposes might be found of service.

195. _Elsner's Green_

is also a combination of tin and copper. It is made by adding to a solution of sulphate of copper a decoction of fustic, previously clarified by a solution of gelatine. To this mixture are added ten or eleven per cent. of protochloride of tin, and lastly an excess of caustic potash or soda. The precipitate is then washed and dried, whereupon it takes a green colour tinged with blue, but without the brightness or durability of the preceding stannate.

196. _Green Bice_,

or Green Verditer, is the same in substance as blue verditer, which is converted into green verditer by boiling. This pigment is one of the least eligible of copper greens.

197. _Green Ochre._

By partially decomposing yellow ochre with prussiate of potash, we have produced a fine dark blue-green, resembling Prussian green, of great depth and transparency. There are, however, difficulties in the process; and the results do not warrant us in p.r.o.nouncing this green superior or equal to a mixture of the ochre and Prussian blue.

198. _Green Ultramarine_

is French or artificial ultramarine before the final roasting. It is a somewhat bright bluish-green, becoming a dull greenish-blue on continued exposure. Chemically, it is not a bad colour; but artists generally have decided against it.

199. _Manganese Green_,

or Ca.s.sel Green. By several methods, manganate of baryta may be obtained either as an emerald-green, a bluish-green, or a pale green. The manganates, however, are decomposed by contact with organic matter; and hence the green would be liable to suffer from the vehicles employed, as well as by being compounded with animal or vegetal pigments.

200. _Mineral Green_

is the commercial name of _Green Lakes_, prepared from sulphate of copper. These vary in hue and shade, have all the properties of the common non-a.r.s.enical copper-greens, and, not being subject to change of colour by oxygen and light, stand the weather well, and are excellent for the use of the house-painter, &c. Having a tendency to darken and blacken by time and foul air, they are not eligible in the nicer works of fine art.

Another Mineral Green adopted in Germany as a subst.i.tute for the poisonous Schweinfurt green, is composed of chromate of lead, carbonate of copper, oxide of iron, and chalk. Valueless for the palette, it has not the beauty of Schweinfurt green, but is recommended as being free from a.r.s.enic. It is not, however, altogether harmless, and should not be used in confectionery or the like.

201. _Molybdenum Green._

A clear malachite green colour, when dried, is produced from molybdate of soda and potash-chrome-alum, or from the molybdate and alum with ammonia. Being more expensive than the chrome oxides and not better, its introduction, for use by artists, would be attended with no advantage.

There is likewise obtainable a copper molybdate, by adding neutral molybdate of soda in excess to sulphate of copper. The precipitate is a very pale green colour, flocculent at first, but crystalline after was.h.i.+ng. Like the chrome molybdate it would be superfluous as a pigment.

202. _Quinine Green_

is rather adapted for a dye than an artist-colour. It is furnished by acting on quinine with hypochlorite of lime, hydrochloric acid, and ammonia, successively. Thus prepared, the green resembles a resin, insoluble in water, but soluble in alcohol, and turned blue by acetic acid. Its alcoholic solution dyes silk green, and also woollen and cotton when mordanted with alb.u.men.

203. _Roman Green_,

brought from Rome some years back by a President of the Royal Academy, appeared to be a mixture of Prussian blue and Dutch or Italian pink. It was a fugitive compound, which became blue in fading.

204. _Silicate of Baryta._

One part of silica heated to whiteness with three parts of baryta, yields a pale green solid ma.s.s, permanent, but deficient in colour when ground. It might be employed in enamelling.

205. _t.i.tanium Green_

has been proposed as a subst.i.tute for the green a.r.s.enical pigments in common use; but, apart from its expense, the colour is very inferior to Scheele's green, &c. t.i.tanium green is a ferrocyanide of that metal, produced by adding yellow prussiate of potash to a solution of t.i.tanic acid in dilute hydrochloric acid, and heating the mixture to ebullition rapidly. The dark green precipitate is washed with water acidulated with hydrochloric acid, and dried with great care, since it decomposes at temperatures above 100.

206. _Uranium Green_

is an oxide of a deep dull green colour, inclining to olive, and nearly black when in lumps. A durable but unattractive preparation, equalled in permanence and far surpa.s.sed in beauty by many cheaper compounds.

207. _Vanadium Green_

falls when ferrocyanide of pota.s.sium is added to vanadic acid dissolved in a strong acid. It is a beautiful green precipitate, but at present simply a curiosity, owing to the rarity of the metal vanadium.

Adopting our usual custom of separating the wheat from the chaff, we point to the opaque and transparent oxides of chromium, Veronese green, viridian, emerald green, Scheele's green, and terre verte, as more or less worthy of being dubbed durable.

As semi-stable, malachite green, bronze, Hooker's green, and Prussian green, must be cla.s.sed.