Field's Chromatography Part 10

There are obtainable from cobalt by different processes rose and red colours of more or less beauty and intensity, but all vastly inferior to those of madder, in whose absence alone they could gain a place on the palette. Durable as a rule, they are in general characterized by a fatal chalkiness, and poorness of hue. More expensive than the madder colours, and without their purity, delicacy, depth, or transparency, cobalt reds have often been offered as pigments, and as often declined. A colour may be good in itself, but if there is something better and at the same time cheaper, its introduction into commerce is out of the question.

99. _Copper Reds._

A somewhat finely coloured red oxide is produced by exposing to a white heat for twenty minutes, a mixture of certain proportions of blue vitriol, mono-carbonate of soda, and copper filings. The product, however, is affected by impure air, and is otherwise not so desirable as an iron oxide.

An interesting account has lately been given by Professor Church of a new animal pigment, containing copper, found in the feathers of the violet plantain-eater and two species of Turacus, natives respectively of the Gold Coast, the Cape, and Natal. _Turacine_, the name proposed for it, is noticed here only because it is the first animal or vegetable pigment, with copper as an essential element, which has been hitherto isolated. The colour is extracted by solution in an alkali, and precipitation by an acid, and is changed on long exposure to air and moisture to a green hue. As the entire plumage of a bird yields not more than three grains of pigment, turacine must be looked upon as a mere curiosity.

100. _Ferrate of Baryta_,

Produced by adding aqueous ferrate of potash to an excess of dilute solutions of baryta salts, has been described as carmine-coloured and permanent. We have not found it to be so--an experience which has evidently not been confined to ourselves; and we cannot help thinking that this is one of those errors which get copied from one chemical work into another, to the special confusion of students. It is but fair, however, to add that in Mr. Watts' Dictionary of Chemistry, the latest and best work of the kind, this ferrate is said to become "brick-red after was.h.i.+ng and drying at 100 C.," and to be only "tolerably stable."

101. _Gold Reds._

Many organic substances added to gold solutions throw down either the metallic gold or the red oxide, which then unites with the organic compound more or less decomposed and forms a red precipitate. Sugar, gum, the decoctions of cochineal, gamboge, fustic, turmeric, sumach, catechu, and Brazil wood, all afford red pulverulent colours. Boiled with sugar, gold solution gives first a light and then a dark red.

Whatever their merits, the excessive costliness of these preparations renders them inadmissible as pigments. At one time, indeed, a gold compound known as purple of Ca.s.sius was so employed, but this soon became obsolete on the introduction of madder purple.

102. _Iodine Pink._

There may be obtained from iodine and mercury a very pretty pink colour, a.n.a.logous in composition to pure scarlet. It is apt to pa.s.s into the scarlet modification, and is in other respects even less to be depended on than that variety.

103. _Kermes Lake_

Is an ancient pigment, perhaps the earliest of the European lakes, and so called from the Arabic Alkermes. It is sometimes spelt _cermes_, whence probably cermosin and crimson, and kermine and carmine. In old books it is named vermilion, in allusion to the insect, or _vermes_, from which it is prepared. This insect is the "coccus ilicis," which feeds upon the leaves of the p.r.i.c.kly oak in the south of Europe. Like the "coccus cacti," it is covered with a whitish dust, and yields a tinctorial matter soluble in water and alcohol. Kermes and the lac of India doubtless afforded the lakes of the Venetians, and appear to have been used by the earliest painters in oil of the school of Van Eyck. The former, under the appellation [Greek: kurno kokino], is said to be employed by the modern Greeks for dyeing their caps red.

Some old specimens of this pigment which the author obtained were in drops of a powdery texture and crimson colour, warmer than cochineal lakes, and having less body and brilliancy. They worked well, however, and withstood the action of light better than the latter, though the sun ultimately discoloured and destroyed them. In other respects, they resembled the lakes of cochineal. As a colouring matter, kermes is only about one-twelfth part as powerful as that substance.

104. _Lawson's Red._

In 1861 it was stated that Professor Lawson had prepared a new dye of great richness, in the laboratory of Queen's College, Canada, from an insect, a species of coccus, found the previous summer for the first time on a tree of the common black spruce (_Abies nigra_), in the neighbourhood of Kingston. Having been but recently observed, a sufficient quant.i.ty had not been obtained for a complete series of experiments as to its nature and uses; but the habits of the insect, as well as the properties of the dye, seemed to indicate that it might become of practical importance. In colour it closely resembled ordinary cochineal, but was rather more scarlet in hue. It was described as capable of being produced in temperate countries. The colouring matter had not then been thrown upon a base, nor do we know that it has since been introduced as a pigment. If it possessed greater stability than cochineal, with equal brilliancy and depth, this dye might form one of those colours of the future, to whose possible sources we would direct attention.

105. _Manganese Red._

Bisulphide of a.r.s.enic combines with basic metallic sulphides forming a cla.s.s of sulphur-salts, called by Berzelius, hyposulpha.r.s.enites. The hyposulpha.r.s.enite of manganese is a dark red precipitate, uninjured by sulphuretted hydrogen, and so far applicable as a pigment. Containing a.r.s.enic, it would of course be poisonous; and would probably be found to fade on exposure to air and light.

106. _Murexide._

The red obtained from this substance created a great deal of interest among printers and dyers on its introduction in 1857, or thereabouts.

For purity and brilliancy of shade it was not excelled by any other colour, but not being able to stand the effects of air and light, its employment was limited. We are not aware that murexide has yet been brought forward as a pigment, and judging from its character as a dye, it would scarcely enrich the palette. Dyes and pigments have much in common, and a fugitive dye cannot be expected to furnish a permanent pigment.

Murexide is produced by the action of ammonia on alloxan, which is itself derived from the uric acid of guano by treatment with nitric acid, and was known nearly forty years back to stain the fingers and nails red. The first murexide sent into the market was a reddish-purple powder, dissolving in water with a fine purple tint, leaving a little residue undissolved. Owing to improvements in manufacture, it is now capable of being prepared almost chemically pure, and with that green metallic reflection peculiar to several coal-tar salts and the wings of certain insects. When sulphuretted hydrogen is pa.s.sed through a concentrated solution of murexide, it is immediately decoloured; a fact which renders it likely that murexide pigments would be as liable to suffer from an impure atmosphere, as from exposure to light and air.

When an alkaline solution of murexide is precipitated by an acid, a light s.h.i.+ning powder results, called purpuric acid. This dissolves in alkalies, and combines with metalline bases to form various coloured compounds, termed _Purpurates_. Among them may be mentioned a red purpurate of lead, a purple-red and a rose-coloured purpurate of mercury, a purple-red purpurate of silver, a dark red-brown purpurate of strontia, a crystalline red purpurate of cobalt, a scarlet purpurate of platinum, a yellow purpurate of zinc, and a green purpurate of baryta.

All of these, however, being more or less soluble in water, and owing their colours to murexide, would be ill adapted for pigments.

107. _Paille de Mil,_

Or African Cochineal, is a substance obtained from Africa. Whether it has received its name of cochineal from its appearance or origin is not clear, but it behaves more like galls and sumac than cochineal, though it does give a kind of red with alumina mordants. The colours it yields are deficient in brightness, and it has otherwise been reported unfavourably of.

108. _Peganum Harmala,_

The seeds of which afford a red colour, has been investigated by the French, but described as inferior to existing reds both in brilliancy and stability.

109. _Persulph.o.m.olybdates._

The metallic compounds formed by the combination of persulph.o.m.olybdic acid with a base are pulverulent, in many cases of a red colour, and for the most part insoluble in water. With barium, the acid furnishes a yellowish-red powder, insoluble in, but made denser by water, which imparts to it a cinnabar colour. With calcium it is said to yield a scarlet, sparingly soluble in water. With chromium, uranium, lead, platinum, and copper, it gives a dark red; that from the last metal turning brown when collected on a filter. It likewise produces reds with zinc, cadmium, iron, mercury, and tin; of which the last is slightly soluble in water.

Molybdenum being a rare metal, and persulph.o.m.olybdate of potash, the salt used in the foregoing reactions, difficult to prepare, it is unlikely that the colours named will rank among the pigments of this generation. Nevertheless, as we have observed before, such fancy products should not be altogether ignored, it being quite as well to have some knowledge of our resources, even though those resources be not at present available. All the rare metals afford coloured compounds: tantalum, niobium, pelopium, vanadium, tellurium, t.i.tanium, yttrium, lanthanum, didymium, glucinum, cerium, thorinum, zirconium, palladium, rhodium, iridium, ruthenium, osmium, indium, thallium, &c.; and it is just possible that some of these may one day sc.r.a.pe acquaintance with the palette.

110. _Red Chalk_,

The colouring matter of which is sesquioxide of iron, is used as a crayon. Some specimens are excessively hard, so much so that they are difficult to crush, even in an iron mortar; while others have the consistence of the softest iron-ochres. They vary too in tint from a fawn colour to the softest brick-red, occasionally being almost as bright as a mixture of equal weights of vermilion and Venetian red. The amount of iron oxide present has been found to range from four to thirty-seven per cent, according to the depth and hardness of the samples. When a specimen of red chalk tolerably rich, but not too rich, in iron oxide is finely powdered and strongly ignited, it offers a remarkable change of colour, becoming a dull sage-green. Perhaps this, if it were permanent, might prove useful in foliage tints.

111. _Red Precipitate_,

Or mercuric oxide, may be obtained either of a brick-red or orange-yellow colour. It is destroyed by impure air, and on exposure to suns.h.i.+ne gradually turns black, being superficially decomposed into oxygen and metallic mercury or mercurous oxide.

112. _Rose Pink_

Is a coa.r.s.e kind of lake, produced by dyeing chalk or whitening with decoction of Brazil wood, peachwood, sapan, bar, camwood, &c. It is a pigment much used by paper-stainers, and in the commonest distemper painting, &c., but is too perishable to merit the attention of the artist.

Chevreul obtained a crystalline substance from Brazil wood, which he looked upon as the pure colouring matter, or as containing the pure colouring matter, and which gave red and crimson precipitates with many salts. Possibly some of these might prove more durable than the roughly made rose pink.

113. _Rouge_,

The rouge vegetale of the French, is a species of carmine, prepared from safflower or carthamus, which is the flower of a plant growing in the north of Africa, India, and other warm climates. Safflower yields two colours--a valueless yellow which dissolves in cold water, and about five per cent of red, insoluble in water but dissolved by alkalies. The red, or carthamin, furnishes a pigment of exquisite beauty, marked by richness, transparency, and free working. Its extreme fugacity, however, militates against its employment by artists. As a dye, its manner of fixing upon fibre is different from that of any other colouring matter; requiring no mordant, like madder or cochineal, and needing no solution, like indigo or anotta, but fixing at once as soon as the cloth is brought into contact with it. But even for a dye the colour is fugitive, fading after a few hours' exposure to suns.h.i.+ne, and sometimes being quite bleached in the course of a day. It is when combined with levigated talc to form the paint of the toilette that the red becomes most serviceable. Possessing a peculiar softness and velvety glow, rouge is an unrivalled--and a most harmless--aid to beauty.

_Chinese Rouge_ and _Pink Saucers_ have much of the qualities of, and appear to be also prepared from, the safflower.

114. _Rufigallic Red._

When a duly proportioned mixture of gallic acid and oil of vitriol is carefully and gradually heated to 140, a viscid wine-red liquid results. If this be poured into cold water, after cooling, a heavy brown-red granular precipitate is formed, soluble in 3333 parts of boiling water. It dissolves in potash-ley, and to fabrics impregnated with alum or iron mordants, imparts the same shades of colour as madder; the colours so produced withstanding soap but not chlorine.

Whether brilliant lakes could be obtained from the potash solution of the red, and whether those reds would be stable, it might be worth while to ascertain.

115. _Sandal Red._

We have kept this separate from other reds derived from woods, because it is said (by Professor H. Dussance) to be obtainable not only equal in beauty and brightness to carmine, but of greater permanence. The process of preparation is as follows:--The powdered root exhausted by alcohol gives a solution to which hydrated oxide of lead is added in excess. The combination of colouring matter and lead oxide is then collected on a filter, washed with alcohol, dried, dissolved in acetic acid, and mixed with a quant.i.ty of water. The red being insoluble therein is precipitated, while the acetate of lead remains dissolved. After being washed, the colour is dried at a low temperature. The Professor affirms that the red so produced is unaffected by sulphuretted hydrogen, or by light and air; and it is stated that the colour which was used to paint the carriages of the Emperor Napoleon, remained as bright at the end of nine years as when it was put on. Possessing such properties, it is curious that the red has never been--in this country at least--introduced as an artistic pigment, the more especially as seventeen years have elapsed since its discovery.

116. _Silver Red._

By adding monochromate of potash to an acid solution of nitrate of silver, a particularly fine ochre-red is obtained. It is, however, apt to be injured both by foul air and exposure.

117. _Sorgho Red._

Some nine years back there was found to be a carmine colouring matter in most parts of the Chinese sorgho, chiefly in the unpressed stem. The red, which is extracted in an impure state, is dissolved in weak potash-ley, thrown down by sulphuric acid, and washed with water. This purified product, soluble in alcohol, caustic alkalies, and dilute acids, has been employed in Austria, Baden, &c., for the dyeing of silks and woollens with the common tin mordants. The colours produced from it are unchanged, they say, by warm soapsuds or light. We do not know whether the red found its way to England, but it has certainly not appeared here as a pigment.

118. _Thallium Red._

The orange-yellow precipitate formed by mixing a neutral salt of protoxide of thallium with b.i.+.c.hromate of potash, is converted by nitric acid into an orange-red. The latter compound, which is a terchromate, is almost insoluble in cold water, 2814 parts being required to dissolve it. If the colour be boiled in a large excess of moderately strong nitric acid it is dissolved, yielding magnificent cinnabar red crystals on the solution cooling. These crystals likewise seem to be the terchromate.