Groups of Pigments

After considerable expansion and some rearrangement, the differences just indicated afford a reasonable basis for a chemical classification which will prove of real service in judging of the degrees of stability, and of the possible interaction of pigments.¹ The mixed and even indefinite character of many pigments, and the chemical solitariness of others, preclude the formation of groups having equal rank and precise group-characters: thus the proposed chemical classification, though convenient, has no pretension to completeness. I suggest the following nine groups:

Group I. - Elements

• Ivory black
• Charcoal black
• Lamp black
• Indian ink
• Graphite
• Silver
• Gold
• Platinum
• Aluminium

All the black pigments in ordinary use consist of or contain the element carbon, and are not subject to change: graphite is a form of carbon and is unalterable and inert. Gold, if pure or nearly free from alloy, is not liable to chemical change, but silver readily tarnishes by combining with sulphur; drawings in silver-point are frequently found to have altered in hue from this cause. The best metal-point for fine drawing is pure platinum.

Group II. - Oxides

• Zinc white, ZnO
• Green oxide of chromium, Cr₂O₃
• Burnt umber, Oxides of Fe and Mn
• Cobalt green, CoO,nZnO
• Cobalt red, CoO,nMgO
• Cobalt blue, CoO,nAl₂O₃.
• Coeruleum
• CoO,nSnO₂
• Red lead, Pb₃O₄
• Venetian red, Chiefly Fe₂O₃
• Light red
• Indian red
• Burnt sienna

These have generally been prepared at a high temperature, and are not easily amenable to chemical or physical change: they are, moreover, not liable to affect other pigments, being practically inert.

Group III. - Sulphides

• Cadmium yellow, CdS
• King's yellow, As₂S₃
• Realgar, As₂S₂
• Vermilion, HgS
• Ultramarine, contains Si, Al, Na, O and S
• Artificial ultramarine
• Green ultramarine
• Red ultramarine

Some of these may give up sulphur to the metallic bases of other pigments. Thus cadmium yellow blackens emerald green, producing copper sulphide. One of these pigments, vermilion, is prone to a molecular change, whereby the red crystalline form passes, without chemical alteration, into the black amorphous variety. The members of this group sometimes contain free sulphur, or injurious sulphur compounds.

Group IV. - Hydrates

• Yellow ochre, Fe₂O₃nH₂O
• Raw sienna, Oxides and hydrates of Fe and Mn
• Raw umber, Oxides and hydrates of Fe and Mn
• Emerald oxide of chromium, Cr₂O₃2H2O
• Mountain blue, Cu(OH)₂

The water present in these compounds exists in two states, essential and hygroscopic. Sometimes a part of the former may be lost, and a change of hue occur in consequence, but the alteration is rare, save in the pigments which owe their colour to the presence of copper hydrate. In the case of raw umber, the water present acts rather in aiding the oxygen of the air, under the influence of sunlight, to oxidize some of the peaty or bituminous matter sometimes present in this pigment.

Group V. - Carbonates

• Flake white, 2PbCO₃Pb(OH)₂
• Whitening, CaCO₃
• Chessylite, CuCO₃,Cu(OH)₂
• Malachite, 2CuCO₃,Cu(OH)₂

Three out of the four carbonates included in this group are liable to suffer change on account of the metal they contain (lead or copper) combining with sulphur, and so forming a brown or black sulphide.

Group VI. - Silicates

• Terre verte, Silicate of Fe, K, Mg
• Egyptian blue, CuO,CaO,4SiO2
• Smalt, Silicate of Co and K

These are generally inert bodies little prone to suffer or cause change. Some of the ochreous earths contain silicates of iron, manganese, and alumina, as well as the hydrates of the two former metals, and so might be placed in this group.

Group VII. - Chromates

• Baryta yellow, BaCrO₄
• Strontia yellow, SrCrO₄
• Chrome yellow, PbCrO₄
• Chrome red, Pb₂CrO₅
• Zinc chromate, ZnCrO₄

This group consists of compounds rich in oxygen. When in contact with some of the more alterable organic pigments belonging to Group VIII. the chromates occasionally lose oxygen. On this account they show a tendency to acquire a greenish or greyish hue, the result of the reduction of the red oxide of chromium to the green oxide. This change is accompanied by a corresponding injury to the colour of the organic pigment which has been involved in the reaction.

Group VIII. - Various Inorganic Salts

• Baryta white, BaSO₄
• Lead sulphate, PbSO₄
• Aureolin, K6Co212NO2
• Naples yellow, contains Pb, Sb, O
• Schweinfurt green, contains Cu, As, O
• Tungsten green, Chromium tungstate
• Manganese violet = Nürnberg violet, Manganese metaphosphate

A number of miscellaneous compounds having no chemical relationship have been thrown into this group. One, a sulphate, is insoluble and inalterable; another, potassium cobalti-nitrite, is generally inert, but, owing to its nitrous constituent, acts injuriously upon some organic pigments, notably on indigo. The members of the group which contain lead or copper may darken in the presence of sulphuretted hydrogen and of some other sulphur compounds.

Group IX. - Organic Compounds

• Indian yellow. Yellow lake. Gamboge. Pure orange. Rose madder. Madder carmine. Rubens madder. Madder red. Purple madder
• Brown madder. Scarlet alizarin. Crimson alizarin. Carmine. Crimson lake. Scarlet lake. Purple lake. Sap green. Verdigris.
• Emerald green
• Indigo
• Prussian blue
• Antwerp blue
• Bitumen
• Bistre
• Sepia
• Vandyke brown B

This group includes many more pigments than any other: not one of its members possesses the permanency belonging to the majority of the mineral pigments, while some are so fugitive that they may even be used for producing a photographic picture by being exposed to sunlight under a negative. This fading is generally due to the combined action of water and oxygen : in oily and resinous media it is lessened, retarded, or even prevented by the hydrofuge character of these vehicles.

1

In a pamphlet by M. Paul de Lapparent there is an ingeniously constructed diagram giving, at one view, a list of pigments which, in the judgment of the author, react upon one another. If we omit from the list red lead, we shall be able to cancel nearly one-third of the black marks assigned to mixtures of two pigments; but we shall still include two conspicuous offenders, namely, pale cadmium and Prussian blue. Pale cadmium is stated to be incompatible with Indian red, Venetian red, burnt sienna, the native earths, ivory black, Prussian blue, and cobalt violet; while Prussian blue is marked as affecting, or as affected by, Venetian red, burnt sienna, deep cadmium, orange cadmium, pale cadmium, aurora yellow, the native earths, ivory black, white lead, zinc white, and viridian. On the other hand, M. de Lapparent allows that vine black is an entirely innocuous and permanent pigment. My experience is not in general agreement with the judgments of M. de Lapparent as embodied in the diagram under consideration. For instance, I do not find that aureolin reacts with white lead or with ultramarine, nor can I admit that Prussian blue deserves the bad character assigned to it. Possibly the purity of the particular pigments with which our author dealt was not assured in all cases. Might there not have been free sulphur in his pale cadmium and free acid in his Prussian blue?