The manufacture of mineral and lake pigments

Chapter LXX: The Examination of Pigments

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When an accurate examination of a pigment is required, the only course is to conduct an exact chemical analysis, which can only be done by an expert chemist in a well-equipped laboratory. But often it is necessary in the course of trade to decide rapidly the nature of a pigment or to detect the adulteration of a dear pigment with a cheaper, and for this purpose it is important to have a method of examination which can be conducted without much apparatus, and which requires no great chemical knowledge. It is quite possible to test the majority of pigments in a simple manner. Few reagents are necessary. For an examination of the mineral pigments the following are generally sufficient: hydrochloric, nitric and sulphuric acids, caustic soda solution and ammonium sulphide.

The examination of a pigment containing organic compounds is somewhat more difficult, especially when it is necessary to ascertain the nature of the colouring matter. In this case additional reagents are required—stannous chloride, alum solution, etc.

Since the present work is intended to meet the requirements of the practical man, the behaviour of the different pigments towards ordinary reagents is given in tabular form, the pigments of the same colour being taken together. Since the colouring matters of organic origin, as they occur in the lakes, require a rather more detailed examination, the testing of pigments composed of inorganic materials only is first given, afterwards the properties of the organic colouring matters, so far as is necessary, will be added.

=Mineral Pigments.=—If the substance to be examined is in the form of a dry powder, it may at once be tested with the reagents mentioned, but if it is a paint or water colour, the oil or gum must be first removed, otherwise it would not be possible to recognise with certainty the action of the reagents.

From water colours, which are ground with gum Arabic or tragacanth solution, it is easy to separate the pigment from the binding medium. The colour is allowed to stand in a tall narrow beaker with a somewhat large quantity of water. After some time the lumps become soft, they are then repeatedly stirred up, the pigment allowed to deposit, and the water, which now contains the binding medium, poured off.

The removal of oil from a paint is somewhat more difficult. A quantity of the paint is placed in a flask with a mixture of equal parts of strong alcohol and ether, or with benzine. The flask is lightly corked, and, after frequent shaking, allowed to stand. The liquids mentioned are good solvents for oils. After a day or two the pigment will generally have deposited at the bottom of the flask. The solution is then poured off, the residue mixed with a small quantity of the solvent and transferred to a filter, where another small quantity of solvent is poured over it. The solvent is allowed to drain off, and the residue is dried. A powder without coherence should be left; this is the pigment free from oil. It can be treated like an originally dry colour.

The pigments are most conveniently examined in test tubes. If these are not at hand the reactions may be carried out on a plate of glass lying on white paper. The powder is placed on the glass and the reagent dropped on to it from a glass rod dipped in the liquid.

=Examination with the Blowpipe.=—From the behaviour of pigments at high temperatures important conclusions may be drawn as to their nature. For this purpose small porcelain crucibles are used; broken pieces of porcelain or an iron spoon may also be employed. An ordinary spirit lamp is generally sufficient as a source of heat. In some cases a higher temperature is required, which is obtained by means of the blowpipe.

The blowpipe is an invaluable instrument in the examination of mineral pigments. By means of it, almost without reagents, the nature of the pigment can generally be ascertained. The reagents necessary in using the blowpipe are soda, borax, and the solution of a cobalt salt.

The following method should be observed in examining pigments by means of the blowpipe. A quantity of the substance, about equal in volume to two grains of rice, is placed in a small hole cut by a knife in a piece of charcoal, where it is heated by the blowpipe flame. The behaviour of substances is different in the oxidising and reducing flames of the blowpipe. When a flame is blown out to a point by means of the blowpipe it may be seen that the flame consists of two conical portions, one inside the other. The inner is known as the reducing flame, because metallic oxides heated in it produce a bead of metal, or, as the change is chemically expressed, are reduced to metal. The outer cone has the opposite properties; metals melted in it are quickly changed into oxides by the action of the oxygen of the air, which has unrestricted access. In examining a pigment with the blowpipe the reducing flame is first used. The nature of the bead of metal, such as is readily obtained from lead pigments, often allows the composition of the substance to be recognised with certainty. If the behaviour of the bead of metal is not conclusive it is further heated in the oxidising flame. The metal is thus converted into oxide, which deposits on the charcoal, and by its colour and volatility or want of volatility enables the metal contained in the pigment to be determined.

A solution of cobalt nitrate or chloride is used in testing for certain metallic oxides. The substances are moistened with a very dilute solution of one of these substances before heating. After they have been heated they show characteristic colours if certain oxides are present.

Several metallic oxides give a characteristic colour when fused with borax. For this purpose a small loop is made at the end of a thin platinum wire; this is moistened and dipped in powdered anhydrous borax. On heating in the blowpipe flame the borax adhering to the wire melts to a colourless glass. In testing a pigment the transparent bead of borax is then dipped into the powder, and again fused in the blowpipe flame. It is of great importance in this test to fuse but a very small quantity of material in the borax bead; some metallic oxides have such great colouring power that, when too much is used, the bead appears quite black, so that its colour cannot be recognised.

REACTIONS OF THE WHITE PIGMENTS.

_On Heating on Charcoal_:—Lead pigments give a lead bead in the reducing flame, which is converted in the oxidising flame to lead oxide, forming a deposit on the charcoal surrounding the hole.

Antimony white gives a brittle metallic bead in the reducing flame. This burns in the oxidising flame with the evolution of white vapours, and is at the same time covered by small shining crystals.

Bismuth white in the oxidising flame gives a rainbow-coloured incrustation spreading far over the charcoal.

Tin white gives a malleable metallic bead, converted by nitric acid into a white powder.

Zinc white is converted into a green mass when moistened with cobalt solution and heated in the oxidising flame.

The more expensive white pigments are frequently mixed with cheaper substances, _e.g._, white lead with finely powdered barytes, chalk or gypsum. Such admixture is distinctly to be regarded as adulteration, since the added substances have not the covering power of white lead. In the case of coloured pigments an addition of a white material is not to be regarded as adulteration, since the addition is made in order to impart a paler shade to the colour.

_On Heating on Charcoal_:—Chrome yellow, chrome red, Cassel yellow, massicot and lead iodide give lead beads in the reducing flame. Chrome yellow and red when fused with soda give red masses soluble in water. Naples yellow gives a lead bead and white fumes without smell.

Orpiment gives an odour of garlic.

Cobalt yellow, when heated with alumina, is turned blue.

Cadmium yellow produces a brown incrustation on the charcoal.

The examination before the blowpipe serves especially for the recognition of lead, chromium, antimony and arsenic. If a pigment blackens on heating, the presence of an organic colouring matter is indicated, such as Dutch pink, weld lake, etc.

The adulteration of pigments free from lead by colours containing that metal is recognised by the blackening produced by ammonium sulphide.

_On Heating on Charcoal_:—Chrome red and red lead give lead beads in the reducing flame. The former gives a red mass when fused with soda, which dissolves to a yellow solution.

Ferric oxide pigments become darker, but give no incrustation.

Antimony vermilion burns with production of sulphur dioxide and white fumes without smell when heated in the oxidising flame; when fused with soda before the blowpipe, it gives a white brittle bead of metallic antimony.

Vermilion volatilises in the oxidising flame and gives a smell of sulphur dioxide.

Mercuric iodide readily fuses and volatilises.

Realgar volatilises. When heated with soda in the reducing flame, white fumes with an odour of garlic are produced.

_On Heating on Charcoal_:—Prussian, Chinese, Paris, Turnbull’s and Brunswick blue are turned black, the residue colours the borax bead pale brown in the oxidising flame, and pale green in the reducing flame.

Ultramarine is unaltered at a high temperature.

Smalts, on long heating in the reducing flame with borax, gives a dark blue bead.

Cobalt blue is infusible; it colours the borax bead blue. The bead loses its fine colour on long heating in the reducing flame.

Mountain blue is blackened before the blowpipe. When the residue is moistened with hydrochloric acid and again heated, the flame is coloured bright green. When fused with borax in the oxidising flame, an emerald green bead is formed.

_On Heating on Charcoal_:—Verdigris, Bremen and Brunswick greens give black residues on charcoal, which produce a bluish-green bead when fused with borax in the oxidising flame.

Emerald green and Scheele’s green behave in a similar manner, but on heating evolve an odour of garlic.

Rinmann’s green gives a blue borax bead.

Manganese green is discoloured in the reducing flame.

_On Heating on Charcoal_:—Lead brown gives a lead bead in the reducing flame.

Manganese brown and pyrolusite brown, when fused with saltpetre on platinum foil at a high temperature, give a bluish-green mass.

Prussian brown and iron brown give a pale green borax bead in the reducing flame, which turns yellowish brown in the oxidising flame.

Chrome brown heated, moistened with hydrochloric acid, and again heated, colours the flame green. It also gives a green borax bead.

Cobalt brown produces a blue borax bead.

Humin substances burn when heated on charcoal.

REACTIONS OF THE BLACK PIGMENTS.

Almost all black pigments consist of carbon, upon which reagents have no action. They should be at once heated on charcoal. If they burn away completely in the oxidising flame they consist of lamp black or carbon obtained by some process of incomplete combustion; if a white, infusible residue is left, the pigment is bone (ivory) black; if the residue is black the substance under examination must be ”neutral tint,” chrome black, or chrome-copper black. The two former give a pale green borax bead, whilst chrome-copper black gives a deep green bead, and when heated, moistened with hydrochloric acid, and again heated, it colours the flame green.

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The manufacture of mineral and lake pigmentsChapter LXX: The Examination of Pigments

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