The manufacture of mineral and lake pigments

Chapter LXXI: Examination of Lakes

Text size

In examining lakes it is necessary to ascertain the nature of the organic colouring matter, the base with which this is united, and also the nature of the substances mechanically mixed with the lake to lighten its shade.

It is most convenient to examine the pigment first for mechanical admixtures. If an effervescence follows the addition of hydrochloric acid, denoting the presence of a carbonate, this is generally calcium carbonate. White lead is rarely used, since it is very heavy and is more expensive. Gypsum and magnesia are occasionally added to lakes; they are recognised by examining the residue left on ignition.

In testing for the metallic oxide which is united with the colouring matter a small quantity of the lake is heated in a little porcelain crucible until all organic substances are completely destroyed. The residue is then examined for aluminium and tin oxides, by means of which lakes are most frequently prepared.

_The remaining red colouring matters give such uncertain
reactions that it is almost impossible to distinguish
between them by ordinary means; they are therefore not
included here._

_The remaining red colouring matters give such uncertain
reactions that it is almost impossible to distinguish
between them by ordinary means; they are therefore not
included here._

It is not easy to decide which colouring matter is united with the metallic oxide, because the organic colouring matters do not give, as a rule, decided reactions. In examining pigments of this kind it is always advisable to test at the same time a pigment of known composition, and to compare the reactions of the two. The colouring matter of the lake is first brought into solution. This is accomplished by the action of dilute hydrochloric acid, which decomposes the compound of metallic oxide and colouring matter, the latter dissolving. A portion of the lake should remain undecomposed, so that the solution has no strong acid reaction by which the action of the reagents to be used would be modified. When the lake has been treated for some time with dilute hydrochloric acid, water is added and the solution of the colouring matter filtered from the residue. Small quantities of this solution are then treated with the different reagents. By a comparison of the reactions given by the solution and by a solution of the pure colouring matter, the nature of the colouring matter under examination may be decided.

In deciding in the first place if a colour is of organic origin, it is treated with hydrochloric acid in the above manner. If a coloured solution results, an organic colour is probably present. Chlorine water is then added to a portion of the solution; if the latter is quickly decolourised an organic colouring matter is certainly present, for they are all decomposed by the continued action of chlorine.

The following reagents are used in testing organic colouring matters: dilute sulphuric or hydrochloric acid, caustic soda solution or lime water, strong nitric acid, which, in consequence of its oxidising properties, gives different reactions to the other acids. Of the metallic salts, alum, stannous chloride and ferric chloride are used, and occasionally copper acetate. Glue solution is also used in the pure form of isinglass or gelatine solution, with which several colouring matters give characteristic precipitates. The foregoing tables give the behaviour of the colouring matters contained in the organic pigments towards the reagents mentioned above.

Previous Next

Comments

The manufacture of mineral and lake pigmentsChapter LXXI: Examination of Lakes

0%9 min left in chapter