CHAPTER LXXI._The manufacture of mineral and lake pigments

EXAMINATION OF LAKES.

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.

REACTIONS OF THE ORGANIC COLOURING MATTERS.

+----------+----------+----------+--------+---------+--------+

+----------+----------+----------+--------+---------+--------+

| | | | | | |

| | | | | | |

| | | | | | |

| | | | | | |

| | | | | | |

| | | | | | |

| | | | | | |

| | | | | | |

| | | | | | |

| | red. | | |red. | |

| | | | | | |

| | | | | | |

| | | | | | |

|Litmus |Turns red.|Unaltered.| —— | —— | —— |

| | | | | | |

| | | | | | |

| RED. | | | | | |

| | | | | | |

| | | | | | |

|Lac, |Yellowish |Violet | —— |Yellow. |Red |

| | | | | | |

|Safflower |Unaltered.|Red | —— | —— | —— |

| red | |solution. | | | |

| | | | | | |

|Madder |Red |Red | —— |Yellow. |Red |

| | | | | | |

| | | |red | |precipi-|

| | | | | | |

|Red wood |Red |Violet | —— |Yellow. | —— |

_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._

| | | | | | |

| | | | | | |

|Sepia |Unaltered.|Brown | —— | —— | —— |

| | | | | | |

+----------+----------+----------+--------+---------+--------+

+----------+---------+---------+----------+---------+--------------+

| | | | | | |

+----------+---------+---------+----------+---------+--------------+

| | | | | | |

|Dutch pink| —— | —— | —— | —— |Chars when |

| | | | | | |

| | | | | | |

|Weld lake | —— | —— | —— | —— | —— |

| | | | | | |

|Gamboge | —— | —— | —— | —— |Readily melts |

| | | | | | |

|Gamboge |Yellow | —— | —— | —— | —— |

| | | | | | |

| | | | | | |

| | | | | | |

| | | | | | |

| | | | | | |

|Indigotin | —— | —— | —— | —— |On heating |

| | | | | | |

|Indigo | —— | —— | —— | —— |On heating |

| | | | | | |

|Litmus | —— | —— | —— | —— |Unpleasant |

| | | | | | |

| | | | | | |

| RED. | | | | | |

|Cochineal |Reddish | —— | —— | —— |Completely |

| | | | | | |

|Carmine | —— | —— | —— | —— | —— |

| | | | | | |

|Lac, |Red | —— | —— | —— | —— |

| | | | | | |

|Safflower | —— | —— | —— | —— |Rapidly |

| red | | | | |decomposed |

| | | | | | |

|Madder |Red | —— | —— | —— | —— |

| | | | | | |

| |red |precipi- |red | | |

| | | | | | |

|Red wood | —— | —— | —— | —— |Much ash |

_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._

|Sap green | —— | —— | —— | —— |White ash |

| | | | | | |

|Leaf green| —— | —— | —— | —— |White ash |

| | | | | | |

|Sepia | —— | —— | —— | —— |Burns with |

| | | | | | |

+----------+---------+---------+----------+---------+--------------+

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.