72

DYEING AND CALICO PRINTING.

material, but it cannot corredtly be called a solution, and the mucilage israther pedtine than true gum.

It is very difficult to give precise directions as to the quantity of thickeningrequired for the preparation of the colours, as this depends partly on theviscosity of the thickeners themselves, and still more on the chemical adtionexerted upon them by the colouring matters. For instance, where a certainquantity of starch is sufficient for indifferent (neutral) solutions, or such asbecome only slightly acid after boiling, half that quantity will suffice whenthose salts are employed which exert a coagulative action, such as concentratedsolutions of sesquioxides, or caustic substances like soda. With sulphatesthe result is different.

When gum is used a small quantity is generally sufficient with the largerclass of mordants (the solutions of sesquioxides), but double the quantitywill be found necessary for indifferent solutions, proto-salts, &c. The morebasic any salt is, the greater is its tendency to coagulate; the sesquioxideshave this tendency, but the proto-salts have not. Those substances whichpossess the greatest thickening power, such as gum tragacanth and starch,are not so suitable for printing purposes as the natural and artificial gums,which are viscous and fluid at the same time.

We may here observe that gum aCts in a peculiar manner with the acetatemordants; for instance, with proto-acetate of iron it has the effect of producingpaler tints than roasted starch. This peculiarity, which is generally believedto be due to the acidity of gum solutions, appears, according to M. C. Koechlin,to be simply a hygrometrical effedt, as when glycerine or sal-ammoniac isadded to the pigment mixed with gum solution, the latter is restored to all itssuperiority of adtion.