THE EFFECT OF OX-GALL UPON COLORS.
By chemical analysis we have seen that glycocholic acid is the principal constituent of ox-gall and in combination with taurocholic acid, glycin, choline and lactic acid is soluble in water. On the other hand, when pure, it is soluble with difficulty and therefore is only of use to us when in combination with other acids, as it must combine intimately with water-colors.
The atomic weight of gall and the insolubility of pure glycocholic acid in water are the most valuable properties of it for the marbling colors.
The first gives to the colors the expanding power, the latter the adhesion to paper.
All the lower fatty acids, among which ox-gall may be numbered, possess properties similar to fatty bodies, namely a less atomic weight than water. They differ from fatty bodies in this, that they mix with water.
The expanding power and lightness of ox-gall are, consequently, peculiarities of equal value. Marbling size forms the base for the colors, which by the consistency of the glutinous mass and by the gall added to them are prevented from sinking to the bottom. They float like drops of oil upon the water and only differ from them by the fact that the colors will spread out on the size, while the drops of oil on the water are contracted to circular spots of small size. If a drop of oil were thrown upon the size it would spread out very quickly.
This purely physical occurrence is caused by the different weights of the bodies and the consistency of the size. The fatty bodies, which are much lighter than water, float upon it but the gravity of the drop itself prohibits its spreading out, as it sinks somewhat and is held together by it. It is different with the size; by the consistency of the glutinous mass the gravity of the drop is antagonized and as a result it flattens and spreads out.
As I have already mentioned the insolubility of glycocholic acid binds the color to the paper. To explain this I will mention a few examples; the saturated alum solution will cause glycocholic acid to curdle. If, therefore, we add to ox-gall which is basic the saturated solution of alum, a soft plaster-like mass is precipitated which consists of glycocholate of aluminium, insoluble in water. This chemical reaction explains the durability of the colors. If we desire to marble books or papers we impregnate them with alum water, allow them to dry partly and take the edge from the size.
In the same moment that the paper soaked with alum comes in contact with the colors, the latter become bound by the formation of glycocholates of aluminium and do not run. They have the same property of fixing colors that resinous soaps used in the paper manufacture for gluing machine paper have. If this resinous soap were not too strongly basic, which is injurious to many colors, it might be applied as a surrogate for ox-gall.
As the resinous soap on account of its binding quality and insolubility could also be used in marbling with such colors, upon which it exercises no injurious effect, I will here give its preparation and former use in the manufacture of paper. If 2-5 ounces of rosin be boiled with 1-1/3 ounces of sodium lye and if you add by teaspoonful from time to time so much lye until a sample of it is dissolved by hot water to a clear fluid, the mass on cooling congeals and forms a quite solid soap. If we mix a solution of resinous soap with one of alum then we obtain an insoluble compound of resinous acids and aluminium. In this way resinous soap is used in gluing paper by adding to the paper pulp, first resinous soap and then a solution of alum. During this process a thin layer of insoluble soap of aluminium is formed around every particle of the paper and thus the running of the ink is prohibited. The same process occurs with ox-gall contained in the colors, when the edge is lifted off on paper or book-edges impregnated with alum, by which the running of the colors is prohibited.