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Marbling Gall: A wetting agent. Makes colors spread on the surface of the size. Some colors need it, some colors don't. If a color is not spreading, or is sinking, give it a drop or two of gall. Also, gall is necessary for some patterns such as the Stone Marble and the Italian Vein. This is not the traditional oxgall, from a cow... genuine oxgall is not strong enough to work with acrylic colors; our gall is actually a type of detergent and works much better (and doesn't smell bad)
Many acrylic colors will need to have some gall added to them to make them spread well. Marbling gall is a surfactant, or wetting agent (actually a type of detergent). It makes the colors float and spread out on the surface of the size. After you start marbling, if you find that some colors are not spreading well, a drop or two of gall should be added to them. The gall should be added into the bottle of paint, never directly into the marbling tray. If it is not added, a lot of the color may sink to the bottom of the tray. Some colors require more gall than others; some don't need any at all. This is figured out mainly by trial and error: if a color does not spread well or sinks, add one drop to the color and test it; then another if necessary, etc. The gall is pretty powerful; use onlyone or two drops at a time.
The weather has a lot to do with whether a color needs gall or not. A change in temperature, humidity, or air pressure may cause some colors to start sinking or spreading too much, even if they were working fine before. So before each marbling session, you should test each color by sprinkling a few drops on the size to see how it is responding. If the color sinks or only spreads out a little, add a drop or two of gall into the paint. If it spreads too much because of too much gall, add more paint. Do not put gall into a full, new jar of color — it is easy to get too much. Always work out of a separate cup with only a small amount (a half inch or so) of color in it, and add your gall to that.
Marbling gall gets its name because marblers used to use actual gall from the gall bladder of a cow. When working with watercolors or gouaches, many marblers still use real oxgall. But real oxgall does not work with acrylics. The marbling gall we sell is not an animal product.
Gall is also necessary in making the Italian Vein and Overmarbling.
Any time you want one color to dominate over all the other colors, that color should have a small amount of extra gall added to it to make it spread more than the others, such as the dark red on the top example here, or the blue in the examples here.
And if you want several different shades of the same color, such as on these Stone Marbles, that is achieved by applying a color to the size, adding a drop of gall into your jar of color and applying more (which will make it a bit paler), then adding more, etc.