Re: ox gall
You have to be careful using gum arabic ( responding to old post, but just saw this )
Usage of gum arabic being added, is noted by art conservationists as creating a layer of paint, this layer of paint does not properly adhere over time and will flake off. If you add some, just add a tiny bit.
Ox Gall is very useful with student grade paints, and with Holbein and QoR brand paints, since they either don't use it, or use a synthetic. ( Da Vinci does not either, but I don't think I've noted the same behavior where it does not disperse as easily in washes, could be a case of misrepresentation, or some other additive which works better )