It is remarkable that only very few recipes for black watercolors mention the addition of specific components which alter the paint properties. In principal, a thorough tempering process is the most crucial success factor.
> Ox gall: One common additive is bile, or ox gall, that is added as a dispersant which aids in the separation of particles and inhibits clumping. For instance, the use of ‘the gall of a Neat’ for the preparation of hartshorn black is prescribed in 1596.[1]
> Sugar candy: Sugar increases the gloss of paint. Also, it helps to keep the color in a shell moist, as Browne puts it: “Black. Grind Ivory with a pittance of white Sugar Candy, which will preserve it from crackling out of your shel.”[2]
> Honey: Honey is deliberately added as a wetting agent, allowing the paint to retain moisture and slowing down its drying time.
[1]Neat is a term for cattle of the bovine genus, like bulls or oxen. Here ox gall is referred to. Anonymous. 1596. A very proper treatise wherein is breefely set forth the art of limming: fol. 7r. https://archive.org/details/verypropertreati00impr/page/n6.
[2] Browne. 1669. Ars Pictoria: p. 80. https://archive.org/details/gri_arspictoriao00brow.