PW24

A common filler, especially in 20th century oil paints, and its use likely continues today. It has two main roles in paintmaking: one as a base for the precipitation of transparent pigments (lakes), and one as a filler or extender. The texture it imparts has been described as "buttery," but also as “gelatinous” or even “rubbery,” which calls to mind our experiences with some vintage (and modern) oil paints. Though used frequently it can cause problems. Getten writes in his 1966 encyclopedia: “It has high oil absorption, and for this reason, tends to increase yellowing in paints.” Similar sentiments are echoed by several other authors and Seymour also includes notes on this. Artiscreation identifies this as "Gibbsite (Natural form of Aluminum Hydroxide)." A common filler and extender, and also used as the basis for lake pigments (e.g., alizarin or phthalo).

In terms of health and safety, Monona Rossol mentions that this may contain soluble aluminum, see her work here. For more information on the hazards associated with aluminum pigments, see Monona Rossol's book, The Artist's Guide to Health and Safety. Aluminum hydrate also adds "buttery" feeling, but at a cost (see below).

The Artist's Handbook. Pip Seymour's book The Artist's Handbook discusses its use as a transparent filler in oil paints. Small amounts of it seem to have an initial benefit to the look of the paint, but may cause oil paint to yellow after it's dried along with other issues. Where they are using this, paint companies rarely disclose this among their pigment codes, perhaps because it is used so widely, and it is reasonable to infer that it shows up in mediums as well. Paint companies may be using the language of "stabilizer" instead of "additive" or pigment since it's transparent. For health reasons as well as lightfastness, we would like to see companies list this pigment where it is being used, especially over certain thresholds.