Oil Paint Additives and Fillers: Humidity Risks and Safer Choices

Costantini, Nodari, La Nasa, and Tomasin studied how relative humidity, light, and carbonate‑based fillers affect the early aging of commercial oil and alkyd paints. They tracked surface chemistry on Prussian blue and zinc white in mock-ups using micro-attenuated total reflectance infrared spectroscopy during accelerated aging at approximately 50, 70, and 90–95% relative humidity, both in light and in the dark.

This review focuses on oil paint additives and fillers, as well as how humidity affects their behavior in modern paints.

They report that hydromagnesite (basic magnesium carbonate) is more reactive than calcite (chalk) under the tested conditions, promoting the formation of metal carboxylates and oxalates. Oxalates formed reliably in dark, highly humid environments. In wall paintings and stone, thin calcium oxalate patinas can sometimes behave as protective skins; however, in drying oil films, oxalates usually mark ongoing hydrolysis and oxidation of the binder. They often appear as tenacious, whitish, matte surface crusts that lower gloss, trap dirt, and complicate aqueous cleaning because most oxalates are water‑insoluble and relatively acid‑resistant. In short, for oil paintings, their presence is generally adverse and indicative of excess moisture and reactive fillers or additives.

Zinc white already contained network‑coordinated zinc soaps (zinc carboxylates) at baseline, and light broadened the ester carbonyl band as photo‑oxidation products accumulated. By contrast, Prussian blue in alkyd showed fewer spectral changes than Prussian blue in drying oil.

Together, the results underscore humidity as a primary driver of hydrolysis and carboxylate growth in additive-rich, modern paint films, and they suggest practical strategies for artists to mitigate risk [p. 1, 4, 8–9, 12].

Why this matters in practice

• Soft, matte, dirt‑catching surfaces
• Water‑sensitivity during cleaning or varnish work (swelling, blanching)
• White blooms or translucent protrusions from metal soaps
• Shifts in gloss and color in extender‑rich blues and transparent organics
• Slow or uneven drying that traps moisture if wrapped too soon

Therefore, keep curing work in moderate humidity, prefer chalk over hydromagnesite where possible, and reserve zinc white for thin additions.