Ultramarine blue is an FDA-regulated cosmetic colorant with a well-documented weak point: research measuring its behavior in dilute acid shows it loses over half its weight without surface treatment, versus a small fraction with it. This page covers that research and where ultramarine blue is regulated and used.
What is it?
Ultramarine blue is a pigment produced by calcining, above 700°C, a mixture that can include kaolin, sulfur, sodium carbonate, siliceous matter, sodium sulfate, and carbonaceous matter (not all of these are necessarily present in every batch) into a complex sodium aluminum sulfosilicate. Per 21 CFR §73.2725, the FDA permits ultramarine pigments for coloring externally applied cosmetics, including cosmetics used in the area of the eye, in amounts consistent with good manufacturing practice, subject to limits of 20 parts per million lead, 3 parts per million arsenic, and 1 part per million mercury; batches are exempt from certification.
Why should I use it?
- It’s an FDA-regulated cosmetic colorant with defined purity limits, documented for use even around the eye area — see “What is it?” above.
- It’s a widely used pigment across construction, plastics, coatings, and cosmetics — see “How it’s used.”
How it’s used
A 2025 peer-reviewed study describes ultramarine as favored for its deep color, stability, and fade resistance, and “widely used in construction, plastics, coatings, art, cosmetics and other fields.”
Limitations — when it isn’t the right choice
- Ultramarine blue has poor acid resistance, and without surface treatment it degrades substantially in acidic conditions. The same 2025 study explains the mechanism directly: in acidic conditions, hydrogen ions react with the pigment’s sulfur ions, generating hydrogen sulfide and elemental sulfur and breaking down the chromophore groups responsible for its color. The study measured this directly — unmodified ultramarine exposed to 1.0% hydrochloric acid lost 51.87% of its mass, with a substantial color shift (ΔE* of 14.94). After surface modification with a silane treatment, mass loss dropped to just 3.70%, with a much smaller color shift (ΔE* of 2.57). This means ultramarine blue needs surface treatment or a similar protective modification before it’s used in any application where exposure to acidic conditions is expected; these exact figures are specific to this study’s test acid concentration and treatment method and shouldn’t be assumed to hold at every acid strength or exposure duration.