Ultramarine blue pigment is notoriously unstable and fades when exposed to light, heat, or moisture. Ultramarine blue is a pigment made of lapis lazuli, a semi-precious stone. The exact chemical composition of ultramarine blue can vary slightly depending on the source of the lapis lazuli, but the main components of ultramarine blue are ultramarine, lazurite, and calcite. Ultramarine is a complex silicate of sulfur and aluminium. Lazurite is a deep blue mineral that gives ultramarine blue its characteristic color. Calcite is a common mineral that is found in many types of rocks.

The fading of ultramarine blue is due to the breakdown of the ultramarine molecule. This breakdown can be accelerated by exposure to light, heat, or moisture. When ultramarine blue is exposed to light, the photons of light interact with the electrons in the ultramarine molecule, causing them to become excited. This excitation can cause the electrons to move to higher energy levels, which then cause the electrons to become unpaired. This process, known as photooxidation or photolysis, creates free radicals, which attack the ultramarine molecule and lead to the breakdown of the pigment.

The same is true for exposure to heat and moisture. The higher the temperature or humidity, the faster the rate of degradation. This is because the heat and moisture cause the ultramarine molecules to vibrate more, which makes them more susceptible to attack by free radicals.

As a result of all these factors, ultramarine blue is not considered a permanent pigment and should not be used in artwork that will be exposed to light, heat, or moisture. However, it can be used in artwork that will not be exposed to these conditions.