Why some pigments last forever: the science behind ultramarine and the most stable colors in art history

Why do some paintings created hundreds of years ago still display rich, vivid colors, while others have faded, yellowed, darkened, or completely lost their original appearance? The answer does not lie only in artistic technique or the skill of the painter, but in the chemistry of pigments themselves. A pigment is not just a color; it is a material whose long-term stability can vary dramatically depending on its chemical structure and its resistance to light, oxygen, moisture, and time.

The most durable pigments in art history are primarily inorganic pigments, meaning they are based on mineral structures rather than organic molecules. Their particles form highly stable crystalline networks that are already in a low-energy, chemically stable state. Because of this, they have little tendency to react further with environmental factors. Sunlight, air, and humidity have far less impact on these materials compared to organic dyes and pigments. In simple terms, they are chemically “locked” in a structure that is extremely difficult to break down.

A perfect example is synthetic ultramarine, a deep blue pigment based on a sodium aluminosilicate framework with sulfur-containing ions responsible for its color. This pigment is highly stable under light exposure and resistant to many environmental conditions. It does not easily undergo photodegradation because its crystal structure prevents the free movement of electrons in a way that would normally lead to color breakdown. One can think of it as a color trapped inside a very strong mineral cage. The main weakness of ultramarine is its sensitivity to acids, which can damage its structure and cause discoloration, but under typical artistic and museum conditions it is considered highly stable.

Other inorganic pigments behave in a similar way. Iron oxides, responsible for natural ochres, umbers, and siennas, are among the oldest pigments known to humanity, used in prehistoric cave paintings tens of thousands of years ago. Their stability comes from the fact that iron in these compounds is already fully oxidized, meaning it is in a chemically stable state that does not easily react further with oxygen in the air. Titanium dioxide, a modern white pigment, is also extremely stable and highly resistant to light-induced fading. Carbon-based blacks, such as charcoal and carbon black, are likewise very durable because elemental carbon is chemically stable and does not easily break down under normal conditions.

However, not all pigments have been so fortunate. Throughout the history of art, many colors have proven unstable and have changed significantly over time. Organic pigments, which are based on carbon-containing molecules, are particularly vulnerable. Their molecular structures often contain bonds that are sensitive to ultraviolet light, which can break them apart and cause fading or color shifts. As a result, many once-brilliant reds, yellows, and lakes have faded dramatically in historical paintings.

One notable example is carmine, a deep red pigment derived from cochineal insects. It was once highly valued for its intense color, but it is very sensitive to light and gradually darkens or loses saturation over time. Similarly, Paris green, a copper arsenite pigment, is both chemically unstable and toxic; it can degrade and change color due to chemical reactions in its structure. Chrome yellows, based on lead chromate, are also known to darken or shift in hue under certain environmental conditions due to chemical changes within the pigment.

The difference between stable and unstable pigments ultimately comes down to chemical structure and energy stability. Inorganic pigments resemble rocks: they are already in a stable, finished chemical state with little tendency to change further. Organic pigments, on the other hand, behave more like delicate molecular systems that are far more sensitive to environmental stress. This is why conservation science plays such an important role in understanding historical artworks and preserving their original appearance as accurately as possible.

Color in art is therefore more than aesthetics. It is a record of chemical durability and the long-term interaction between matter and time. Some colors have survived unchanged for millennia, while others have disappeared from paintings even though they were once just as vivid as the most permanent pigments. This is why the study of pigments is also a study of how time itself transforms what we see.