From the moment an artist completes an oil painting, the painting begins to change color. The shift is gradual, taking decades, or even centuries, but eventually colors fade, darken, or become more transparent over time.
Oil paint is made of pigment particles suspended in an oil binder. The variety of substances used as pigment make some colors susceptible to aging.
For example, mineral-based pigments such as verdigris, a green color made from copper, changes to a dark brown over time. This pigment, which was widely used in the 15th century, left many great works much darker than intended.
In contrast, pigments produced from organic dyes, such as rose madder and indigo, are susceptible to fading when exposed to light. The loss of color from a yellow gamboges glaze, based on plant resins, can make green foliage in Dutch flower paintings appear blue.
Environmental contaminants can also be damaging to colors. For example, hydrogen sulfide in the air converts white colored, lead based paint to black colored lead sulfide.
As if that weren't enough to keep restoration experts busy, natural resin varnishes meant to protect paintings, turn yellow with age as well. This makes an artist's white highlights seem stained, causes blues to appear green, and reds to appear orange.
Proper storage, out of direct sunlight, and with controlled temperature and humidity, is the only thing that can slow the aging process. Preservation is important because shifts in color and opacity change the way we perceive paintings. Illusions of depth and three-dimensional form depend on subtle transitions in color and tone that are often obscured, and sometimes lost as a painting ages.