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Why did 12th- and 13th-century church windows often use deep blues and rubies?

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The north rose window at Chartres Cathedral in France, featuring intricate stained glass with vibrant geometric and biblical designs in a large circular and five tall vertical panels. — stained glass The stained-glass rose window in the north transept of Chartres Cathedral, France.

In the 12th and 13th centuries, artisans composed stained glass windows using deep, rich colors—specifically intense rubies and blues—primarily as a logical response to the architectural environment of the time.

Because church interiors were relatively dark, the light that penetrated through the window openings appeared exceptionally brilliant, even harsh, in contrast to the surrounding gloom. To balance this high-contrast environment, artists utilized a palette of saturated, deep colors that could withstand and modulate the intense light. These colors were achieved by adding metallic oxides to the glass while it was in a molten state: cobalt was used to create blue, while copper was used for ruby.

As architectural styles evolved and church walls were opened up to admit more light, the stark contrast between the interior and exterior diminished. Consequently, the dense, saturated colors of the earlier period became less effective, leading to the development of lighter, more muted color harmonies in the 14th and 15th centuries.12