For centuries the deepest, truest blue in a painter's kit was also the most expensive pigment on the palette, and sometimes it cost more than gold. Ultramarine was ground from lapis lazuli, a stone mined in the Sar-e-Sang valley of Afghanistan and hauled overland across half a world before it ever reached a European workshop. The name itself remembers the journey: a colour from beyond the sea. Turning that rock into usable paint was slow, wasteful work, and the scarcity was priced into every brushstroke.

The expense reshaped the art it touched. Renaissance patrons did not simply hope for good blue; they wrote ultramarine into their contracts by name, specifying it as they might specify a quantity of silver. Reserving the finest blue for the robes of the Virgin was an act of devotion, but it was also a visible display of budget, a way of signalling that no cost had been spared. The colour carried a message about the person paying for it as much as the figure it clothed.

What ended the reign of ultramarine was chemistry. In the 1820s, chasing a prize offered for exactly this, chemists worked out how to make a synthetic version, and the monopoly of the mine collapsed. Within a generation the colour that had announced wealth became just another tube of paint, affordable to anyone. It is one of the cleanest illustrations of a pattern the exhibit keeps finding: a colour whose whole meaning came from its scarcity, and which lost that meaning the moment scarcity ended. The blue did not change. What changed was that it stopped being rare, and a pigment that had once been a boast became, quietly, ordinary.