In the National Gallery in London there is an unfinished Michelangelo, The Entombment, with a hole in it. Not a tear. A deliberate blank, lower left, a patch of bare brown panel where a seated figure should be. The figure is the Virgin Mary, and the usual reading is simple and a little heartbreaking: the blue for her robe never came, so he stopped.

To see why a painter would leave a masterpiece open rather than finish it in some other colour, you have to know what that blue was. Ultramarine was ground from stone, and the stone was lapis lazuli, and through the whole medieval and Renaissance period essentially all of it came from one place: the Sar-e-Sang mines in Badakhshan, in the mountains of northeastern Afghanistan. One valley, half a world away, feeding every blue robe in Christendom. By the time the rock had been dug, carried overland and by sea for months, and refined into powder, it was, weight for weight, worth more than gold.

The refining is half the cost. Lapis is not pure blue. It is blue flecked through grey and white and glinting pyrite, and if you simply grind it you get a disappointed slate. The recipe written down by Cennino Cennini around 1400 is a slow act of patience. You grind the stone and knead the powder into a dough of wax and resin and oil. Then you work that dough under a mild lye solution, pressing and pressing, until the pure blue particles sink into the water and the grey stays trapped in the wax. You do it again, and again, and each pressing yields a paler, cheaper blue. The first wash is the treasure. A craftsman could spend days to make a few spoonfuls.

So the blue arrived expensive, and everyone treated it accordingly, and the price is still visible in the paintings if you know to look. Patrons wrote ultramarine into their contracts by weight, specifying the ounces and the grade, because it was the single most expensive line in the commission, costing more than the master’s own time. And because it cost what it cost, it went where the money was meant to show: the robe of the Virgin, the mantle of Christ, the one figure the whole altarpiece was built to honour. The hierarchy of the painting and the hierarchy of the price are the same hierarchy. The blue is a receipt. Michelangelo’s bare patch is a receipt too, for a sum never paid.

Painters hoarded it. Even when they had ultramarine they seldom used it alone. They would underpaint the blue in cheaper pigments, azurite or smalt or indigo, and lay the ultramarine over the top as a thin final glaze, a costly skin over a common body. A whole technique grew out of not being able to afford very much of one thing.

Here is the turn that moves it from a story about money into my kind of story. The cheap blues underneath did not keep. Smalt, which is ground blue glass, fades and greys. Azurite can creep toward green. Indigo is a fugitive that leaves over time. Natural ultramarine does none of this. It sits at the top of the lightfastness scale, and there are paintings five centuries old whose ultramarine is as deep and true as the day it was ground. So the most expensive material was also the most permanent, which means wealth did not only buy brilliance. It bought survival. In many of these paintings the only blue that still tells the truth is the blue someone paid a fortune for, while the budget blues around it have drifted toward grey and green. What we read as the Virgin seated in timeless, perfect blue is partly an accident of accounting. Hers is the pigment the patron paid enough to keep.

The era ended quickly. In 1824 a French society offered a prize to anyone who could make ultramarine synthetically. By 1826 Jean-Baptiste Guimet had done it, and by 1830 he was turning it out of a factory. The blue that had cost more than gold for seven hundred years fell to the price of any other colour. Now it sits in every child’s paintbox, still called ultramarine, “beyond the sea,” the last trace of the mountain it used to come from.

The Badakhshan source and the pigment’s history are from Wikipedia on ultramarine and Sar-i Sang; the synthetic breakthrough is in Jean-Baptiste Guimet. The reading of the Michelangelo Entombment’s bare area as the missing ultramarine is the common one, though the exact reason he stopped is inferred, not documented.