The colour of Ru ware is famously — and infuriatingly — impossible to photograph. What looks like a soft sky-blue on a still morning shifts toward grey-green in a northern light, and toward almost nothing under the photographer’s flash. The reason is not mood or pigment. The reason is iron.

A Ru glaze carries, by weight, between one and three per cent of iron oxide. That tiny fraction is the entire colour budget of the piece. The rest is silence: feldspar, quartz, a little clay, and a small dose of bone ash. The glaze is a near-colourless liquid before it enters the kiln.

The reduction moment

What happens inside the kiln is one of the most studied and least reproducible reactions in ceramics. At roughly 1,250 degrees Celsius, the kiln atmosphere is deliberately starved of oxygen. The burner is throttled. Smoke curls out. The flame turns from a clean blue to a soft, sooty orange.

In that starved atmosphere, the iron in the glaze is forced to give up some of its oxygen. The compound that forms is not the bright orange of common rust; it is a reduced form — a quieter cousin, magnetite — that scatters light in a particular way. To a human eye, that way looks like the colour of a clear morning after rain.

The kiln is not a tool that produces sky-blue. The kiln is a question, asked in fire, that the iron in the glaze is sometimes willing to answer.

Why three per cent

Below one per cent, the glaze fires to a clean ivory with no colour. Above three per cent, the surface becomes muddy, then almost black, then metallically shiny in ways that betray the iron rather than refining it. The window between one and three is the Ru window. The window is also surprisingly narrow on the way up: a single per cent more iron and the bowl is no longer a Ru bowl. It is a black-burnished iron-stone.

This is why a contemporary master can fire a kiln for ten years and still set aside a third of the pieces that come out. The colour is not painted on. It is the answer to a question that is asked in fire, in a piece of earth, every single time.

What the buyer is buying

When a contemporary Ru bowl sits on a low wooden table in a Belgravia flat, the colour it holds is the residue of a particular chain of decisions: which iron, which particle size, which glaze recipe, which kiln, which stoke, which minute of which day. There is no batch. There is no repeat.

This is also why provenance is not paperwork in this field. Provenance is the chain of decisions. To own a Ru bowl is to inherit a small piece of one particular conversation between a maker and a kiln.

And on a still morning, the bowl will catch the light, and the colour will look, briefly, like the sky it was named after.