A layer of pigment laid on thickly gives a flat surface, however good the pigment. Light goes in, strikes the metal ground and comes back by the same road. The eye reads a coloured patch.

Four thin layers, each dried completely before the next, do something else. Light enters through the first, returns in part, passes through the second, returns again. Each layer holds a little back and lets the rest travel on. What is seen at the surface is not the colour of the pigment, but the sum of the journeys light has made through it.

Why it cannot go faster

A layer that feels dry to the touch is not dry. The bonds go on forming for hours after the surface stops being tacky. A new layer laid on then sinks into the one beneath; the two mix at the boundary, and the boundary was exactly where the depth was being born.

The result is not a visible mistake. It is an absence: the dial looks right and yet flat, and the explanation does not show in a photograph.

The fifth layer

Depth increases up to a point, then falls. After the fourth layer, each new addition begins to steal light rather than return it: the layer on top becomes a filter for everything under it. The composition goes out, without any single element being wrong.

On a disc of four centimetres, where the whole scene occupies a few dozen square millimetres, the threshold is felt before it is seen. That is why a dial worked in layers is looked at, between layers, under three different lights and from the distance at which it will be worn.

The rest is patience, and patience cannot be shortened.

The processor was receiving more tasks than it could carry. At Houston, the decision that the alarm would not stop the descent was taken within seconds, by a man who knew the system sheds its non-essential tasks and recovers. The descent continued.

Two possible images

A dial that carries the Apollo 11 mission has a choice between two things.

The first is the print in the dust, or the module against black. It is recognised from three metres, asks nothing of anyone and adds nothing to what the viewer already knows. It is an image consumed instantly.

The second is the moment when it could all have stopped. It has no form of its own: there is no photograph of alarm 1202. It has to be built out of something else, out of the rhythm of the numerals, out of what is shown and what is left in shadow, out of what is read only on second looking.

What a dial asks of the person wearing it

The second choice has a cost. It asks the man who wears the piece to know what he is looking at; otherwise the composition stays a handsome surface and nothing more.

On silver, worked in micro-painting and hand engraving, the choice shows in the distribution of weight: what is pressed and what is allowed to fade. Whoever knows the story finds the order. Whoever does not know it sees a dial in no hurry to explain itself.

The crew came back with 21.55 kilograms of lunar rock and soil. That figure fits on a dial. The second in which it was decided that the alarm would not stop the descent does not fit; it is suggested.