Before the sun comes over the marsh
You climb the ladder in the dark. The scaffold smells of wet lime and pine. Below you the octagon of San Vitale is a well of shadow; above you the half-dome is still bare grey plaster with a red drawing on it — a drawing you are about to bury forever.
The church was begun in 526 by Bishop Ecclesius and paid for by a banker with an eastern accent, Julianus Argentarius. The chronicler Agnellus later put the bill at 26,000 gold solidi. Nobody living in Ravenna has ever seen the emperor who will be shown on this wall. Justinian never came. Neither did Theodora.
Your job is not to paint them. Your job is to build them out of about ten thousand pieces of glass, each roughly the size of a fingernail, and to decide — one at a time, ten thousand times — which way each piece should face.
That decision is the whole art. A mosaic set flat is a picture. A mosaic set at angles is a machine for catching light.
Make one gold tessera
A gold tessera is a sandwich. A slab of coloured glass at the bottom; a leaf of hammered gold about half a micrometre thick — a hundred times thinner than a hair; and a wafer-thin lid of clear glass called the cartellina, less than a millimetre, fused over the top so the gold can never wear away.
The trick most people miss: the cartellina is so thin that the colour you finally see is the colour of the glass underneath the gold. Greenish glass gives cold gold. Amber glass gives hot gold. Choose.
Three coats, one red drawing, and a clock you cannot stop
Tesserae are not glued on. They are pushed into wet lime. Two rough coats go on first, scored so the next will grip. The middle coat carries the sinopia — the full-size drawing in red ochre. Then the fine coat, mixed with powdered marble, goes on last — and only as much of it as you can cover before it sets. Miss your patch, and tomorrow you chisel today off the wall.
Scientists analysing Ravenna's fifth- to seventh-century wall mosaics have found not just line drawings but full painted layers hiding under the glass — colour laid down so that the mortar showing between the tesserae would never read as a grey gap.
The setting wall
Here is the secret of Byzantine gold. The mosaicists did not lay their gold flat. They tipped it — sharply, and downward, aiming the little mirrors at the person standing on the floor below. Then they nudged each one slightly differently, so no two fire at once.
Push the tilt to zero and the wall goes dead: a flat gold sheet, like painted tin. Push it to twenty or thirty degrees, add a little disorder, and drag the lamp — the wall breathes.
Drag anywhere on the wall to move the lamp.
How much light comes back, angle by angle
Computed live from the lamp you just moved. Every bar is the whole wall re-set at that tilt and re-measured. Move the lamp and watch the peak walk.
Light returned to a viewer on the floor
Relative brightness (peak = 100), by downward tilt of the gold
Green cheeks, red eyelids, and a face that only exists at ten metres
Press your nose to a Byzantine face and it falls apart into lumps of olive green, brick red, chalk white and dull pink, laid in stripes that look like nothing at all. Step back and your eye does the mixing for you. The mosaicist is painting on your retina, not on the wall.
Let the wall assemble itself
On the south wall of the presbytery stands the Empress Theodora, holding a jewelled chalice, a fountain playing beside her and a courtier lifting a curtain as if she were about to step out of the wall and into your church. She never came. The mosaic is her stand-in — and it is still there, 1,479 years later.
Everything below is drawn from scratch in code: my own interpretation of the panel, not a photograph of it.
Tap a marker
Seven things in this panel are worth arguing about. Tap the gold rings to hear them.
The wall's whole life, in twelve stops
Six things that are true
Four places where the story wobbles
A good exhibit tells you what it does not know.