You are Michelangelo's plasterer on the Sistine scaffold, Rome, 1511. Before the sun sets, the wall will decide what stays forever.
It is still dark when you climb the ladder. Sixty-six feet below you, the chapel floor is a pool of candle-light; up here on the bridge there is a bucket of lime putty, a hawk and trowel, a jar of brushes, and a man who has not spoken since yesterday.
He tells you where to spread. That patch — no bigger than a door — is today. Everything he paints on it in the next few hours will bind into the wall and outlive both of you. Everything he paints after that will sit on top like dust, and fall off within a century. There is no undo. There is no evening session. There is only the day the plaster gives you, and the word for it is giornata.
So: mix it, spread it, and watch the clock in the stone.
Fresco is not paint stuck to a wall. It is a rock taken apart and put back together — with your colour trapped inside. Click through the four moments of the lime cycle.
Lime putty alone would crack as it shrinks. Sand gives it a skeleton. Water makes it spreadable — and, if you are greedy with it, ruins everything. Set the mix; the trough will tell you what it thinks.
Wet lime is fiercely alkaline. Some pigments shrug at it and live for five hundred years; some turn brown, green or black; some simply refuse to work at all and must be laid on later, dry, where they will slowly flake away. Tap a jar to read it — then let five centuries pass.
Here is your patch of vault and the master's cartoon, pricked and pounced onto it in dots of charcoal. Spread the plaster, then paint. The plaster is your timer: too early and the colour sinks into a milky blur, too late and it just sits on the surface waiting to fall off. Watch the band under the clock.
Muddy the earths with black and fresco goes dead and grey. Michelangelo's answer was to refuse the shadow: where a fold turns away from the light, he changes the colour completely. Green becomes yellow. Rose becomes acid green. It is called cangiante — "changing", like shot silk. Drag the slider and watch the drapery decide.
Every Mass burned candles. Every brazier smoked. Old restorers brushed on animal glue to "revive" the colours, and it darkened like caramel. By 1980 the ceiling was the colour of strong tea, and the world had decided Michelangelo was a sombre painter of browns and greys. Then a team put a swab to it. Drag across the fresco to clean it yourself.
You need: plaster of Paris (craft shop) or ordinary wall filler, a plastic lid, water, a cheap brush, and watercolours or food colouring.