Scagliola

Ground gypsum and dry sanding make a very fine dust. Mask on throughout, and grind wet wherever you can.

Use a seasoned wooden board or a slate slab. A support that moves will crack the scagliola layer.

Pass the selenite gypsum through a fine sieve. Every lump becomes a spot that will not take a polish, and it shows immediately on the finished surface.

Dissolve animal glue in warm water and mix the gypsum with that. It is the glue that makes polishing possible — gypsum and water alone stays porous and never takes a mirror finish.

Divide the mix and add earth pigments to each part. Prepare every colour before you start: once the ground layer is laid you have to work fast.

Apply the base layer to the support and level it. It must be dense and free of voids: every bubble becomes a crater when you come to polish.

Wait until it has set but the slab is still slightly damp. That is the right moment to incise.

Transfer the drawing and cut the lines and fields out with gravers and scrapers, 3-4 mm deep.

Press coloured paste into the recesses, colour by colour, leaving it slightly proud. It sinks as it dries, so it pays to overfill.

Bring everything to one plane with the scraper. The design is now inside the mass, not on top of it: this is not a painting, it is an inlay.

Every bubble or shortfall has to be refilled and scraped again. On serious work this cycle repeats several times.

Work from pumice through to very fine abrasives, always wet. This is the longest stage of all and it cannot be shortened.

Finish with tripoli and linseed oil on felt, working at length. The surface becomes hard, glossy and cold to the touch, like stone.

Gypsum fears water: scagliola is for interiors. A scagliola floor washed with buckets of water is ruined, and no wax will prevent it.

An economic answer to a luxury problem. Commesso di pietre dure — the inlay of marbles and semi-precious stones perfected in Florence under the Medici — gave extraordinary results at prohibitive cost: rare stones, water-powered saws, years of work. Scagliola emerged as the alternative in sixteenth- and seventeenth-century Italy, with recognised centres at Carpi and in Tuscany, and spread across Europe. The name comes from scaglia, the chip of selenite you start from.

The chemistry, briefly. Selenite is natural gypsum, calcium sulfate dihydrate. Firing drives off part of the water to give the hemihydrate; remixing returns the water and the material recrystallises as a dense mass. It is the same reaction as ordinary plaster of Paris — the difference lies in the fineness of the grind and, above all, in the animal glue replacing plain water, which fills the pores and lets the surface take a polish.

Why it is not a painting. The design does not sit on the surface, it runs through the thickness. You can grind half a millimetre away and the pattern is unchanged, exactly as in stone inlay. That is why good scagliola deceives: it lacks the surface sheen of varnish and has instead the depth of the material itself.

A necessary honesty. It is often said that scagliola imitates marble. More precisely it imitates commesso hardstone inlay, and it is a declared imitation rather than a forgery: no craftsman ever pretended to sell scagliola as stone. And it has a limit stone does not — gypsum dissolves in water. That is why you find it on altars, altar frontals, table tops and interior floors, and never outdoors.