The Methodin Seven Steps

Every wall starts in the yard behind the bottega, not on the scaffold. We slake quicklime ourselves: water into the tub first, the lumps in after, then a long hoe and a lot of stirring while it spits and boils. The milk of lime runs through a sieve into the pits, settles, and stays there under a skin of water for at least 36 months before any of it goes near a wall.

Ageing is the whole point. Young putty is short and lean and cracks as it dries; three-year putty is fat, slow and smooth as cream under the trowel, and it sets into a harder, more open stone. We do not lay anything younger, which is one honest reason our lead time runs to months.

The oldest pit was slaked in February 2011, in drums in the old cellar in town, and came across the city with us in 2013. Its board still says so in Tommaso's handwriting. The putty in it is now so fine that we use it only for the faces in frescoes, a bucket at a time.

Sand comes from the river, washed and graded: coarse for the rough coat, fine for the last. We mix by hand and by volume, and judge the mix the old way, between thumb and finger.

Most walls we are called to are not suffering from age. They are suffering from the last repair: a cement render put on in the 1970s, cement pointing raked into lime joints, a gypsum skim, three coats of plastic paint. Each one seals the wall, and a sealed wall keeps its water inside, where it does the damage.

All of it comes off by hand: lump hammer and bolster for render, plugging chisels for cement joints, scalpels for paint. Never a grinder. A grinder cannot tell cement from the old lime underneath, it scores the face of the stone, and it fills a house with a dust nobody should breathe.

Taken slowly, cement that never bonded often lets go in whole sheets, and the old lime behind it is sometimes still sound. What is sound we keep, consolidate and plaster over. What is soft and salted goes back to the stone.

Then the wall is brushed down dry and left open to breathe for as long as it needs: days in a dry summer, weeks in a damp chapel. Small areas of historic plaster that only need mending are patched instead, charged by the conservator-day.

The intonaco is the skin: a few millimetres of fat, three-year putty and fine sand, laid over the damp arriccio and closed with the trowel until it shines. Lime only takes pigment, and only binds well to the coat beneath, while it is wet. So we lay only as much as can be finished that day, the giornata, and not a hand's breadth more.

At the end of the day whatever cannot be finished is cut back at a slant to the arriccio, and the edge goes where nobody will look for it: along a cornice, into the shadow of a beam, round the outline of a figure. Next morning's plaster is pressed up against that bevel.

On an old fresco you can still count the painter's days this way, if you know where to look. On our plain walls you should not be able to find one.

Where a wall is to be coloured, lime wash in natural earth pigments goes on in two coats over the cured plaster, thin as milk, and dries several shades paler than it goes on. It costs €24 to €32 per m².

Lime does not dry so much as turn back into stone, taking carbon dioxide from the air, and it does it slowly. Dry it too fast, with a heater, a through draught or a June sun, and it cracks and dusts. So for 7 days after laying, one of us comes to mist interior plaster every day, and we ask you to keep the windows as we leave them that week.

Outside, the weather decides. We lay exterior lime only from April to October, never below 5 °C and never with a frost in the forecast. Crews start at 7:30 all year, and from June to August they stop at 16:30 because of the heat.

Two weeks after the last coat we come back, tap the wall again under raking light, look for hairlines and soft spots, and put right anything that needs it. Only then do we ask for the final 10%.

It is also why we turn down fast-track jobs. A wall that skips its seven days will tell you so within a winter, and we would rather lose the job than lay that wall.