Lime plaster, and how it holds light
Materials
Paint sits on a wall. Lime plaster becomes the wall, and the difference is easiest to see in the last hour of the afternoon.
We specify lime plaster on almost every project, and clients almost always ask why, usually while holding a paint chart.
The short answer is that it is not a coating. Lime is mixed with sand and water, laid in thin coats onto a wall that has been wetted first, and then left to carbonate: it takes carbon dioxide back out of the air and turns slowly to stone. The wall ends up as one material rather than a surface with a film over it, which is why a knock leaves a mark instead of a chip.
Mixing and laying
Ours is mixed in Lisbon by a plasterer we have worked with since 2013, from aged lime putty and a local sand that carries a little iron. He lays three coats: a scratch coat to key the wall, a body coat to true it, and a fine finish coat burnished with a steel trowel while it is still green. The last pass is the one that decides everything.
Colour is not painted on afterwards. Pigment goes into the finish coat, so the wall is coloured the whole way through and shifts very slightly across its own surface, darker where the trowel pressed and paler where it lifted. In a photograph that reads as a faint cloud. In a room it reads as depth.
What it does with light
A painted wall gives back the same light all day, because the surface is flat and the film is even. A burnished lime wall does not. Morning light rakes across the trowel marks and finds every one of them. By the middle of the day the wall goes quiet and almost matte. In the last hour it takes the low sun and holds on to it, and the room warms without a lamp being switched on.
That is the argument for putting the plaster where the light actually falls: the wall opposite the window, the return beside a door, the underside of a stair. In north facing rooms we lighten the mix by a shade, because there is no low sun coming to give it back.
It marks. A chair back leaves a grey arc, a wet glass leaves a ring that fades over a week, and a moved painting leaves its own outline for a month. All of it sands out with a fine block and burnishes back, and none of it needs a painter. We leave a labelled bucket of the finish mix with the house at handover, and one page on how to use it. What it will not tolerate is standing water, so it stops at the splashback and at the shower, where we run stone or tile instead. Everywhere else it is the cheapest surface on the drawings to repair, and the only one that improves while nobody is touching it.