Lime, and time.

Lime is slow. It sets by taking carbon dioxide back out of the air, which means a wall finished in March is still quietly hardening in September. For a studio that works to programmes and handover dates, that slowness was at first a problem. A year later it is the reason we use it almost everywhere.

Across Atelier Mar, Casa do Arco and two private interiors in Lisbon, we have now applied a little over nine hundred square metres of lime plaster. We kept a diary of each wall: the mix, the weather on the day, the number of coats, the time between them and, later, every crack.¹

Part 01How it cures

The first weeks decide everything. Lime that dries too fast never carbonates properly; it stays soft and dusts off on your sleeve. So the walls are misted, sometimes three times a day, and shaded from direct sun with hessian. Our plasterers call this “feeding the wall”. It feels old-fashioned, and it is, but nothing we tried in its place worked as well.

Colour changes as it cures. A wall mixed with a warm ochre pigment will look almost grey when wet, then pink, then settle into its final tone over six to eight weeks. Clients need to be told this in advance. We now hang a cured sample board on site from the first day, so nobody panics in week two.

Part 02Cracks, and how they heal

Lime cracks. Hairline cracks appeared in every one of our walls within the first month, usually near openings and at the junction with timber. The surprising part is that most of them closed again. Free lime in the plaster dissolves in moisture and re-deposits in the crack as calcite, a process restorers call autogenous healing.²

The cracks that did not heal were wider than half a millimetre, and nearly all of them sat over a change of background material. The lesson is not about the plaster at all. It is about what lies behind it, and about putting a hessian scrim across every junction before the first coat goes on.

Part 03How it ages

A lime wall at one year looks better than a lime wall at one month. The surface softens, the trowel marks become less obvious, and the faint unevenness in tone starts to read as depth rather than as a mistake. Hands polish the corners. Sun bleaches the south walls by a shade. Nothing about it looks new for very long, which is precisely the point.

We still use paint in places: kitchens that need scrubbing, bathrooms that will see steam every morning. But wherever a wall is there mainly to hold light, we now reach for lime first, and we tell clients to expect it to keep changing for years. Most of them, it turns out, are glad to hear it.³