On hot-mixed lime

Every few weeks someone asks us why we bother with hot-mixed lime when there are perfectly good bagged NHL products on every builder's merchant shelf. The short answer is that we don't always bother; we use hot-mixed lime where the original mortar was hot-mixed, and NHL where NHL was there before us. The long answer is about what hot-mixing actually does to the finished joint, and why that matters for a building that was pointed in the same material three hundred years ago.

Hot-mixed lime starts with quicklime — calcium oxide, fresh from the kiln — mixed on site with aggregate and water. The water reaction is violent and exothermic; the mix steams and spits and expands. Done right, the heat drives the binder into every grain of the aggregate, creating a mortar that's structurally closer to the sharp-edged medieval lime it's replacing than anything you can get out of a bag. Done wrong, it's a safety hazard and a waste of good stone.

Why does this matter for a building? Because a mortar joint is not a neutral filler. It breathes. It carries moisture through the wall and releases it into the external air at a rate calibrated — by original masons, over centuries of empirical trial — to match the building's wider fabric. A replacement mortar that's harder, or more waterproof, or slower to carbonate than the original mortar creates a mismatch. The original stones and bricks are softer than the new mortar; water routes around the mortar and through the stone; the stone spalls. A pound of wrong lime over a hundred square metres can cost tens of thousands in consequential damage over ten years.

So when do we bother? Roughly: any building older than about 1800, any wall with more than forty square metres of historic pointing, any time a chartered surveyor's report mentions crystallised salts or frost damage at mortar junctions, and always when Historic England or the conservation officer ask for a like-for-like replacement. For newer buildings, internal work, and smaller patches, bagged NHL 3.5 from a reliable supplier is usually fine — we use it on our own projects for exactly these cases. The point is never dogma; the point is matching the intervention to the fabric.

If you want to see hot-mixing done on site, book onto the Lime Plastering Day at the studio. Martha runs the mixing and it's the single most useful thing you can watch in ninety minutes.