Calvexia

The byre stands on a slope west of Hexham, built of buff sandstone rubble laid in a soft lime mortar that had washed out of the north gable to a depth of 40 mm in places. Someone in the 1970s had pointed the lower third in a 1:3 cement mix, and the stone either side of those joints had begun to spall, with the face coming away in thin, biscuit-like sheets. Hard pointing does this to soft stone. Water and salts can no longer leave the wall through the joints, so they leave through the stone instead. The gable is 6.2 m wide and 4.8 m to the eaves, roughly 30 m² of wall, and we gave ourselves a fortnight at the end of August to rake it out and repoint it.

We chose hot-mixed lime: lump quicklime slaked directly in damp sand, with no putty and no bagged NHL. It is almost certainly how the original mortar was made. The small white lumps you find when you break open an old joint, the so-called lime inclusions, are a sign of it. A hot mix is sticky and workable, it bonds well to weak stone, and it sets slowly by carbonation, so it stays softer and more porous than the sandstone it sits against. That is the point. Our ratio was 1 part quicklime to 3 parts sand by volume. The sand was a sharp, well-graded 0–4 mm from a quarry outside Haltwhistle, with about 10% of a coarser 6 mm grit added to match the texture of the surviving joints.

Quicklime is calcium oxide, and it reacts with water fiercely. The middle of a mixing heap steams hard and passes 100 °C, and a fleck in the eye is a hospital visit, not an inconvenience. We wore sealed goggles, nitrile gloves under leather gauntlets and long sleeves, and kept a bucket of clean water and an eyewash bottle beside the board at all times. The method was simple. We made a ring of damp sand on a mixing board and tipped the quicklime into the crater. We covered it with more sand, then added water from a rose-headed can, a little at a time, until the heap began to hiss and crack. Once the hissing slowed, we turned it through twice with a shovel and passed it through a 6 mm riddle to catch any unslaked lumps.

Some people use hot mortar straight off the board. We bagged ours in heavy-duty sacks, tied them off to keep the air out, and left them overnight. Next morning we knocked the mortar up on the board with a mortar rake, beating rather than wetting it, and it came back to a stiff, buttery working consistency without a drop of added water. Extra water at this stage is the commonest mistake. It weakens the mortar and makes it shrink as it dries.

Raking out was done with a plugging chisel and a lump hammer. We did not use an angle grinder anywhere near the sandstone, because one slip takes the arris off a stone for good. We cut each joint back to two to two-and-a-half times its width, and never less than 25 mm. The cement pointing came out in short sections, worked from the joint’s centre outward with a 6 mm chisel. The evening before pointing we brushed and blew out the dust, then soaked the wall with a garden sprayer. We wetted it again an hour before starting, so the stone would not pull the water out of the fresh mortar.

Mortar went in with a pointing iron in layers no thicker than 10–15 mm, each pressed hard into the one below. Where a joint opened into a void, we packed it with sandstone gallets cut from offcuts rather than filling it with mortar alone. We finished each joint slightly back from the stone face, never proud and never smeared over the arrises. When the mortar was green-hard, taking a thumbprint without smearing, usually three to five hours later, we beat it with a stiff churn brush. This compacts the surface and opens up the aggregate so the joint looks like sand bound with lime, not a smooth grey line.

Day two went wrong. A dry easterly came in after lunch and the joints at the top of the gable dried in under four hours, showing hairline cracks along every stone edge. We cut out about 2 m² and did it again. From then on we hung hessian on battens 50 mm off the face and misted it three times a day for five days, longer in the wind. Six weeks on, the joints have gone from grey-buff to pale and ring hard under a key at the surface, but deeper in they still give to a fingernail. Carbonation will carry on for months. We stopped work in mid-September, before night temperatures could drop below 5 °C. In all we used nine 25 kg sacks of quicklime and just over a tonne of sand.