[Draft written for review, edit before publishing]
Walk along almost any nineteenth-century brick street and you will see two kinds of repointing. In one, the mortar sits slightly back from the face of the brick, matches it in colour, and has worn in the way the brick has worn. In the other, hard grey ribbons stand slightly proud of the wall, and the bricks on either side of them have lost their faces — spalled, crumbled, or gone powdery at the arrises. The second wall was repaired more recently than the first, usually with cement, and the repair is what is destroying it.
What mortar is actually for
The common assumption is that mortar is glue: it holds the bricks together, so stronger is better. In a solid masonry wall this is close to backwards. The wall mostly stands up through compression and geometry — mass, bearing, and friction — and the mortar’s jobs are more subtle. It beds the units so that irregular bricks or stones bear evenly instead of point-loading each other. It accommodates movement, because walls move with temperature, moisture and settlement, and something has to absorb that. And it manages water: it lets moisture that has got into the wall come back out through the joint.
That last one is the heart of it. A traditional solid wall is not waterproof and was never meant to be. It gets damp and then dries, and the mortar is the sacrificial, permeable, slightly soft part of the system through which it dries. Mortar is meant to be the component that fails, because mortar is cheap to replace and bricks are not.
How Portland cement took over
Lime mortar — burnt limestone, slaked with water, mixed with sand — is the mortar of essentially every masonry building before the late nineteenth century. Portland cement was patented by Joseph Aspdin in 1824, but it took decades to become the default; through the second half of the century mortars were commonly gauged, a mostly-lime mix with some cement added for early strength. By the early twentieth century cement-rich mortar had largely won, for reasons that are perfectly good when you are building something new: it sets fast, it sets in cold and wet weather, it is strong in days rather than months, and it suits a steel or concrete frame where the masonry is a veneer rather than the structure.
The problem is that those mortars were then used to repair buildings designed around the opposite set of properties, often by people doing their best with the only material on the shelf.
Three ways hard mortar damages soft masonry
It is harder than the brick. Hand-made brick, soft sandstone and limestone are all weaker than a cement mortar. When the wall expands and contracts, the stress goes into the weakest material — so the brick cracks and spalls at its edges instead of the joint taking the movement. You can watch this happen along a parapet: the joints are pristine and the bricks are losing their corners.
It is much less permeable. Water still gets into the wall, through the top, through a failed flashing, by driving rain. With a permeable lime joint it leaves through the joint. With a dense cement joint the only available exit is the face of the brick or stone itself, so evaporation — and with it the salts carried in the water — happens inside the unit. Salt crystallising just below the surface is what blows the face off.
It is rigid. Lime has a small but real ability to accommodate movement and even to re-heal fine cracks, as free lime dissolves and redeposits. Cement does not flex; it cracks, and the crack is a clean path for water straight into the wall. A hairline crack in a hard joint can admit more water than a soft, slightly open lime joint ever did.
Repointing mortar should never be stronger than the units it sits between. The joint is supposed to be the part that gives way.
What lime does differently
Non-hydraulic lime sets by carbonation: the calcium hydroxide in the mortar reabsorbs carbon dioxide from the air and slowly turns back into calcium carbonate, which is chemically where it started as limestone. That is why it is slow, why it needs air, why it cannot be used in thick lifts, and why it has to be protected from frost and from drying too fast while it cures. It is also why it stays comparatively soft, permeable and forgiving for its whole life.
Hydraulic limes sit between the two: limestone with clay impurities, burnt and slaked, sets partly through reaction with water and partly by carbonation. The European classification gives three broad grades — NHL 2, NHL 3.5 and NHL 5, roughly in order of strength — which makes it possible to choose deliberately rather than by habit. A sheltered interior wall of soft brick wants the weak end; an exposed chimney in a Boston winter wants more.
Repointing, briefly and properly
Most of the damage I have seen was done with good intentions and an angle grinder. The sequence that does not do damage runs roughly like this.
- Only repoint what needs it. Sound mortar that is merely old is not a defect. Repointing a whole elevation because one parapet is failing is a loss of historic fabric for no gain.
- Find out what is there. A mortar analysis — the binder identified, the aggregate washed out, graded and looked at — gives you the ratio, the sand colour and the sand size. That is what makes a new mix match, not a colour chart.
- Rake out by hand, to roughly two to two and a half times the joint width so the new mortar has something to key into. Hand tools and a hacking chisel, not a grinder: a disc widens the joint and takes the arrises off the brick, and that damage is permanent.
- Match the profile. Match the original tooling, and finish the joint back to or slightly behind the face of the unit, never smeared over it. A joint struck proud of the brick sheds water onto the face and hides the real joint width.
- Cure it slowly. Damp the joint before and after, keep it out of sun and wind, protect it from frost. Lime that dries before it carbonates is just coloured sand.
How to tell what you are looking at
A few field tests cost nothing. Scratch the joint with a key: lime will mark and shed a little sand, a cement mortar will resist. Look at the aggregate — lime mortars usually carry visible, locally sourced sand and often lumps of unslaked lime. Look at the relationship between joint and unit: if the joint stands proud and the brick is eroded, somebody has already made this mistake for you. And look at where the wall is wet, because water is the whole argument.
I started paying attention to this while drawing brick walls for Thresholds in Brick, and again on the long elevations of Reuse of the Ropewalk, which has had every kind of repair applied to it in sequence. The same logic holds for objects as for walls: in Conserving the Cast Collection the repairs that caused the most harm were made with a material stronger than the thing it was repairing.
The rule worth remembering
Match the original as closely as you can, and if you cannot, err softer, weaker and more permeable than the masonry around it. A joint that is a little too soft will need redoing in thirty years. A joint that is too hard will quietly destroy the brick, and brick cannot be repointed.
