Lime Mortar

It’s no accident that property is sometimes called ‘bricks and mortar’. Mortar is not just a filler, but an essential ingredient of any traditional building. One of the easiest ways to harm an old building is through the inappropriate repointing of mortar joints. A carefully formed traditional mortar joint is – in William Morris’s words – both useful and beautiful. It provides an attractive bedding for the masonry blocks but also a route through which rainwater and trapped moisture can evaporate.

Mortar has two main ingredients: aggregate and binder. The aggregate in an old mortar is generally a mix of sand and fine gravel, well-graded so that particles are of a range of sizes. The aggregate is held together by a binder. In mortars used in modern building, the binder is normally Ordinary Portland Cement. Ordinary Portland Cement has been in common use for over a century. It’s the construction equivalent of fast food: easily acquired, simple to mix, useable at almost any time, but probably not good for your old building’s health.

Modern cement-based mortars tend to be very hard. Traditional buildings – and traditional mortars – are softer and more flexible. The softest traditional mortars are earth-based – and these are more common than was once appreciated – but equally common are lime binders.

Lime was produced by burning calcium carbonate in a lime kiln. The calcium carbonate came from materials such as limestone and chalk, or even from oyster shells in coastal areas. In the kiln, the calcium carbonate was placed between layers of a fuel such as timber, coke or coal. Lime burning required careful temperature control, but ultimately produced a reactive calcium oxide or quick lime (‘quick’ meaning ‘live’, rather than ‘speedy’). This was sometimes slaked to produce a lime putty (calcium hydroxide), but it’s increasingly understood that much traditional building mortar was ‘hot mixed’, where the quick lime and aggregate were directly combined, generating considerable heat when water was added.

In rubble wall construction, lime mortar joints are generally wide and a suitable mortar will contain quite large aggregate particles.

For ashlar stonework or gauged brickwork joints will be very slender and a finely-sieved aggregate will be needed. Choosing the right mortar, in terms of constituents and preparation, will be crucial to the end result. Even in a benevolent climate such as Jersey’s, lime mortars are vulnerable to inclement weather and need to be protected against rain or frost w and ideally work should be carried out when the weather is dry and mild.

Jersey does not have limestone, but lime was still burnt for mortar. Some limestone was imported but lime for Jersey’s kilns often came from shells, obtained on the island or imported as ballast. Lime kilns existed in many places in Jersey and can still be found at St Cyr in St John, and at Augrès Manor where the National Trust for Jersey carried out conservation work in 1977.