Heritage hot lime (quicklime) repointing.

Most people think their houses were built from just stone and lime mortar. However, in many locations, lime wasn't actually used to build with until the late 1800s.

Above: a personal favourite of mine with blended sands and limestone chippings. 

Circa 1600s earth built barn cottage in Bledington originally pointed with quicklime mortar. Like many, this became an OPC victim. Our job was to restore the windows and reinstate a like-for-like mortar. 


After: at the moment of completion. It needs to spend a few months drying out and carbonating. 


Close up: We used Lincolnshire chalk lime and a blend of 5 sands to achieve this look. Choice of sand was very important as this is a north facing elevation and we did the work in mid-winter.


Faringdon earth built house reinstated with hot lime mortar: before.

After: flexible and porous whilst protecting vulnerable stone. 

Therefore, if your house was constructed before this time then it's highly likely it will have been built from what's called common mortar otherwise known as earth-lime mortar. 

Littlemore: Although non-listed with no legal requirement for lime mortar this came to us through my client's surveyor who was concerned about the amount of cement and its potential detrimental impact. He strongly recommended hot lime mortar.


The lime mortar was sand slaked in situ. This means the sand and lime were mixed on-site which meant it reached boiling temperature on the street. It was applied as soon as the quicklime had stopped reacting with the water.

Furthermore, earth-lime mortar is simply sub-soil and sand mixed with quicklime (around 10% - 20%). As this is easily eroded by the elements it has to be protected in some way. This is where lime mortar comes in. Traditionally it was simply mixed on site from quicklime and sand (sand slaking) and then used to point between the stones to keep everything watertight, breathable and capable of dealing with seasonal ground movement and thermal movement without cracking. 


During the mixing process, the resultant exothermic reaction is mortar which gets very hot. 

To see this in action then please got to:

https://www.youtube.com/watch?v=cbzKOuI2Cxo

The process of adding water is called slaking and is an essential part of the process. Here, the expansion of the quicklime results in a sticky, porous and flexible mortar which is perfect for repointing old stone and brick. 


The quoins deteriorated because they had been repointed with cement. Unfortunately this will eventually rot all cut stone. Costly to replace so we repaired with our own breathable stone repair mortar. This is our own blend with every batch mixed differently to recreate the natural colour variations inherent in sandstone. 


Below: new mortar and the remains of aged lime wash. 

So, when you look at an old stone house, with all its aged lime, what you're actually seeing is a property built with earth and stone but which was pointed many years ago with a hot lime mortar and then lime washed. 

Before: Buckland Grade 11 listed thatch cottage. 


After: Buxton quicklime and a blend of sharp and finely ground sand with added pozzolan, animal hair and unburnt lime. 

 

https://www.youtube.com/watch?v=5oWSysf7XVI

We're currently seeing a revival concerning hot lime mortar because of extensive research into how this traditional mortar works. Basically, when water is added to quicklime it will increase in volume. What this means is a very lime rich mortar which is extremely adhesive and also highly vapour permeable (breathable). 

Before: Bampton Grade 11 listed earth built upper front elevation repair.


After: We only did the upper section as the base of this property had been repointed in cement. Unfortunately it was such a rich mix of sand and OPC that its removal would have damaged the property. The only course of action was to leave it to eventually be rejected by the building.


Close-up: this is in the process of carbonating but will lighten up over time as it reabsorbs the carbon content which was burnt off when the limestone was in the kiln. 


Before: Grade 11 earth built listed house in Bledington. Gable end repoint using stone dust and Buxton quicklime. 


During:


This was completed over the toughest months of winter. We broke for a week during the lowest temperatures but apart from that were not hampered by the weather. Flush coverage means this elevation is now well protected for many hundreds of years to come. 


If you would like to know more about how important historic mortars are to your house then follow the SPAB link below:

https://www.spab.org.uk/sites/default/files/documents/MainSociety/Advice/Mortars-SPAB-conference-report.pdf


In sum, if you have an old house then hot lime is the best treatment as it is a true like-for-like-repair which allows for maximum pore size which will actually pull moisture out from property allowing it to expel damp which comes up from the ground. It will also pull away rainwater from the surface of the stone which means harmful salts - which damage stone and brick - are given far shorter dwell times. 

Because of its extremely flexible nature it will cope very easily with structural movements. It will also expand with the stone as the sun's rays heat it up in the summer which means it won't crack. If it ever does then it will self-heal and bond back together naturally.

If you would like go into the matter in greater depth then I would recommend reading 'Hot Mixed Lime and Traditional Mortars. A practical guide to their use in conservation and repair. 

This is written by a colleague of mine who is very prolific in heritage conservation work and also involved in research at York University.



 

For hot lime mortar repointing in Oxfordshire, Milton-under-Wychwood, Bruen, Windrush, Burford, Highworth, Idstone, Bibury, Ashbury, Ascott under Wychwood, Buckland, Faringdon, Witney, Bampton,