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Author Topic: Clay mortar  (Read 1355 times)

chrismahon

  • Joined Dec 2011
  • Gascony, France
Clay mortar
« on: September 07, 2023, 02:25:11 pm »
As requested [member=152775]arobwk[/member] .Most people will be familiar with 'lime mortar', a lime and sharp (washed) sand mix used for pointing or rendering stone builds with no damp course. Unlike cement mortar it 'breathes', so lets out moisture which has risen up through the wall which otherwise would be trapped in the building leading to damp issues. Here to vary the colour naturally they mix red and white sand- there is no yellow.


But there is an earlier type of lime based mortar used before sharp sands became available post 1790. Clay mortar was made using subsoil clay together with river sand which is basically very small pebbles. The subsoil clay here is yellow, so that's the colour of the mortar. I developed it by experimentation, not realising that it was once in common use. There are variations used in this 1769 house (think some of it is older) for binding the stone structure and rendering the internal walls, but the external walls were never pointed as I am doing now, just rendered with an off-white river sand/ lime mix.


Lime mortar is used a lot here to re-seal stonework. Fairly quick to apply, but without structural improvement. The use of clay mortar is confined now to restoration on historic buildings. It's rather time consuming to make and use but has advantages. It breathes well and is a good insulator. It also blocks electromagnetic radiation, so no mobile signal. The downside is the water resistance. Over time with frequent exposure the lime leeches out and it crumbles, so use is confined to the exterior of the South and East walls or the interior. Our rain 99% of the time comes from the Northwest. I have seen a South wall rendered with it- 250 years old and still OK.


After trying variations of mix of clay to sand from 1:2 to 1:5 which resulted in cracking or being too white I settled on about 30%. To retain colour consistency and workability the proportions have to be EXACT, so only dry weights, not volumes. I'm using 0.25Kg of live lime in 70-75cL of water, 0.7Kg of 100% clay, 0.2Kg white fine sand and 2.1Kg of river sand. The fine white has had to be added because of variations in the second batch of river sand (insufficient fines). Just 2.3Kg of river sand would be normal. Note that just a tiny amount of water in excess wrecks the batch, so start with 70cl and add during mixing 1cL at a time. This gives me two days worth of pointing at 2 hours work a day.


The mortar is 'deep pointed' in 3cm stages if necessary. Each layer is driven into the gaps when partially dried using a hammer and drift. The top layer can be brush finished but I prefer to create a smooth finish with a jewellers hammer.


The clay comes from about 2 feet down here and has to be sorted and sometimes soaked and dried to remove all the stones which would otherwise corrupt the mixing percentages. The previous owner left bags of clay, a sample of sand with a note in French and a full 50Kg tub of live lime- but no instructions. He had started to experiment as well, but passed away before completing them. Takes me 7 days for 1 m2 so I expect to finish sometime in 2025, subject to weather of course.

Forestlens

  • Joined Jul 2020
  • North Devon
Re: Clay mortar
« Reply #1 on: September 11, 2023, 08:47:21 pm »
Fascinating, thanks for sharing, but it sounds like a labour of love!

chrismahon

  • Joined Dec 2011
  • Gascony, France
Re: Clay mortar
« Reply #2 on: September 12, 2023, 11:19:34 am »
Not sure there is much love involved in it [member=203979]Forestlens[/member] as it is being done out of necessity. The South wall is in an unstable condition due to severe water damage- overflowing of a blocked gutter. The clay mortar binder between the stones has very little lime in it and so has no resistance to weathering. It was exposed due to serious mistakes by previous owners. The original lime/sand rendering was covered with a new cement render. Of course this stops the building from breathing and it gets damp. It was either removed, or more likely fell off due to poor keying and the sheer weight, taking what was left of the original render with it. Nothing was then done, which wouldn't really be much of a problem if it wasn't for letting the guttering get blocked. The flow down the wall quickly washed away the binder and stones have fallen out. Basically the wall is getting necessary structural repair, rather than a cosmetic surface skim to hide the damage. I'm not underestimating the instability- tap the new mortar into the joints and material falls on your head from above!

Forestlens

  • Joined Jul 2020
  • North Devon
Re: Clay mortar
« Reply #3 on: September 12, 2023, 01:01:34 pm »
We’ve been there! Just spent a small fortune on re rendering and pointing with lime render/mortar on our barns! But they now look very smart!

 

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