Earth Mortar

[JohnB]

The stone walls of my house are held together with earth mortar. It's a yellowish colour, the same as the clay in the former clay pit in what is now my wood, so I assume was acquired very locally. A quick Google didn't come up with any useful information about earth mortar, and I'd like to know more about the subject. When removing the small patch of cement render I've done so far, I came across what appears to be traces of lime. Would the mortar have been pointed with lime, or would this be traces of an original render or limewash? From the 1967 photo I have, it looks like it was unrendered stone, but it's a greyish colour, while the stone is actually red. Inside, it looks like the gypsum plaster has been put on top of a layer of old lime plaster.

The mortar will need patching up in places, both inside and outside, so how should I do this?

I'm hoping to externally insulate the house, in a breathable way of course . Inside I like the idea of using earth plaster, if I have the right stuff on site, maybe leave some bare stone if it looks ok, or maybe limewash the stone. I'm after minimal use of resources and energy efficiency, rather than historical accuracy.

 

[MdB]

Look closely at your earth mortar. Does it have any lime lumps in it? Mine does which I believe is fairly standard. I'm not sure why as it doesn't seem to be doing much as the mortar doesn't have much (any?) strength. I also remember being told that clay isn't a good mortar and it needs to be mixed with sand and other stuff to make it usable. This is from vague memory so could be wrong.

I'm patching up my wall (much worse state than yours) with lime mortar as the wall could do with being held together a bit better than it is and it is easy for me to mix up. There are also signs of lime plaster in the odd few places on our walls (under the cement) and I guess yours would have been the same.

 

[biffvernon]

Yes, you really need quite a lot of sand mixed with the clay. The basic theory ought to be that the load is taken by the hard, strong grit and sand grains touching each other and the spaces between these grains is filled with silt and clay to stop the coarser material shifting when a load is applied. This means you only want about a quarter to a third of the material to be fine stuff. Too much clay will be weaker and will crack as it dries.

Lumps of lime in the old mortar are probably there because they just got swept up and form part of the aggregate, but they won't contribute anything useful. Adding fine hydrated lime powder to the mix would strengthen it but would not be visible to the naked eye, other than by making the mix a relatively paler colour.

In an old building one can never be sure whether the builder knew what and why he was doing. Some buildings fell down, so we now have a sample biased towards the more well built.

Pointing with lime adds a great deal of weather-proofing to an earth mortared wall.

 

[JohnB]

Pointing with lime adds a great deal of weather-proofing to an earth mortared wall.

So if I'm removing patches of render around the major cracks, would it be advisable to point the exposed areas with lime mortar, as it will be some time before I get on with seriously dealing with the whole outside?

 

[MdB]

I would definitely.

 

[another_richard]

If you have read Robert Tressell's Ragged Trowsered Philanthropists you would know that earth was a key ingredient in mortar by Slobiton and Bodgit or Sloggem and Makehaste if I recall the name of the company.