Cob lime-stabilized cob

[Michael Saunby]

> Hi Michael,
> 
> thanks for your reply. Regarding the plaster you mention, which would be the percentages of the ingredients? I already was thinking in the possibility of using the same composition as for SCEBs, but of course with more water. As for pigments, we have a lot of different rocks here in Chihuahua, Mexico, which could be ground and added for this purpose. Now, talking just to myself: the animal hair obviously is added to avoid cracking, and the lime should avoid their biological breakdown, shouldn't it?
> 
> Uwe

The tufts of cow hair (about 5/8 inch long) I find in plaster when I'm doing 
repairs here, is in perfect condition.  This house is over 300 years old, though
the plaster is probably only 100.  I was suprised to find the same stuff had been
used to plaster the back of the fireplace, but of course lime makes a very good
fireproof cement.  The old bread ovens here are made from a very white (so I 
guess high lime content) plaster of some sort.

In The Devon Historic Buildings Trust leaflet "The Cob buildings of Devon 1,
History, Building Methods and Conservation" it says that "Internal plasters made
up of sieved earth and hay or animal hair remained in use, even in very grand
houses, right up to the 1850s."  As for external coatings the leaflet says, 
"External cob walls, especially those of barns and the rear (usually north
and east facing) walls of farmhouses and cottages, were often left unrendered.
Alternatively they could be coated with limewash which would, with repeated
applications over the years, form a protective layer up to 3-5 mm thick. Where
a render coat was applied it would be made up of lime and river sand with animal
hair added to strengthen the material and act as a binder.  Rough-cast renders
were applied to many rural buildings during the late 19th century but are 
thought not to be a traditional finish for cob walls"

Well it might not be traditional but the external walls of this house are in
much better condition to the walls of the unrendered cob shed which stands next
to it.  To improve matters, and reduce the number of holes in the wall with
creatures living in them (we've put up bird boxes to compensate for the loss),
I've applied a thin coat of plaster with the following mix: 1 part lime putty, 
1 part coarse sand, 3 parts soil.  The soil was chosen for its higher sand and 
grit content.  I've applied a thin coat, except where there were holes to fill,
by pressing it in with a trowel. Before doing this the surface of the wall was 
very spongey and easily damaged.  I'll finish it off with a coloured limewash.


The Devon Earth Building Association give this mix for Roughcast Render:
2 x coarse sand
1 x grit (up to 4mm dia)
1 x lime putty
Then 1 x bucket mix to 1/2 bucket of teased hair.
 
For a smooth render retain the grit but replace half the coarse sand with
fine sand.

Rough cast should be applied by hand casting (using a harling or dashing trowel).
There is a knack to this and I haven't got it, so I'll stick to the trowel for now.


Another bit of info on the use of lime, from "The Cob Buildings of Devon 2, 
Repair and Maintenance", "According to contemporary accounts, from the middle
of the 19th century it became common practice in some parts of Devon to add lime
to cob mixes, presumably in order to expedite the building process.  However,
recent experience would indicate that for new building, or for the reconstruction
of large sections of cob wall it should rarely, if ever, be necessary to add
stabilisers to the soil.  The use of lime may, however, be justified in certain 
cases as a means of controlling shrinkage when carrying out minor repair works.
The addition of 5 to 19% non-hydraulic lime to the mix can reduce shrinkage by up
to 50%; it will help the material to achieve a faster set and, ultimately, 
greater strength.


Well I hope all that is of some interest.

Regards,
 
Michael Saunby