[GSBN] Straw Preservation in Clay _ Research Documents

Mon Sep 30 09:37:17 UTC 2013

Hi Anthony

I can't point you in the direction of studies about this as such - but I can highlight some texts written historically about the fibres in cob or dealing with fibres in historic cob. (You may already be aware of all of these - but I might aswell share). 

I also have personal experience with 150 year-old-plus cob houses with the straw beautifully intact. Your question is, is it still performing from a tensile strength point of view? A good question that I have not measured. However, my gut feeling is that as the walls are not displaying signs of distress, I could surmise that the straw is still carrying out its function adequately.

This cob cottage (above) appeared on the first Irish ordnance survey map of 1838, so it is at least that old - but obviously could be much, much older. As you can see, the straw is still golden.

Sorry for the quality of the photo above, but you can just about make out the straw in this cob too. This building was first identified on a map dating from 1812. However, many of the construction details and the layout of the original part of the building (of which this cob is part) suggest that the cottage may date from the early to mid-1700s. So this straw is somewhere between 200 and 300 years old.

Both of these buildings are in Ireland - Co. Kildare and Co. Meath - so definitely a rainy climate. And I have come across others around the country with the straw intact.

I find that the old cob buildings can sometimes be deceptive. Any cob that is exposed to the elements will lose its straw, probably as much as an inch or so below the visible surface, so people wrongly assume that all of the straw has rotted away. However, my experience is that if you break off a chunk of exposed cob, you will find good straw within the wall, which has been preserved because it was completely encased in earth.

http://www.mayglass-2000.ie/ - This is a link to a project carried out by the Irish Heritage Council in 2000 on a cob house in Co. Wexford. It was documented in a book, but unfortunately the book is no longer in print and I don't own a copy. There may have been some tests carried out to determine the stability of the cob walls. I have no idea - but you could maybe contact the Heritage Council in this regard - http://www.heritagecouncil.ie/architecture/initiatives/mayglass-farmstead-project/.

The English Heritage Research Transactions: Earth - The Conservation and Repair of Bowhill, Exeter: Working with Cob, July 1999

p.14 "....as recent experimentation has shown, it (straw) provided shear resistance in the newly dry cob wall, reinforcing the material under compression". The test referred to is a study on cob blocks for MJA Greer's thesis at the University of Plymouth 1995 - 6. This is a test on newly-made cob. but the university may since have carried out tests on historic cob. The University of Plymouth, England is quite active in the research of earth as a building material, as is the University of Bath, England. Cra-Terre in Grenoble, France carry out a lot of testing too.

The same English Heritage book also has footnote 28, p.102.

"An early suggestion that fibre might also distribute shrinkage as the cob dried appeared in Duncan 1947, 124 where it is noted, "My own theory ... is that the straw, on being easily compressed, takes up any shrinkage in the wall and ditributes it around the mass of the wall, so the no exterior cracks are caused." Parts of this book are an excellent commentary on the subject in Devon. A similar view is given by William-Ellis and Eastwick Field in their 1947 revision of Cough William-Ellis's Building in Cob, Pise and Stabilised Earth.

Tests in California in 1929 with unconsolidated 'poured adobe' or 'mud concrete' as it was termed showed that: ..........Mixing straw lessend the tendency to cracking ....."  There is more which I haven't written here. (Long, 1929, 9, 26).

p, 24 "It may also be noted that Alfred Howard is critical of modern straw, considering that it is left on the stalk too long, and is therefore more brittle, and less strong than in the past. Nitrates can weaken it, and it is also crushed by mechanical harvesters"

I know thatchers who complain about the quality of Irish reeds for thatch and one of the big problems is the level of nitrates in the water as a result of overuse in farming. They feel the thatch grows too fast and is therefore weaker and more prone to rot. At this stage, a lot of materials for Irish thatching are imported from countries like Turkey. I don't know if the effect in cob would be so significant as the straw is encased and therefore not exposed to the weather. But it may be that it is weaker in tension. In our own practice, we reject straw which looks battered and flattened - we look for open, round stalks - as "structural" as possible.

The idea that straw from the "olden days" was better may stem (excuse the pun) from practitioners' experience of the durability and strength of straw in historic cob walls. If the straw historically was "better" and current testing proves that our inferior, modern straw is still excellent at adding strength to contemporary cob, then could we assume that historic cob walls are by default stronger than anything we can build nowadays?

Cottage Building in Cob, Pise, Chalk and Clay by Clough Williams-Ellis, 1919

p.41 "The timber built into old cob does not seem to decay. The walls are usually so dry, especially when plastered, that the wood is well preserved. The straw in the interior of old cob walls is often as bright as when put in. The straw in cob performs in a similar function to hair in plaster. Heather has sometimes been used instead of straw with good results."

p.36 "The following is an extract from an analyst's report on a sample of typical old cob walling:"

    (There follows a list of constituents, of which straw is 1.25% - surprisingly low, I think)

".....the straw (acts) as a reinforcement".

Earth Construction - A Comprehensive Guide, Hugo Houben and Hubert Guillaud, 2008

p.82 refers to tests on fibres and their ability to improve the strength to cob. It could be that they have since started to look at samples of historic cob too. It might be worth getting in touch with them at Cra-terre.

As I've said, nothing quite touching on what you are looking for, but some more anecdotal evidence and maybe some leads you could chase.

Kind Regards

Féile Butler
MRIAI B.Arch Dip. Arch Conservation Grade III

Mud and Wood

Grange Beg, Skreen, Co. Sligo, Ireland

T:  +353 (0) 71 930 0488 

M: +353 (0) 86 806 8382

E : feile at mudandwood.com

W: www.mudandwood.com

  ----- Original Message ----- 
  From: Anthony Dente 
  To: Global Straw Building Network 
  Sent: Friday, September 27, 2013 9:44 PM
  Subject: [GSBN] Straw Preservation in Clay _ Research Documents

  Hello Everyone,

  I hope your Septembers are coming to a pleasant close.  I'm curious if any of you could point me in the direction of reports documenting the long term preservation of straw in clay plasters, cob walls, or similar.  Most importantly it's tensile characteristics.

  I have been aware for some time of the "common knowledge" that wicking qualities of clay's hydrophilic nature preserves such biodegradable materials and that this is the backbone of many clay building techniques.  I have even spoken with colleagues and friends who say they have chipped open buildings and felt these qualities themselves.  Though I am unable to locate studies/reports documenting significantly old structures (preferable in rainy climates) that have had boring tests done and confirm this.

  Thanks for any and all of your help,
  Anthony

  Anthony Dente, PE, LEED AP
  Kevin Donahue Structural Engineers
  Berkeley, CA
  814.502.6001

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