[GSBN] Straw Preservation in Clay _ Research Documents

Mon Sep 30 21:16:17 UTC 2013

This is fabulous, thank you so much.  I will report back after my digging.

~Anthony

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

On Mon, Sep 30, 2013 at 2:37 AM, Feile at Mud and Wood <feile at mudandwood.com
> wrote:

> **
> 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*
>
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> ********
>
> ******
>
> ******
>
> ***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 <dente.a at gmail.com>
> *To:* Global Straw Building Network <GSBN at sustainablesources.com>
> *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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