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How durable is straw bale with clay plaster?

The clay plastering of the FE workshop is completed and we are between coats on the exterior lime wash. The only reasonable thing to do is use our spare plaster experimenting bale to carry out some destruction tests!

So we have had most variants of the predictable jibes and japes about straw as a building material: it will burn, it will rot, the wolf will huff and puff, blowing the workshop across the field… Even the local farmer selling us the bales warned us not to put anything electrical in the ‘workshop’ when it was done, for the sake of fire safety. That’s all complete nonsense, and luckily, there is now a reasonable amount of reliable information regarding the performance of straw bale construction [1].  Straw bales, finished with a sufficient render are extremely resistant to fire, and if constructed properly (with vertical pins, pre-compressed etc.), can be considered load bearing, and support the weight of at least one upper story. Knowing that in advance, this little destruction test is a bit of fun, and a chance to examine our particular clay plaster mix.

Cutting a cross section in the clay plaster

Below is our ‘test bale’, we used it to try our various mixes of clay plaster. The one this side and on top is the mix that most closely resembled the mixtures we ended up using, so that’s the section we are most interested in. It consists of a 1cm first coat, 1cm second coat and a thin 2-3mm final skim coat. The right hand side is polished (hence smoother), the left is as trowelled…

We used a diamond coated cutting disc to remove a quadrant and give us a nice edge, so that we could get a nice view of the compositional make up of the different layers. We were also able to see how well bonded together they were, and how tightly they held to the straw itself. Here’s a little vid of the cutting…

The dissection was quite interesting revealing the different colour clay used for the first and second coat, and how the straw content changed (we chopped it much finer for the second coat, and put the same volume in, meaning it was more thoroughly distributed). The final coat, as you would expect, is very different, with no straw, but small hairs for tensile strength. That it was all well stuck together and solid, confirmed what we already sorta knew, but was encouraging none the less.

 Fire Resistance Testing

Ok so the straw bale’s trial by fire took loads longer than expected. We already know the clay plastered over straw was quite good – it has passed “ASTM E 119 standard” tests with flying colours (The ASTM E 119 is basically a recognised way  of doing a standardised test on building elements, by having a furnace, that reaches about 2000 degrees C,  flaming one side of a test wall, while measuring the temperature on the other for 2 hours. Not only must the temperature rise be less than 120degrees C (250F), but after the full 2 hours the wall must then maintain structural integrity, while being immediately blasted with water from a fire hose (that can knock a person off their feet!) [2]. Anyway, building a wall just to burn it down, noble as it is, would be a bit much for us, so we are left with our solitary test bale.


We used a fairly heavy duty propane torch, one we use for plumbing, heating up stubborn bolts and other workshop tasks. It can heat steel up to a bright yellow (1100 degrees C/ 2010F) fairly quickly, so we thought it would be up to the task…

The blackened patch below where I am heating in the photo is where we started –  this became tedious as very little was happening. As you can see the central focus of the flame brings the plaster to a cherry red, but after a good while of blasting there was no signs of cracking. In the ASTM E 119 tests carried out by EBN in 2006 the plaster exposed to the furnace developed cracks after 20 minutes. We had suspected that this ‘crack time’ would have been reduced a lot using this localised blasting method (In the ASTM tests the furnace burners run parallel to the wall – they don’t actually blast it like we were). The addition of various fibres in our plaster probably help). So after this became particularly tedious we shifted to attacking the edge of the plaster, as in the photo above.

Again this wasn’t as dramatic as expected. The loose straw quickly burnt away, leaving the tightly packed straw smouldering.

This tallies with what ‘official’ tests seem to suggest: that the straw, when kept tightly packed in a bale, behaves much like large section timbers – the outside surface charring and forming a somewhat protective layer that makes it difficult to get it outright blazing.

After some time of this, we became hungry and I feared the gas would run out so we stopped for lunch. That’s when memories of my dad using some crazy flaming beast of a thing to start blazing fires that burnt large swathes of dried bracken from farm fields… Some searching about later…

The funny machine is warming up. It uses a pressure vessel at the back to vaporise paraffin at the nozzle. It’s a little worse for wear – the pressure gauge doesn’t seem to work any more, and you have to pump the tank a lot to get seemingly very little increase in pressure. With the constant nagging feeling that I might unwittingly over pressurise it, and be blasted by an exploding tank of paraffin, I think we ended up running it without enough pressure to get good vaporisation for the most part. Still, we got it going after a fashion.

Here’s the vid of the fire resistance testing:

All in all, I suppose we have encouraging results, tinged with a slight disappointment that the bale didn’t do anything very dramatic. Instantaneous implosion and chain lightning were notably absent…


  1. [1]see for example, this report by The Architecture & Civil Engineering Dep. at The Universit of Bath,  “Compression load testing straw bale walls” (2004)
  2. [2]One of the most recent tests carried out by  The Ministry of Industry and Trade of the Czech Republic in June 2011 to this standard can be found here: (but is is Czech). This english report documents this kind of test but deals with straw bale rendered with cement/stucco, rather than clay Also see for a really good video and explanation of these testing procedures (the second half of the video is a test on earthen plaster), and see this page for a good set of resources relating to all kinds of straw building tests:

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