Plastering party: 18th-20th May

Duel purpose post here: a little update on workshop happenings and an open invite to come and learn/do some clay mixing and plastering on the straw bale walls. So the weekend: the scene is set, we have some bale walls crying out for plaster, a lovely load of clay earth and other ingredients to mix some plaster with, now all we need is your enthusiastic hands and feet to help put the whole lot together!

This should be really good fun: play with mud, learn new skills and experience different, natural building materials, check out the invention studio so far, and enjoy the beautiful surroundings at the Golden Hill Centre. We’ll start Friday evening and carry on through ’til Sunday, but you are most welcome to join us at any point over the weekend. There are a fair amount of accommodation options – floor space, parking space for vans, fields for camping etc. We’ll get the food in, just let us know if you plan on coming via the contact form so we can plan for numbers and tell you what to bring etc.

The clay render so far…

We have been quite busy preparing for the clay rendering-athon. The render is really quite an integral part of the wall, both improving the structural stability, fire, water, insect and rodent resistance of the bale wall. While we are not so worried about the structural strength – we have a semi-structural timber frame design which stabilises the whole thing – it will be good to get this done for all the other reasons. Plus it means lots of playing about with/in the mud! We are doing the inside first, with a clay plaster, while still researching and pondering what to put on the ‘outer’of the walls (which is still covered from all but the most horizontal rain). Initial investigation into lime render (the standard approach to outside straw bale walls in the UK) ran into the  cost problem: -from our preferred, local(ish) supplier, Tŷ-Mawr, the total was going to run just over £650, which is more than we have spent on the whole wall thus far! That could almost certainly be reduced quite a bit, as the quote was was for enough material to do two 9mm haired base coats and one 9mm top coat, which seems a touch excessive. So yeah, compared to the clay plaster, which is dug up on-site and free, it seems a lot, especially since the walls aren’t really outside proper. We will have to investigate and maybe test some various clay lime mixes, or even consider just using clay and protecting it with a number of coats of lime wash, or some kind of vapour permeable silicate paint?

Clay plaster – Testing the soil

First step is to find some soil that has enough clay to hold the plaster together and that can be dug up relatively easily without creating too big a nuisance with holes in inconvenient places. So we went wandering round collecting some sub soil to do some testing. The sediment-jar test helps identify how much clay to sand/ gravel there is, whilst playing about with plasticised lumps in various ways gives an impression of how well it will stick together (cigar/worm/sausage tests; drop and splat tests etc). Here are the results of the disc (or as we say biscuit) tests: all of the samples dried to be extremely hard and difficult to crush, which indicates a good amount of clay. There is some shrinkage and crackage we can see in the reddish middle sample, which is another good indication. So it seems, there is a good amount of clay about the Golden Hill Centre. Unfortunately a lot of it is mixed in with a fair portion of shale (sample on right), other areas are hard to get to (middle sample) or not in a great location for hole digging (left sample). Happily we manage to find a spot with a good amount of clay, not too much shale, in a field which needed a drainage channel digging :) So we grabbed ourselves a barrow load and started testing different mixes of plaster on a test bale. If we used just earth it is likely it would not all hold together and possibly crack too much(though there is a fair bit of sand in the soil too), hence we add straw for tensile strength and sand to weaken or dilute the clay, that is the idea anyway…

Here we tried 1:1 earth and straw (looking good so far) 2:1 earth and straw (quite cracked); 1:1:2/3 earth, straw, sand (too feeble and crumbly) and 1:1:1/3 earth ; straw, sand (looking ok). After 6 days we are still waiting for it to dry properly to pass final judgement (!)

Harvesting time

Although in quite a difficult place to access, we were still loving the colour of the middle sample above, so we are planning to use this for the top skim coat of plaster. Everyone likes a bit of digging and wheel- barrowing afterall… but we won’t need as much of this stuff.

It looks pretty idyllic down in the mining area…

…however, that is a fair old hill to carry a barrow up (yes, it is too steep roll it), all good exercise!

When the soil makes it back to head quarters we set it out to dry for a bit as this makes it easier to get it through the riddlers (sieves for soil). We’re trying to lose of the bulk of the stone as this won’t be much help in making nice plaster.

We constructed a more industrial sized riddler with the aid of some scrap bits of rabbit hutch and fancy knot work from Dave (which we all had to stop to gaze at in awe!).

So a fair bit of riddling later, and we have a nice horde of clay earth, ready to be mixed up with chopped straw and sand.

Posted: 04|27|12 at 9:31 am. Categorically squeezed into: Workshop.
Tags: clay plaster, earth plaster, natural building materials, render, soil testing, straw bale walls
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