We had our first plastering party, hoorah! A really good fun weekend, with lovely people and lovely mud. Here’s a quick low down on what we did and learnt.

Since our testing of different clay plaster mixes and digging mission (documented in the previous blog post) there was quite a bit of

preparation to be done: Trimming the bales with the combined efforts of chainsaw and hedge cutter. The chainsaw was good for monstering off larger chunks and ‘shaping’, whilst the hedge cutter being a touch more controllable, gave a straighter all-over finish.

We stapled jute scrim over some of the timber uprights, to provide a key for the plaster. Other bits of timber, like the horizontal pieces shown below, had ‘wood wool’ (a kind of stringy porous wood fibreboard) attached. The wood wool gives an excellent breathable surface for the earth plaster to stick to, added thermal and acoustic insulation, better fire resistance, and isolation of the plaster from the timber (a buffer zone so that inevitable movements in the wood do not cause immediate plaster crack-offage). On the down side, the wood wool’s bonding agent – the thing keeping the interwoven wood fibres together – is cement… So a bit of a compromise there – hopefully worth it if it averts claytastrophe later down the line.

The clay plastering party

And then the real fun could begin! In true weekend style we started with pizza, wine (for some) and some late night stomping to the beat in the mud. The first step is to get the clay and water to bond well together, and allow it to plasticize. It’s  bit like kneading bread really, the texture changes quiet a lot the better mixed in it gets, and if you leave it to settle for a few hours (or even overnight) before mixing in the rest of the ingredients, all the better. We used our last four remaining straw bales, and an old tarp to create a stomping pit, put on some music, and began.

Our careful studies show it works best without shoes on…

Stomp stomp. Having a tarpaulin allows you to regularly flip and fold the clay-straw over upon itself which makes further stomping mix it up much better.

So yeah, we might have gotten to bed a bit late on Friday night/Saturday morning, but we could start again after a sleep with pre-stomped clay to mix into plaster ready to slap onto the walls.

We also mixed up some buckets of clay slip, which after some experimenting through the weekend, was best done by adding extra water to the pre-stomped clay and working it in with the hands. Using the paint mixing attachment on the drill was fun, but got the drill good and muddy, and was really only good at re-stirring well mixed potions. 

In the morning we got down to working the slip coat into the bales – this is kinda like a glue between layers: it provides a good sticky bond for the first coat and also helps to wet the straw so that it doesn’t immediately suck the moisture out of the plaster. There were a few different ways to apply this sloppy stuff: grinding/rubbing it in with gloved hands, or using a stiff bristled brush, or my favoured method the soft bristled brush. Whatever was chosen, all ways result in lots of mess and runny mud splatters!

With slip on the straw it is all preped and prime to receive the actual plaster. As with slip, application preferences varied, but the principals remained the same for the first coat, grind it into the bales as much as possible, aiming for a plaster thickness of about 1cm. This was a good hands on activity.

More volunteers arrived arrived Saturday, and more again Sunday, so we had enough people to be applying and mixing plaster in parallel (with a little more riddling soil, cutting up straw and carting materials about here and there too).

More clay plaster mixing details

After the test bale (finally!) dried out, results were pretty conclusive, 1:1 clay /chopped straw (av. length about 1 inch) mix won, not too cracked (as was the lesser straw content mix) and not too crumbly like those we added a fair amount of sand too. We added a sprinkling of sand to each load so we could maintain Sam’s cooking style of adding a bit of extra something! The extra water added at this stage was of a similarly ‘by eye’ (or foot) style – the earth we had stored up was at different levels of moisture as we moved down the bag. More water made mixing easier, but too sloppy could result in long drying times and a weakened overly cracked finish. Way too sloppy and it would be hard to work onto the bales. Generally we deemed stomping perfection reached when the batch rolled up to a nice sausage of even consistency.

A most important lesson of the weekend? Good stomping tunes are essential for happy stompers, look at the love going into this clay…

Plastering really shows up any unevenness in the bale walls, even though we thought we’d filled holes, trimmed and bashed the bales into relative straightness before hand, a fair amount of packing out with high straw content mixes were needed in places, though we like the organic curvy shapes we’ve created too, and well, a few lumps here and there add character!

We had lots of enthusiastic help over the weekend (THANK YOU!), it was great to share the experience, cake, talk, walks, bit of sunshine and generally get good and muddy together!

We reckon we’re most of the way there with the first coat now- ace progress. The freakishly hot burst of sunny weather has helped drying considerably, and the evidently thinner patches of plaster are already well on their way to full dryness. We are still umming and arring about whether to do another coat like the first before going to the finish coat or not. We will probably end up doing another full coat in some of the more craterous patches to smooth things out, and leave the rest. Either way, we now need to get experimenting with different finish coat recipes to find the best. It will probably be some mix of the prime orangy clay we have been saving, fine chopped hair (any suggestions for a free/cheap source) and fine plastering sand.

Watch this space for the next plastering weekend!

Posted: 05|30|12 at 2:45 pm. Categorically squeezed into: Workshop.
Tags: clay plaster, straw bale walls, Strawbale building
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