Clay plastering our straw bale house – getting silky smooth

 In Blog, Student Post

This week has been full steam ahead plastering.

We agreed last week that we would do longer days this week to try to get it all done, ready for the electricians’ final fix the following week, so with earlier starts and later finishes we went for it…

Each day, we took different jobs in pairs and then jumped in to help other pairs if we finished before them. There were various tasks to make it all happen.

Some walls and window/door reveals still needed some more dubbing out to bring the surfaces level to a flat surface, and plumb – our mantra being “flat not smooth”. The flatter we got this layer, the better finish we could get with the final finish layer of plaster, needing it to still be a little rough to give a key for the next coat.

Dubbing out plaster recipe

The dubbing out mix was made up of a ratio of:

  • 1 bucket of wet clay
  • 2 bucket of medium length straw
  • 3 bucket of sharp sand 

Plus some water, added until the mix had the right consistency, evident by its sound and movement changing in the mixer, the mixer becoming lighter to tip out, and testing a small amount.

Slightly longer lengths of straw were used in this mix than the other mixes, but cut shorter than the full lengths we used for the previous matrix layer. That had needed the long lengths to bond material joints together, such as between the stick frame and the bales. We applied the dubbing out layer by hand, which I really enjoyed, I found it easier to feel if I’d brought surfaces up to a flat level, as well as a discerning eye, a long level and using a template for the curves (the ‘arc of destiny’) of the window and door openings.

Once the walls had been brought flat (not smooth) the top coat of finer plaster could go on. Preparation first… the area would be cleaned of loose dirt and we would check and make good all the masking tape around windows, window sills and skirting boards was all in place and correctly so. Skirting boards had been fitted by Jodie and Joe through the week.

The wall was then sprayed with a fine mist of water, and using a hawk and trowel, getting on at least a meter square of plaster, to 10mm thick, before smoothing over. We didn’t want to over work the plaster as this brings the moisture to the surface along with the clay particles, which leaves the back of the mix without its balance of bonding material, making it harder for it to stick to the wall. Smoothing over was then done with a larger rectangular trowel. This also showed up small areas that dipped and needed a bit more plaster, so these areas could be filled in with a small trowel at this stage.

After some time for the moisture to begin to soak through and bind with the wall, the surface could be really smoothed with a round edged stainless steel trowel to create a lovely smooth finish.

The masking tape needed to be removed at the end of the day, whilst the plaster was still wet, with a straight edge of a trowel against the wall – we had plastered just over the tape, so when it was removed it gave a really crisp finish to the edge.

Lime stabilised clay plaster recipe

Out in the cuisine d’ete, each wall was plastered with a lime plaster with a hawk and trowel as above, which will later be covered with a coloured lime wash. We had a different mix for this, made as above but with the following ratios:

  • 1 powdered clay
  • 4 sharp sand
  • 1 short cut straw
  • 0.2 lime

The latency of the lime was rubbed back after a day or so – this is a waxy layer that comes to the surface. Removing this will allow the lime wash to take properly.

Clay plaster ceiling recipe

The pottery workshop ceiling needed finishing, which was a large team job, getting up on ladders and the scaffolding tower, and into the tricky space above the storage room and toilet to finish right up to the gable wall. This took some serious flexibility for those brave souls on this part of the job – lying in a tiny space, with tricky corners, plastering upside down. The ceiling mix was different again, with the following ratios:

  • 4 plastering sand
  • 1 powdered clay
  • 1 finely chopped straw

Plaster board had first been fixed to the ceiling, the joins being covered with a webbing, then the whole surface painted with slip to help the plaster take, as clay sticks to clay.

Mixing was done by a team everyday to keep the plastering teams going and in their flow. Some filling of holes was required where skirting boards had been fixed to walls and the boards around the inside of the windows.

Final windows!

The last of the handmade oak windows were fitted – the large bi-fold glass doors at the end of the building in the jewellery workshop. These really gave this workshop a sense of being a room – closing off what had been quite an open wall. The open space helped the plaster to dry though, and we set heaters and fans going over the weekend to continue the drying process, ready for next week.

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