Japanese plastering workshop

[GaryR]

I'm attending a week long workshop on Japanese plastering near Berea, Kentucky, USA. Berea has a long history as an artist and craftsperson community and also a vibrant community of back-to-the-land and natural building aficionados. The workshop is hosted by South Slope Farm, (online search for "The Year of Mud") which also hosts classes on Western timber framing and Japanese timber framing. The class is taught by Kyle Holzhueter, who is an American who has lived in Japan for the last 20 years and is a plastering master there.

Here is the farm from a hill up the slope. The topography here is very "Lord of the Rings." Limestone knobs and dense forest. Class is taught in the pavilion, the large building on the left, and the kura, the smaller storehouse to the left of the pavilion. Both building were partially plastered and students in the workshop are doing further plastering on the buildings as part of the training.




The pavilion. Partly western timber framed and partly Japanese timber framed. The walls are meant to be mostly open. Oak framing with sassafras ceiling and engawa (veranda) boards. Rafters were blackend with oil tinted with soot. The lath in the gable end is pine. That area was plastered with its base coat of earth plaster and straw today. You can see the corner shear wall (plywood) has already been lime plastered.





We made earth plaster yesterday and today from soil removed to make a Japanese style ground gutter next to the pavilion.



The dry soil was shoveled into wheel barrows. The clumps were broken up with sledges and the then screened with 1/4" mesh for the base coats or 1/8" mesh for the brown/scratch coats. The sieves were rolled back and forth over a section of PVC pipe. Very clever.



The straw was either baled or bagged and run through a cheap chipper/shredder depending on what length fiber was needed. Longer for base coat, shorter for brown/scratch coat.



Then earth and sand was mixed in vertical mixer. This little guy is getting a big workout.




So far it has been learning about earth plasters and the base and brown coats. When to add sand and when add fiber, how to use a hawk and trowel. I'm still learning how to get the mud from the hawk to the trowel and onto the wall instead of my shoes. A humbling experience.

Here is a classic Japanese hawk from below. Plywood and scrap wood. About 1 foot by 1 foot, a little wider horizontally.



We will move on to various finish coats including shikkui lime plasters. More pictures tomorrow. The class runs through Thursday.

 

[SamQ aka Ah! Q!]

I'm still learning how to get the mud from the hawk to the trowel and onto the wall instead of my shoes. A humbling experience

Oh yes. You get an incompetency certificate in the first two minutes. It takes ages, just ages, to achieve adherence. You are in good company, Gary.

 

[wallace]

What, no animal poop in the mix?

 

[AndyP]

Plastering to me is one of those dark arts that I bet would be fun to be taught how to do properly.

 

[Cabinetman]

All praise to the instructor for people paying to plaster his walls for him!
Problem is I suppose he had better instruct well to get a good finish as either way he can’t have poor work done lol.
On a side note to the non American readers I bought some of the galvanised mesh the other day and it’s strangely called Hardware Cloth over here. ( for vermin deterrent in the new workshop )

 

[GaryR]

What, no animal poop in the mix?

Yes, we will be using a manured plaster for repairing some rain damaged outhouse wall.The expert judged that the walls failed because the lime/earth plaster that was used had too much sand in it.The rotted plaster was dug out and replaced here with just clay and straw. The whole works will be covered over with a manure plaster, I think.


(appropriately enough).

 

[GaryR]

Today I got to apply a brown/scratch coat to my first wall panel, solo.

The panel had already received a lime/sand plaster base coat some weeks ago. Our task was to apply the scratch coat to a defined and consistent reveal line that would be 2 mm inside of the final finish coat plane. Since the surrounding timbers were different thicknesses this required some head scratching, averaging, and compromises. The blue tape is in place to both establish the boundaries of the scratch coat layer and also protect the timbers.

The average thickness of the scratch coat was to be about 4-5 mm applied in two thin coats. The first coat was applied around the perimeter and then the field filled in. Then repeated with the second thin layer up to the tape boundary. Then the tape was washed cleane and the surface leveled with repeated gentle passes with the trowel. First vertical left to right and then horizontal left to right and top to bottom. Low spots revealed during that were filled in with extra mud. One the surface was deemed flat and smooth enough the tape was washed again and removed.

The expert did all of this on his demonstration panel in about 20 minutes. I took me two hours. And he did come along and fix my mistakes. It will dry to a lighter color.

These panels will get a 2 mm finish coat of Ohtsu, which is clay, lime, and very short straw fibers. (Edit: actually, instead of straw we used toilet paper! It is similar enough to washi paper that would be used in Japan. We chopped up several rolls and then stirred them in water to make a pulp, then stirred that into the soil.) The lime adds durability.

 

[Cabinetman]

Nice one Gary, there’s quite a bit more to it than I had imagined.
Why’s it called a scratch coat?

 

[duke]

Scratch coat in the tiling industry the first rough coat to allow the second coat a good bonding surface.

 

[GaryR]

I'm not sure, but perhaps because it is often scratched after laying on to provide a better key for the finish coat? Scratch is a Western term, rather than a Japanese term. And it was not scratched in the method we saw today. It was referred to as the brown coat to distinguish from the base coat and the finish coat.

 

[duke]

And also referred to as rough scratched surface for a wall rendering.

 

[GaryR]

Yesterday the whole class and staff (17 people althogether) gathered to plaster the farm's kura or storehouse. It already had a base coat of sanded earth/lime mix done in April.
We started in the morning with 2 mm layer of 1 part shikkui (hydrated lime, powdered seaweed glue, hemp fibers) plus 1 part sand. Then a second layer of the same, leveled as smooth as possible. We let that set during lunch. In the afternoon we added two 1.5mm layers of plain shikkui, no sand, but with added vegetable oil. The oil reacts with the lime to make a wax that aids waterproofing.

After the final layer was troweled flat and smooth we compressed it with a finish trowel. It looks fantastic.

Scaffolding nearly set, trim taped. The corners were made of built up shikkui/sand yesterday.



And at the end of the day:

 

[GaryR]

And with the scaffolding down:




And the last task today was to do some repairs to the outhouse using manure plaster. Clay soil, cow manure, chopped straw. A much more rusting look. The manure plaster is the darker stuff at the bottoms of the walls. The previous plaster there was crumbling so the owners decided to have a little fun with a contrasting repair and whimsical design.