Clay Plaster - Scratch Coat

I'm finally starting on the clay plaster!  I've enjoyed the Light Straw Clay (LSC), but I've also been looking forward to covering it up.

This is the before photo.  Note the 3/8" plywood at the top of the wall covering the blocking for the kitchen cabinets.  The plaster will be flush with this.

 luckily, the mortar mixer fired up after several months of non-use without major drama.  Clay: this is the same clay soil I used for the floor, screened through 1/2 wire mesh, soaked in a bucket for several hours (several days is better), and given a preliminary mix with my double auger mixer.  Concrete sand (vs masonry sand)  is used because it has varying grain sizes that lock together more compactly.  I'm still not sure about the wheat paste: it didn't seem to make the mix stickier or more workable, as is claimed.  That will warrant some further experimentation.  One mix is enough to keep one person busy for the rest of the day. This turned out to be five 5-gallon pails filled 1/2 full.

Ready to start applying.  The LCS has been pre-wet to the right of the taped electrical box.  I used my paint sprayer, just spraying potable water and that turned out to be a great way to get controlled coverage (and good for me to practice my wrist technique).  I made one pass 10 minutes or more before stariting, and a second pass right before plastering.  I started out using the trowed to apply the plaster to the wall, but we quickly found that hands work better.

Two sections done.  This is called the scratch coat, as I'll explain later.  The goal is to get the best possible adhesion to the LSC, so we paid the most attention to pressing the plaster into the interstices of the LSC. Apply a handful smearing it up the wall.  Get a section roughly coated, and then come back and press it down with a sort of jiggly motion to lock it into place.  (Same as applying lime plaster to lath.)  The downward motion leaves some lumpiness that is easy to smoothout with some light upward or sideways strokes.  

We put some effort into leaving a uniform thickness, which is really pretty easy if the wall was well formed.  These sections were quite flat, but some later sections were less regular, so the scratch coat sort of half evens out the variations.  The straightening coat, coming next, will finish that process.  This scratch coat was only just thick enough to cover the majority of the straw.  Just a few stalks are left visible on the surface.

The corner was a bit of an experiment.  I started from the top with expanded metal lath stapled into the wood studs.  The lath started out thick at the top (3" per wall) and got progressively thinner toward the bottom.  You can see about 2" per wall in this photo.  The very bottom involved burlap placed into a thin layer of wet plaster.  You can see that it's pretty wavy, and it was a pain to get it to wrap the corner, so I'm leaning toward the lath as my final solution.  Since this corner is in my pantry closet, I'm less concerned with the look of it.

Regarding the exposed studs, I will deal with them with the straightening coat.  First, I'll place a skim coat of plaster over the wood, bridging the adjacent scratch coats.  Then I'll press a 6" strip of burlap into that wet mix and let it dry for several hours or perhaps overnight.  Then I'll come back  and lay the scratch coat over the entire wall, getting it as smooth and level as I have the desire and patience for.

 So that's what we completed by the end of the day.  Not too shabby, and more rewarding than the adobe floor, which involved a lot of loud mixing and laborious troweling.Clay plaster is still laborious, but at least you are standing up about 2/3 of the time.

The final task is to scratch up the surface to give leave a good texture for the next layer to adhere to.  Unfortunately, my closeup photos didn't come out.  At Bob's suggestion, I tried out a hair pick and it was the perfect tool!  Just pull horizontally and you get a nice pattern of 1/16" to 1/8" grooves.  I tried a diamond pattern, but that seemed to rough up the mix too much.

This is the wall after two days of drying.  I find it interesting that two of the sections are notably wetter than the others, since they all seemed to be the same thickness of clay.  I guess not.  In the  darker panels, you can see the scratch patterns: diamond on the left, and horizontal on the right. After 4 days, it was completely dry.  Interior conditions were in the low 60's with relatively low humidity.

Compressed Earth Block Workshop

This natural building technique is relatively new to me, so I wanted to make some test blocks to see if they would work out for the finished surface of my porch. My first attempt was a failure after using the very clayey soil that I've used for the adobe floors because I couldn't get the cement mixed in with the soil adequately.  I found a source of a looser soil that still has a relatively high clay content.

Regardless, the first step is to pass the soil in the 1/4 screen to loosen it up. 

Interestingly, the water content in the soil was pretty high in the morning, but

by afternoon, it had dried out much more than I would have expected.

This is the mixing bucket.  We tried three recipes with varying ratios of cement (1:7, 1:11 and 1:15), and you can see the scale's readout that allowed us to measure precisely.  

This is one of the 1:11 mixes.   After mixing this thoroughly, we also added water to get the mixture to the optimum moisture content (more water was needed as the day went on).  It took some practice, but we found that if you squeeze a handful of mixed soil, it should form a coherent ball, and that ball should shatter when dropped onto a hard surface from about 6 feet high.

This is the simple mold: 2x4 sides screwed onto a 2x8 base.  The first blocks were pretty thin, so we ended up filling it, compacting it slightly by hand, and then re-filling.  Next time, I'll try 2x6 forms to save that step.  (FYI, to make a large number of blocks, you can buy a human-powered press, or gas-powered pneumatic presses.  Since I'm only going to need a few hundred, that's not worth it for now.)

Here, we're compressing the blocks with a hand-tamper hammering on a 3" thick board that fits inside the molds. It only takes about a minute of non-strenuous pounding, and you can feel when you are getting diminishing returns.  You can also hear the difference when it's time to stop, but that is pretty subtle.

I was surprised how hard it was to get the blocks out of the mold.  

We needed to remove two sides, or it was just too sticky to come out whole.

Knocking it with a hammer helped free the block.

 An array of block after a few days of drying.  Since the recipe includes cement, they should be kept shaded and moist, and they won't achieve their full strength for a month.  I've left a couple random pieces out in the rain, and they've held up with no visible degradation.  Once they've all cured, I'll try some compressive tests on dry samples and soaked samples, comparing the results of different cement contents.  

Many thanks to Kate and Bob for the photos and assistance!