Adobe block and adobe plaster are the keystones to all of our buildings. The Bear Cave is all adobe, while the straw bale buildings are plastered with adobe and the main house has an adobe floor.
Building our own adobe cottage by hand meant just that - using hand tools and ancient techniques to create our building. Sadly, living high on an alluvial plain, our Blacktail bajada, we do not have usable clay deposits.
As a consequence, we purchased our clay from a nearby locally owned clay mine. Ultimately, our three buildings were to require over 150 tons of clay for block, wall plaster, stem walls for the utility building, and adobe floors in the main house and the Bear Cave.
As the clay brought in had been screened to 3/8" minus at the mine, we did not bother to further screen for our building blocks. We used a couple primitive tests to determine what, if anything, needed to be added to our material to produce usable building blocks.
The tests included the "jar test" described in a number of books, where a pint jar is filled about half full of material and then topped off with water. The jar is shaken until all the material is in temporary suspension and then put down to settle until the water is clear. The material - clay, sand, debris, and coarse sand - settles in layers and the ratios of clay to sand and aggregate can be estimated.
The second test was to make a test block or two after the jar test. The test blocks were allowed to cure for a week. One of the blocks was held waist high and dropped flat on the ground. If it didn't break, we felt it would work on the walls of the Bear Cave. We then started production.
The clay/aggregate mix was shoveled into block poly tubs and mixed with water using hoes until it would hold a vertical "face" when pushed with a hoe. The stiffer mud is less likely to either crack or shrink. For these blocks, we did not add straw, cement, or other binding material. The aggregate naturally in the block served well.
Later, for plaster, footings, and other uses, we used straw, asphalt emulsion, Portland cement and even aged horse manure as binding and waterproofing.
To make uniform blocks, we leveled and raked the ground for the block building site. After the mud was shoveled into the form, we troweled the top surface and, after the adobe had begun to dry, loosened the edges from the wooden form much like running a knife around a cake tin.
In the hot and dry Arizona desert summer, the forms could be pulled after an hour or so without damage. This will require a bit of trial and error depending on your locale and weather conditions.
We generally left the block flat overnight and then tipped them on their sides and scraped off debris that might be clinging to the underside of the block with the edge of a trowel.
The blocks were then stacked in rows for curing for a week or so before use on the wall. The more care you spend trimming waste from the bottom of each block and making each block uniform, the better and easier will be the actual building of your wall.
A good backlog of block cured on our site before we even built the stem wall. Ultimately, our adobe cottage (320 sq. ft.) took about 2500 block. We made our block a bit smaller (10" x 12") than normal (10" x 14") so the reduced weight would make handling easier.
Once a stockpile of curing block was on the ground, we began the process of building.
Whatever the ultimate use of your DIY adobe block, we wish you the same satisfaction we had in building our adobe house.
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