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Clay, Sand, Silt, and Straw


Over Memorial day weekend I went to Southwest Solar Adobe (SWSA) school in Belen NM about 30 miles South of Albuquerque. I had a blast and the below pictures from the school were stolen from a classmate. The gentleman sitting on the wall in the first picture is Joe, our instructor.


While at SWSA I learned how to make adobe the modern way, how to stabilize adobe, how to install insulation on the outside of an adobe structure and why, the basics of stacking the bricks to make a wall and a basic Roman arch, and finally how to test soil for the right makeup to ensure that any bricks make would pass code compliance testing.


The first lesson at SWSA is that all soil is made of four things: clay, sand, silt, and organic matter. Organic matter is exactly what you think it is Leaves, sticks, grass, etc. Organic matter is a no go in adobes using the modern method so it must be screened out. The other three ingredients need to be in specific amounts for a good adobe. 10 - 30% clay, with the rest sand and silt but no more than 45% of the sand silt mix should be silt.


The modern way to make adobe uses a 3 to 1 ratio of sand to clay. The sand is basically playground sand with no real remarkable properties other than needing to be washed. The clay can really be any that you can find as long as it isn't super expansive as that can cause brick failure after several freeze-thaw cycles. Expansive clay can be identified as clay that when added to water it swells up to a much larger volume in the neighborhood of 3 times its dry volume. The clay needs to be screened through a 1/4" (6.5 mm) screen to get all the big lumps out and any unwanted large stones.


This ratio of sand and clay can be mixed by any method. In class we did it by hand in a wheelbarrow (a TON of work) but can be done in a mortar or cement mixer. Water is added to the mix until it gets sticky and when you try to shovel it suction forms on the shovel. For comparison it reminded me of thick mashed potatoes in consistency. This same mix if not put into forms as described below makes adobe mortar


The mix is then put in wood forms approximatly 10"x14"x4" (254 x 355 x 101 mm) that are pre-wetted with just plain water. All of the air is pushed out of the corners to ensure a well formed brick. Once the form is filled the form is immediatly removed leaving a very soft brick behind. The soft brick is then allowed to dry to 24 to 48 hours on the widest side. After that the block is tipped up on the 4" side to dry and cure which depending on the weather can take 10 to 30 days. At the end of the cure period you have one complete brick that weighs between 30 and 35 lbs (13 - 16 kg).


According to New Mexico code the first course of Adobe mortar and bricks must be fully-stabilized. This is defined as after being submerged in water for 24 hours the brick must not gain more than 3% weight. There are two primary ways to make adobe bricks fully stabilized using portland cement or asphalt emulsion.


Portland cement is the easiest to acquire but has some drawbacks. The drawbacks include quantity needed and chemistry incompatibly with clay which are the main two. Quantitiy needed is between 10-15% by weight for each brick. This means that an 80lb (36kg) bag of portland can only make at best 26 adobe bricks. This can end up being a large expense at 14$ a bag. The chemistry incompatibility comes from cement setting hard and brittle while clay will always expand and contract with moisture. This can cause fractures in the brick making spalling in the brick possible.

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Asphalt emulsion is harder to find but has several benefits when compared to portland as a stabilizer. The Asphalt is completely compatible with the clay in the mix as it does not set hard. A much smaller quantity is needed as well with 3-5% by weight of dry mix and it is much cheaper averaging 5$ per gallon. The only real draw back to emulsion is the need to mix it with the water before adding it to the dry mix.


I mentioned above that the method of make adobe described at SWSA was the modern version. The more traditional and acient version is screened topsoil and straw in the same method with as above but a different ratio. This ratio is not hard and fast as it depends on the topsoil but it is somewhere between 10 to 1 and 5 to 1 soil to straw. There are buildings hundreds of years old still standing using this mixture including most of the pueblos in New Mexico.


Sampling my soil at SWSA saw that I would need both clay and sand to make modern adobes from my land which would come with an additional cost both for materials and delivery. Upon visiting several local quarries/sand pits I also can't locate clay in large quantities to be delivered. This means for me I am going to try traditional adobes with lots of up front testing to see they would be more viable and cheaper. I'll keep you posted on how my testing goes and if it works I'll submit some bricks to lab testing for code compliance.


My foundation is coming along as now the rebar is in the footing trenches, the under slab insulation is down along with the gravel needed. The steel mesh has been put on top of that and the last thing needed is the in-slab heating and then inspection and the pour should be not too far behind.


Stay Groovy



Recently acquired adobe making supplies


Most recent look at foundation




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