What’s in a block?

13 Dec


While Ed spent two days working on a door jamb at Cedar Springs Ranch (Terlingua), I experimented (like the good scientist that I am) with making my first adobe blocks: solo. All the necessary supplies from El Punto were loaded into the truck to take with us: wheelbarrow, new mortar hoe (like a garden hoe, but with holes in the blade), two block forms (a double and single), a bucket of lime, gloves, mask, and two empty buckets. The other constituents – sand, clay and water – were yet to be gathered. And I had a source in mind: my favorite black arroyo bottom.

Dark sand in arroyo bottom.

An arroyo runs through Cedar Springs Ranch carrying tons of black volcanic particles: sand.  After a rain it all glistens like a black milkyway poured from an invisible bowl. The black or dark gray contrasts nicely with the rest of the desert floor which is dull white to light beige, and provides a wonderful contrasting backdrop to wildflowers blooming in or next it. I’ve always wondered what adobe blocks would look like made from this sand and this was an opportunity to find out.

Know Your Mud

I disregarded the general rule of thumb when evaluating any potential source for making adobe blocks. Know the ratios of sand, clay, and silt in your native soil. The best evaluative technique is to do a water and jar test, or soil hydrometer test. Fill 1/3 of a quart glass jar with the mixture (sifted and free from rock) and bring up to 2/3 volume with water, leaving about an inch of air space at the top. Add a drop of detergent, which acts as a surfactant and helps separate particles. Cover and thoroughly shake for several minutes. Let sit undisturbed for at least 24 hours to allow the contents  to settle into layers (sedimentation).

Sand settles at the bottom within a matter of minutes. Silt settles on top of sand within three to four hours. A color and size difference between the two can usually be seen and measured, unless the mix contains little silt. Clay can take days to settle. I usually wait for the water to appear relatively clear. Any debris floating on top of the water is organic matter.

Unlike the loamy forest and garden soils I was used to in the Pacific and Atlantic northern areas, the soils here in the Chihuahuan desert are characterized by higher ratios of sand to clay with little organic matter. It may even have lime in it from abundant limestone. Silt is usually associated with river or stream beds. But since this entire region was once the bottom of an ancient seabed, silt can be found at various levels in all soils here.

Soil types

Soil types

After the sedimentation process, measure the depth of each layer of soil particles.  You can then determine the percentage of sand, silt, and clay in your mix. For example, if you have a 1/2 inch deep layer of sand on the bottom layer in the jar, and the overall depth of the soil is one inch, then your mix has about 50 percent sand. For all practical purposes, the ratio of sand to clay is the most important, but does not have to be exact for making adobe blocks. In other words, a window of margin is unavoidable and practical. Regardless, the clay content should not exceed 30% (or ~ 1/3 of the total ingredients) and the sand should never be less than 50%. Various opinions cover silt content, but most recommend keeping it to a low percentage, less than 25%. Strong blocks can be made from 60-75% sand and 25-35% clay.

Ribbon Test

As one gains more experience and becomes familiar with different soil types, the ball or ribbon test is a quicker way to evaluate native soils. The ribbon test is performed by kneading a ball, slightly larger than a golf ball, of wet soil in the palm of the hand. The soil should be kneaded with the fingers until it has fairly uniform wetness throughout. With the thumb and fingers, work the ball and form a ribbon.

The following guidelines can aid evaluation of the soil in the ribbon:

  • If the ribbon is long, flexible, and can sustain its own weight, it is primarily clay.
  • If ribbon is weak, breaks readily, but forms a ball which withstands much handling, it is a clay loam.
  • If the clay loam is soapy or slippery feeling, and appears powdery when dry; it is a silty clay loam.
  • If the clay loam has lots of visible sand, it is a sandy clay loam.

Now, if soil will not form a ribbon and presents a broken appearance:

  • If wet soil is friable, somewhat gritty and sticky, and the ball handles without breaking, it is loam.
  • If the dry soil feels soft and floury, when wet it feels slippery, and the ball does not break, it is a silt loam.
  • If sand is visible and the ball cannot withstand handling without breaking, it is a sandy loam.

Obviously, if the soil is loose and single grained, if individual grains can be easily seen, and the aggregate ‘ball’ crumbles when touched, it is sand.

I did a quick ball and ribbon test after wetting a handful of the black arroyo soil. I let it sit wet for about five minutes and then worked it in my hand. The ball was gritty and slippery at the same time. But I could not differentiate if the latter was attributed to clay or silt. So I did the third way to evaluate soils for making adobe blocks: I made some test blocks.

Let’s Make Blocks!

Sand and aggregate convey strength to adobe block and clay is the binding agent, the ‘glue’ blinding all the sand and aggregate together. Because we plan to use semi-stabilized blocks (for weatherization) for most of our structures, I added a level shovel-full of cement to a five-gallon bucket full of the dry arroyo mix in the wheelbarrow. I added water and mixed well, recalling my instruction on mixing cement and mortar several decades ago when building my Maine cabin. My mix had a wetter consistency than what Ed usually uses for his blocks. I remembered that Helen, of New Mexico Earth Adobes (Alameda, NM), explained that their adobe mud is poured slightly wetter than what we have seen elsewhere. The mud is pliable, flows into the four corners of the molds and requires little to no packing or floating on top. When adobe mud is too dry, it is not pliable, requires hand packing, and the block may cure with a honeycomb effect (voids in the block). The wetter mud can set up in the forms for an hour or more before the forms are pulled off.

One important task I neglected to do before pouring the mud into the forms was to thoroughly wet the forms. I paid for this negligence later; the mud stuck to the inside wooden surfaces. I had to use a large knife to coax the drying block edges from the wood and they cracked in the process. After that, I now soak the molds in a trough of water right before using.

The next morning when I inspected the blocks, I was disappointed when they crumbled easily in my hand. Too much sand and not enough binder; clay. The slippery texture of the test ball and ribbon was mostly likely silt. So it was back to the adobe mixing for another try. This time, I needed a good source of clay. I planned to recycle the three blocks I made providing sand by adding it to a mix of a higher percentage of clay.

Ed and I went out on the Kawasaki Mule with an empty five-gallon bucket and scouted for a source of clay. Not an easy task in an area that is mostly limestone, sandstone, gravel and sand. But we did come upon a small hillside that looked promising. After returning with 3/4 of a bucket full, I went to work. With the two forms, the double and single molds, soaking in a trough of water, I screened the bucket full of new mix with a 1/4″ screen on top of the wheelbarrow. I noticed that exerting some pressure on the aggregate pulverized it into smaller pieces. Surprisingly, only a small amount of large aggregate was discarded and the colors were interestingly dark brownish-red to rosy pink. Even small flat and angular crystals could be seen. ‘This should be interesting,’ I thought to myself.

I pulverized one of the volcanic sand blocks in with the newly found clay, added a half shovel of lime and water, and began mixing. The wet color was an interesting rosy color despite the added dark sand and gray cement. When it looked well mixed, I retrieved the two forms to lay on cleared gravel and shoveled in the adobe mud, although the mix only filled two and 1/2 molds (2.5 blocks). I was pleased at how well they filled the mold corners and edges with minimal hand packing, and the tops were like poured soft ice cream.

Small cured test brick from new clay source.

After an hour and 1/2, I pulled off the forms easily and the blocks retained their shape well. The forms were washed and brushed clean to dry for the next batch. If these turned out good.

The next morning was a treat when I examined the blocks. They were solidifying nicely and felt dense without any crumbling. And they were a beautiful light rosy blush. I immediately decided this was going to be a good mix and it was time to get more of this clay! We returned to the same hill with two five-gallon buckets. I screened one bucket at a time and added one of the crumbly blocks from the arroyo (which were now getting more difficult to pulverize) to each batch. Ed helped me mix this time (tough on my low back) and we made three blocks from each bucket of new clay. We let the blocks sit in the molds for an hour before pulling off and returning to their water bath. I think they would have done better to sit for another 30 minutes before pulling the forms, so we let the last batch sit longer.

We stayed the weekend at Cedar Springs with Randy and his dogs, enjoying company, sharing meals and stories around the campfire. Before leaving, I placed the blocks on their edges inside the large metal building to protect them from weather and animals. We retrieved the blocks a few weeks later (Thanksgiving weekend) and stacked them under the the ramada at El Punto where they will have the special privilege of being incorporated into the arched window. They are still strong and beautiful; an aged beige with a rosy blush, and I can’t wait to make more.

One helpful addition to the adobe tool set would be a cement mixer!

Blocks from the new clay source.


6 Responses to “What’s in a block?”

  1. Chris Miller December 16, 2012 at 1:28 pm #

    Very informative post! Thanks for sharing. Me, Nicole and the girls have been talking about some adobe brick projects at the homestead. Everyone whose been there says the soil is perfect for it but I want to do the soil test anyway. I just took a scoop from the backyard here in AZ to see how it settles out. Thanks again!

  2. Mike Glynn January 21, 2013 at 7:17 am #

    I would love access to that Adobe facebook page. I have property in the Solitario area but suspect my clay is bentonite. I had hopes of building adobe but hear this is not a good idea with this type of clay.

    • Macrobe January 21, 2013 at 9:52 am #

      Thanks for visiting, Mike! You have heard correct: bentonite is a undesirable additive for adobe blocks and plasters. It is a type of clay that has large colloidal surface area with high adsorptive properties; it attracts and holds water and other molecules, such as proteins. Because of these traits it has an extremely high expansion and contraction rate. Adobe and plaster mixes with a high content of bentonite clay will cause the dried blocks and plasters to excessively shrink and crack. A small amount of bentonite can be included in an adobe block mixture, but the final percentage is controversial, ranging from 0 to ‘less than 20%”. On the other hand, bentonite can confer compressive strength to earthen mixes. In architect and professor Gernot Minke’s seminal book, ‘Building with Earth’, engineering tests demonstrated that a mix enriched with 17% bentonite clay increased compressive strength of compressed earth blocks (versus non-compressed adobe blocks). However, water content is crucial. Although bentonite clay has very high binding strength, it also has very high swelling and shrinkage. Therefor, too much can cause shrinkage and cracks which weaken the total strength of the blocks, both compressed and non-compressed. Adding sand (or stabilizers) to mixes with some bentonite clay can reduce shrinkage and cracking. One local here made blocks used for her house containing up to 3% bentonite clay, but also increased content of washed angular sand (not beach or sand with rounded conformation).

      The best way to determine an optimal mix is to do ball tests and make experimental blocks. We are still doing that 😉 (Mike, we are also in the Solitario area and the majority of our place is steep bentonite slopes down into Black Creek Draw).

      If you send me by email your Facebook link, I will add you to the the FB Adobe Doings page.

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