Friday, October 23, 2009

Making adobes

On the second day of the workshop, after we had outlined the curve of the vault onto the end walls and put up the guide strings, we went to Ojinaga, Mexico, to meet with Don Santos, who has an adobe brick yard there. Don Santos has been making adobe bricks for over thirty years. He doesn't use any machines; just hand labor. He makes adobes in two sizes: 18"x 12"x 3" adobes, which are used for the walls; and 10" x 7" x 2" adobes, which are used for the vault. We were picking up a load of the smaller bricks to take back to Texas. But first we needed to trim and clean them a bit. We did this with trowels and hoes. The objective was to brush off any dry loose dirt adhering to the adobes, and also to trim them a little, removing any bits of dried adobe that were protruding from the edges.

Don Santos and his crew also showed us how to make the mixture of clay, manure, and straw that is used to make adobes. There were large piles of dry clay and manure in the yard, which are mixed together before water is added to make a slurry. This mixture is "kneaded" with a hoe until it starts to cohere.

Then finally the straw is added. The mixture is wheel-barrowed to another part of the yard, where it is scooped into a mold that makes four adobes at a time.

The adobe maker uses his finger to score each adobe from corner to corner, and then he lifts the mold off carefully. This indentation on the back of each adobe helps it to adhere to the mortar.

The adobes have to dry for a few days. During that time they are turned, so that all sides get dry. There were some dry ones there that we loaded onto a truck to take back to Texas. You can see that any kind of fiber, including corn husks, can be used to create tensile strength in the adobes.

Back in Texas, we started laying up the first course of the vault, following the catenary curve we had traced on the end walls. The catenary curve is good because it transfers the outward forces of the vault down to the ground effectively, as you can see in this drawing that our teacher made.

The first course of adobes leans slightly toward the end wall, at about a seventy degree angle. It follows the outline that we had traced of the catenary curve on the inside, and the strings are about the width of an adobe block away from that curve, to guide the outside of the vault wall.

To make the adobes stick to the end wall and to each other, we used a mortar made of sand and clay and water. This mortar was mixed in a regular cement mixer, and the proportion of sand and clay was approximately equal. We put some water in the mixer first, and then while it was turning, we added ten shovelfuls of clay. When that slurry seemed thoroughly mixed, we dumped it into a wheelbarrow and then added ten shovelfuls of coarse river sand.

This simple mortar is amazingly strong. You slap it onto the top of the wall and onto the end wall, and then slap an adobe into the wet mortar. In a few minutes, it's difficult to pull that adobe off! The hydrophilic properties of the adobe bricks cause them to stick tightly to the mortar.

In the next post I'll show the further progress of the vault as it rose from the top of the adobe walls.

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