You can perform a fairly accurate soil test by filling a jar with 1/3 earth and 2/3 water. Then shake the jar and let it settle overnight. In the morning, measure the thickness of the layers.
The following excerpt from The Owner Built Adobe House by Duane Newcomb is from chapter three, "Making Adobe Bricks," and is reprinted with the permission of the author.
Modern adobe bricks consist of a mixture of clay, sand, straw, and emulsified asphalt.
Clay holds your bricks together just like the cement in a concrete block. It contains primarily an aluminum salt and is made up of extremely fine particles. There are several different kinds of clay, but you can use any one of them for making bricks. The so-called adobe soil of the Southwest actually contains too much clay to produce good bricks. What you need is a sandy clay or a clay loam. Soil with too much clay produces too many shrinkage cracks. Soil that is too sandy crumbles easily. If, after a soil test you find that your ground has too much or too little clay, you can bring in sand or soil with a higher clay content as needed.
Sand, the second ingredient used in making adobe bricks, actually is an inert filler held together by clay, much like gravel in concrete. You can use almost any type of sand in making bricks except beach sand, which contains too much salt. Add extra sand to your soil only when you need to lower the clay content to meet code requirements.
Straw, the third ingredient, doesn't add strength but binds a brick together and allows it to shrink without cracking.
Emulsified asphalt (a petroleum residue utilized in paving) makes the bricks water-resistant. In ancient days the Babylonians succeeded in making waterproof brick. The art was lost for centuries, then rediscovered about fifty years ago. Without the addition of emulsified asphalt, unprotected bricks soon weather away. With it, the bricks stand for many years without appreciable damage.
In New Mexico and nearby areas where the bricks are to be plastered over and not exposed to the weather, they are generally left unstabilized (not treated with emulsified asphalt). Adobe-makers create the bricks as people have been doing for centuries and simply lay them up into walls with a mud mortar. You can do this where the codes approve. Bricks exposed directly to the weather must be treated.
Uniform Building Code Requirements
The Uniform Building Code is a set of construction standards that has been adopted for many areas of the United States. Bricks meeting the Uniform Building Code standards are strong and durable, cannot be damaged by rain, and will withstand most stresses.
Most counties across the United States follow the Uniform Building Code requirements, and many lending institutions will not finance dwellings that do not comply with the code. Here is a summary of the Uniform Building Code requirements for adobe bricks.
 The clay content of the soil used in producing adobe bricks must be greater than 25% and less than 45%.
 Bricks are to be stabilized with emulsified asphalt and shall not absorb more than 2.5% water by weight (based on the dry weight of the adobe brick).
 Bricks shall not have more than three shrinkage cracks. No shrinkage crack shall exceed 3 inches (7.6 cm.) in length or be more than 1/4 inch (0.6 cm.) wide.
 The minimum compressive strength acceptable is 300 pounds (136.1 kg.) per square inch (6.5 sq. cm.).
 The average modulus of rupture for five bricks must be 50 pounds (22.7 kg.) per square inch (6.5 sq. cm.), with no one individual brick testing out less than 35 pounds (15.8 kg.) per square inch (6.5 sq. cm.).
Testing the Stabilized Bricks
Before you actually start making bricks, you should determine the amount of emulsified asphalt needed for your particular soil. There are several types of emulsified asphalt you can use (RS-1, SS-1, Dupont S-9). Local paving contractors should sell this product ... or, if not, they can tell you who does. You can have this item delivered to your site, or you can haul it yourself. To hold enough for your brick building, you will need at least one 50-gallon drum. You can generally buy empty 50-gallon drums at surplus stores. (At times I have also found them at the local garbage dump.)
When buying emulsified asphalt, you should shop around for the best bargain. I have found it selling for from 15¢ to $1.00 a gallon. Prices vary greatly, and you can often get a price break by buying 100 gallons or more.
Generally you will find 1/2 gallon (1.9 L.) of emulsified asphalt per cubic foot (.03 cu. m.) of soil fairly satisfactory (that's 1/2 gallon for every 3 1/2 bricks). Here is how to determine the amount of asphalt needed for your particular soil:
 Construct a wooden box measuring 6 inches (15.2 cm.) by 6 inches by 6 inches. This is 1/8 cubic foot (0.004 cu. m. ).
 Fill the box with soil, measure 8 ounces (0.2 L.) of emulsified asphalt into a plastic measuring cup, and add to soil.
 Hand-shape this mixture into a small, 3 inch by 3 inch by 3 inch (7.6 cm. X 7.6 cm. X 7.6 cm.) brick. Label and set aside.
 Repeat this procedure with boxes full of soil mixed with 12 ounces (0.4 L.), 10 ounces (0.3 L.), 6 ounces (0.2 L.), and 4 ounces (0.1 L.) of emulsified asphalt. Label each brick as to the amount it contains.
 Put these bricks on a cookie sheet and dry in the kitchen oven (200° to 400°F) for several hours. Before removing them all, break one open to make sure they have dried all the way through.
 Immerse your test bricks in water for several hours. Bricks with enough emulsified asphalt will not soften along the edges. You want to use no more than the minimum amount of asphalt needed to waterproof your bricks, since too much will weaken them.
Following is a chart that will show you how much emulsified asphalt to use per brick:
Emulsified Asphalt Per Brick
Using a 1/8 Cubic Foot Test Box
oz. of asphalt Gal. per cu. ft. Measuring cup
per 1/8 cu. ft. box of soil oz. needed per brick
4 1/4 9.1
6 3/8 13.7
8 1/2 18.2
10 5/8 22.9
12 3/4 28.0
14 7/8 32.0
As soon as you have determined how much emulsified asphalt to use, go ahead and make a limited number of bricks. When your bricks are tested by a commercial lab, they will also be tested for water absorption, as required by the building department. However, any brick that passes your water test will absorb less than 2.5% moisture by weight. If you own a small scale, you can also make your own moisture test.
It is possible to stabilize your bricks with Portland cement in the ratio of 1 part cement to 12 parts soil. These bricks. while quite strong, are not waterproof and will not meet Uniform Building Code requirements.
Burnt adobe bricks are also used in some areas. These are simply kiln-fired adobe bricks. They are attractive and durable but extremely absorbent, and will flake severely in areas where frost is a problem.
Following are two simple tests you can conduct at home to give you a general idea of just how strong your bricks are:
 Once your bricks have been cured, try to break one by hand. Twist it with your hand, then put it down and stand on it. If your brick survives this test without breaking, it is probably strong enough to meet minimum requirements.
 You can also make a simple compression test using a sample adobe brick and two 2 X 4's nailed together. Start testing at the distance shown for 250 pounds (113.5 kg.) per square inch (6.45 sq. cm.). If the brick doesn't break, then move out and test for 300 pounds (136.1 kg.) per square inch. If the brick passes this test, it is probably acceptable.
Crushing distance for bricks:
Area of 250 lb 300 lb.
test block per sq. in. per sq. in.
4 sq. in. 3 ft. 10 in. 4 ft. 7 in.
6 sq. in. 5 ft. 9 in. 6 ft. 11 in.
As soon as you have produced a hundred or so bricks, have a commercial test made for compression and for modulus of rupture. I simply took two bricks to the lab and asked them to give me a written report. Then, when I finished my brickmaking, I selected three more bricks at random and had them tested. These written test reports were enough to satisfy the building department requirements.
Making Your Adobe Molds
You will, of course, need some sort of mold to produce bricks. This can be a single mold or a multiple mold producing two or more bricks. The mold can be made of metal, wood, fiberboard, or anything else. In practice, I found a five-brick wooden mold worked best for me.
The standard size adobe brick used in post adobe construction and in 16-inch-wide (40.6 cm.), double-brick, solid-wall construction is 4 inches by 7 1/2 inches by 16 inches (10 cm. X 19 cm. X 41 cm.). The standard size brick in New Mexico and similar areas where the bricks are covered seems to be 4 inches by 10 inches by 14 inches (10 cm. X 25 cm. X 36 cm.).
Make the brick dimension the inside dimension of your molds. The molds, however, should be made 4 1/4 to 4 1/2 inches (11-12 cm.) high, since the bricks generally slump when removed from the mold. My bricks actually ranged from 3 1/2 to 5 inches (9-13 cm.) in height, a variation that produced an interesting pattern in the finished walls.
Make all wooden molds from hardwood, Douglas fir, or southern pine. These woods will withstand the wear and tear of brick making. The molds should be painted or shellacked inside to provide a slick surface. Rough surfaces make the bricks difficult to remove from the molds.
To build a five-brick mold, cut two pieces 4 1/2 inches wide by 1 inch thick by 45 inches long (12 cm. X 2.5 cm. X 114 cm.). Cut ten pieces 4 1/2 inches by 1 inch by 16 inches (12 cm. X 2.5 cm. X 41 cm.). Put the outer frame together with wood screws. Add two divider strips between each two brick openings and make the bottom opening (for each brick) 1/4 inch (0.6 cm.) wider than the top. This allows the brick to slip out easily. Add regular drawer handles at either end of your mold or make them out of scraps of wood. Also protect the four edges of your mold with sheet-metal reinforcing.
Some adobe builders also make eight- to sixteen-brick molds out of 2 X 4's. Any mold, however, that makes more than eight bricks at a time requires two persons to lift it off.
A clever innovation for making adobes is an 8 foot by 8 foot (2.4 m. X 2.4 m.) outer shell made of 2 X 4's. Pour the mud into the form and level it with a 2 X 4. After the mud sets for about two hours, remove the outside 2 X 4's. Cut the adobes to size with a piano wire stretched across a 2 X 4 bow. This method is much faster than most others.
In addition to molds, you can buy machine presses for casting bricks by hand. The CINVA Ram is a light, portable adobe press developed by the Inter American Housing and Planning Center (CINVA) Bogotá, Colombia. A similar style of press is the Ellson Blockmaster manufactured by Ellson Equipment Ltd.
The bricks made from these presses are more uniform in size than hand-poured bricks. Since they are made under pressure, they are also almost twice as strong as cast bricks. Generally, however, making bricks with a press is slower than casting with a form.
As soon as you have pretested your soil for clay content, established the amount of emulsified asphalt needed, and constructed your molds, you are ready to start producing bricks.
You now need a smooth, level surface on which to cast bricks, a mud mixer, emulsified asphalt, a bale of straw, a garden hose and nozzle attached to a water source (or a large barrel of water), a shovel, a pick for breaking hard ground, a rake, a trowel, and a large contractor's wheelbarrow.
To lay out my adobe bricks, I leveled a 30 foot by 40 foot (9 m. X 12 m.) area with a small tractor. I found this most satisfactory. Without leveling a large piece of land, however, you can cast bricks on a flat 8 foot by 3 foot (2.4 m. X 0.9 m.) casting surface made of 3/4-inch (1.9 cm.) plywood and 2 X 4's, or you can cast directly on large, level, 3/4-inch pieces of plywood.
Next, to mix your adobe mud, I recommend that you purchase a contractor's plaster mixer. This is a large gas-powered mixer with turning blades used by contractors for mixing mortar and plaster for house construction. A cement mixer does not mix the adobe mud thoroughly enough. I feel strongly that a plaster mixer provides the best method for mixing bricks. After much searching, I found a rental equipment operator willing to sell one of his used plaster mixers for $300. If you intend to buy a used one, start with rental equipment operators, then call masonry contractors and firms specializing in used machinery.
When you are ready to start production, get everything — your pile of dirt, bale of straw (from a local feed and farm supply store), 50-gallon drum of emulsified asphalt, and plaster mixer — as close together as possible. In addition, try to keep the distance between the mixer and the brick-casting area as short as possible.
When you are ready to obtain soil from your site, you should try to blend the different layers of soil to make a uniform mix. One way is to combine the different layers in a pile with a shovel, and then transfer this mixed soil with the shovel into your mud mixer. I don't care for this method because you must move the dirt twice. I found the best method is to create separate piles of each layer with a tractor, then shovel equal amounts from the various layers into the mixer.
If you intend to mix the mud by hand, pile the soil in a 3- to 4-inch (8-10 cm.) layer, add water, and "puddle" into a thick mud with a hoe. Mix thoroughly. When it is uniformly wet, add a 3-inch (8 cm.) layer of straw, and mix. Place in a wheelbarrow with a shovel, mix in the emulsified asphalt, and pour in the molds.
If you are using a plaster mixer, put 7 to 8 inches (18-20 cm.) of water in the bottom, then add about 80 shovelfuls of soil. Add enough water to make a stiff mud. Beat the mud until the lumps are gone, then slowly add emulsified asphalt ... about 2.5 gallons (9.5 L.) per 20 bricks. Let this mix a couple of minutes (the asphalt will not darken the mud). Finally, add the straw: about 1 part straw per 5 parts of soil. I simply cut the straw in 4- to 6-inch-long (10-15 cm.) pieces and placed it in a wooden box. When I had the box full, I dumped it into the plaster mixer. When the straw is added, the mixer slows down slightly.
The final mud mix should be thick enough to allow the brick to stand by itself once you take off the mold. With a few tries you will learn exactly how thick to make your own mud mix.
To pour the bricks, I laid newspapers directly on the ground and placed four five-brick molds on top of the newspapers. (Do not pour the bricks directly on the ground, since the wet mud will form a solid unit with the soil and spoil the brick.)
Next, I poured the stiff mud into the wheelbarrow and dumped the mud from there into the molds. My contractor's wheelbarrow holds just enough mud to completely fill one five-brick mold. The plaster mixer itself holds enough to fill four five-brick molds. I then worked the mud up and down with a rake to fill out the sides and corners of the mold. When I was sure I had the mud into all the corners, I turned the rake over and leveled the brick surface even with the top of the molds. Many people use a trowel for this, but I find a trowel much too slow. As soon as I had the bricks leveled and fairly smooth, I slowly lifted off the molds. If you have made the mud stiff enough, the bricks will stand by themselves with only slightly bulging sides. Do not allow the mud to set for any length of time before removing the molds, as this makes the bricks stick to the molds.
You must now clean your molds. I simply sprayed them with a hose, then scrubbed the molds down with a stiff brush. You can also completely immerse the molds in a tankful of water and clean them with a brush.
Then cover the newly made bricks with a few sheets of newspaper held in place at the corners with pieces of mud. In my early brick making days I did not shade the bricks ... unfortunately, I often ended up with large cracks in each brick. I found that the newspapers slowed down evaporation and gave me almost perfect bricks every time. You will need to experiment with this. If your bricks dry in the open without cracking, fine. If not, utilize some sort of shade. You can make a more permanent shade than newspapers with 2 X 4's and plywood.
One other consideration: The rough surface left on top of the bricks by raking may in itself cause cracking. If you find this to be true with your own soil, smooth the bricks with a trowel instead of the back of a rake.
If a few bricks crack, don't worry about it. You will need many part-bricks. I found, in practice, that I didn't have nearly enough part-bricks and frequently had to cut whole bricks to make the walls come out even between the posts.
As soon as your bricks have dried for three to four days, stand them on end so they can dry on either side. After about six weeks, the moisture content will be down to about 4%. You can then stack them in groups. Simply place them on edge, three to four bricks high, against a center pillar. In my early brick making days I frequently stacked them seven to eight bricks high on wooden fruit pallets. This resulted in considerable breakage among the bottom bricks. Bricks stacked on edge store well for long periods of time.
To protect your bricks from the weather, I recommend covering the piles with a piece of plywood, asphalt felt paper, or black plastic.
Also, as I mentioned earlier, it is important to build and store your bricks as near your building site as possible. You must move approximately 2 tons (907 kg.) of bricks for every 10 feet (3 m.) of wall. To move this much weight any distance will consume a tremendous amount of time and energy. My own bricks, stored 100 to 200 feet (30-70 m.) from the building site, required almost an hour's hauling time for every ten feet of finished wall.
As you get into production, you will soon learn how many bricks you can make in a day. The first day I began, two of us produced exactly twenty-five bricks. I soon discovered that brick making is not an exact science. It always takes several tries to "work out the bugs." In the beginning I couldn't get the mud to fill out the sides of my bricks, many bricks had too many cracks, and others sagged so badly when I pulled off the molds that they couldn't possibly be used. You will be able to iron out difficulties like these after a few working days.
I found two people working together could produce about 225 bricks a day. Alone, I could make 125 to 150. Since I estimated 5,000 bricks were required for my 2,700-square-foot (250 sq. m.) house — you need approximately 100 to 125 bricks for every 8 feet (2.4 m.) of wall — I figured on thirty-three to thirty-five working days to make all the bricks. If you have only weekends for brick production, 5,000 bricks will require many weekends to produce.
Although making bricks is hard physical work, there is a great deal of satisfaction in knowing that you produced every last brick yourself, and in some ways I was almost sorry to see the brick making part of building my adobe house come to an end.