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 sandy clay or a clay loam. Soil with too much clay produces too many shrinkage
cracks. Soil that is too sandy crumbles easily. 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 the bricks together and allows them 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, but the art was lost for centuries--to be rediscovered about fifty years ago. Without
the addition of emulsified asphalt, unprotected brick 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 brick as people have been doing for centuries
and simply lay them up in the walls with a mud mortar. You can do this where the codes approve. Bricks exposed directly to
the weather must be treated. Cement can also be used as a stabilizer/waterproofer.
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, 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 percent and less than 45 percent.
- Bricks are to be stabilized with emulsified asphalt and shall not absorb more than 2.5 percent 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 that 1/4-inch (.6 com) wide.
- The minimum compressive strength acceptable is 300 pounds (136.1kg) 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 com).
- Write to the Building Officials Code Administration International, 4051 W. Flossmoore Rd., Country Club Hills IL 60478
/ Website: www.bocai.org.
Testing The Stabilized Bricks
Before you actually
start making bricks you will need to determine the amount of emulsified asphalt needed for your particular soil. Local paving
contractors should sell this product, or if not they can tell you who does. You will also find manufacturers listed on the
Web. Search for emulsified asphalt. 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. Prices
vary, and you can often get a price break by buying 100 gallons or more.
When buying emulsified asphalt, you should shop around for the best bargain. Prices
vary, and you can often get a price break by buying 100 gallons or more.
Generally you will find 1/2 gallon (1.9L) of emulsified asphalt per cubic foot (.03
cu m) of soil fairly satisfactory (that's 1/2 gallon for every 31/2 bricks). Here is how to determine the amount of asphalt
needed for your particular soil.
1.
Construct a wooden box measuring 6 inches (15.2 cm)
by 6 inches by 5 inches. This is 1/8 cubic foot. (.004 cu m).
2.
Fill the box with soil, measure 8 ounces (.2L) of
emulsified asphalt into a plastic measuring cup and add soil.
3.
Hand-shape this mixture into a small, 3 inch (7.6
cm) by 3 inch by 3-inch brick. Label and set aside.
4.
Repeat this procedure with boxes full of soil mixed
with 12 ounces (.4L), 10 ounces (.3 L), 6 ounces (.2L), and 4 ounces (.1L) of
emulsified asphalt. Label each brick as to the amount it contains.
5.
Put these bricks on a cookie sheet and dry in the
kitchen over (200 degrees to 400 degrees F) for several hours. Before removing the all, break one open to make sure they have
dried all the way through.
6.
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 need 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.
Amount of Emulsified Asphalt Needed per Brick
Utilizing a 1/8 Cubic Foot Test Brick
-----------------------------------------------------------------------------------------------
Measuring Cup
Measuring Cup
Oz of Asphalt
Gal per cu ft oz Needed
Per 1/8 cu ft Box
of Soil
per Brick
-----------------------------------------------------------------------------------------------
4
1/4
9.1
6
1/8
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 limit number of bricks. When your bricks are tested
by a commercial lab, they will also be tested for wart absorption, as required by the building department. However, any brick
that passes your water test will absorb less than 2.5 percent moisture by weight. If you own a small scale you can also make
your own moisture test.
Burnt adobe bricks
are also used in some areas. These are simply kiln-fired adobe bricks. They are attractive and durable but are extremely absorbent
and will flake severely in areas where frost is a problem.
Strength Tests
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 4s nailed together as shown.
Start testing at the
distance shown for 250 pounds (113.5 kg) per square inch (6.45 sq com). 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.
Compression Test
-------------------------------------------------------------------------------
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
6ft 11in
-----------------------------------------------------------------------------------------
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 brick making,
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, 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 (40.6 cm) wide, double-brick, sold wall
construction is 4 inches by 7 1/2 inches by 16 inches (10cm x 19cm 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 (10cm x 25 cm x 36 cm).
Make the brick dimension the inside dimension of your molds. 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 (9-13c,) 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 1/4 inches wide by 1 inch thick by 45 inches l9ong (12cm x 2.5 cm x 114
cm). Cut ten 4 1/2 inch by 1 inch pieces 16 inches long (12 cm x 2.5 cm x 41cm). Put the other frame together with wood screws.
Add two divider strips between each brick opening and make the bottom opening (for each brick 1/4 (.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 4s. 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 in an 8 foot by 8-foot (2.4 m x 2.4 m) outer shell made of 2 x 4s. 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 2x 4s. 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), Bogota, Columbia. CINVA Ram Inc., P, O. Box 59032,
Orlando, FL 32859. /website: www.cinva-ran,com.
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.
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 he mud to set 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 thankful 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 you bricks dry in the open with cracking, fine. If not, utilize some sort of shade. You can make a more permanent
shade than newspapers with 2 x 4s 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 percent. 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 you 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 me) from the building site,
required almost an hour's hauling time for every 10 feet of finished wall.
As you get into production you will soon learn how many bricks you can make 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 of the molds that they couldn't possible 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
we required 700 bricks for my 2,700 square foot (250) sq m) post adobe 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, 5,00 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 come to an end.
