Adobe Mini-FAQ

Sun May 4 16:03:41 CDT 1997

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<DT VALIGN=TOP><IMG SRC="dot.jpg"> What does "Adobe" mean?
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<DT>The word adobe is in wide use in the United States of America and Spanish speaking countries. It can mean adobe bricks, the soil that used to make them, the mud plaster, the building that is built of adobe bricks, and the architectural style, which has several sub-divisions. The word itself is believed to come from an Arabic word atob, which means muck or sticky glob or atubah "the brick". The "adobe" style of architecture migrated from North Africa to Spain, so the name adobe must have come with it. It many other countries, the word adobe is meaningless, and it is more accurate to say "earthen brick." Other forms of the same material with different details and names, such as rammed earth, Pisé, Jacal, Barjareque, or puddled mud are sometimes referred to as adobe.
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<DT VALIGN=TOP><IMG SRC="dot.jpg"> Can adobe be used only in deserts or arid climates?
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<DT>Unfortunately, most people think "adobe" buildings are found only in deserts or arid climates. Actually, earth wall buildings (adobe, rammed earth, and other types of earth building) have been used in almost every country and climate in the world. Earth is mans oldest building material. Various forms of earth-wall buildings are used in different climates. For example, it takes several days of dry weather to cure adobe bricks so they may be handled and used to build a wall. In a damp rainy climate that may not have enough dry weather, damp earth is packed into forms (rammed earth) or placed by hand in a wall shape,(commonly called "puddled").
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<DT VALIGN=TOP><IMG SRC="dot.jpg"> What are adobe bricks?
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<DT>Adobe bricks are soil and water are thoroughly mixed together into a plastic condition, shaped in a mold (or by hand), and dried by the sun. The resulting bricks can be almost any shape or size, and are laid up with a mortar made of the same material, soil and water. In the summer it takes about a week of hot dry weather for the bricks to cure adequately for handling and use. Other seasons or bad weather may take longer.
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<DT VALIGN=TOP><IMG SRC="dot.jpg"> Is it necessary to add straw to the mixture when making adobe bricks?
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<DT>No, it is not necessary in most cases! See explanation following...
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<DT VALIGN=TOP><IMG SRC="dot.jpg"> Are special soils required to make adobe bricks?
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<DT>Almost any soil will make usable adobe bricks. Nearly all naturally occurring soils have rocks, sand, silt, and clay, in varying proportions. If the rocks are too large, approaching the thickness of the brick, they should be removed. Some soil has no rocks at all, and may be mostly sand, which can be coarse or fine. Silt is a flour fine powder of ground rock. The clay is the "glue" or binding agent that holds the mix together and makes the bricks more resistant to wetting. The clay can be one of several types, some of which expand when wet, shrink when drying, and some are inert. The majority of soils found around the world have too much clay, which may produce shrinkage cracks on drying. This can be corrected or reduced by adding either sand or straw. The quantity to be added must be determined by trial and error. The best test for determining the suitability of a soil for bricks is to make a few test bricks. The proportions of sand, silt and clay can vary widely, and still make a usable brick.
 In some old adobe buildings (150 years +/-) an analysis of the percentage of clay in bricks removed from the wall has varied from 0% to 32%. The variety of clay (expansive or inert) used for bricks has little importance because when it has dried, the brick size will not change appreciably. The mortar will (as in all masonry) make up the differences in size of individual bricks. The tradition of straw being a necessary ingredient for adobe bricks has come from this experience, as most natural soils have too much clay instead of too little.
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<DT VALIGN=TOP><IMG SRC="dot.jpg"> Will rainfall destroy an adobe wall?
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<DT>The direct effect of rainfall is not as damaging as is commonly imagined. Vertical surfaces in areas with rain up to 25 inches per year will only erode at the rate of approximately 1 inch in 20 years. Horizontal surfaces such as the top of a wall, on the other hand, can erode much more rapidly (2-3" per year.) This is normally dealt with by annual maintenance or the placing of a protective cap (of brick, wood, concrete, or tile in some cases) on the tops of the walls.
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<DT VALIGN=TOP><IMG SRC="dot.jpg"> Adobe is a good insulator, isnt it?
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<DT>Not in the technical sense of the word insulation. The actual insulating value (the resistance to the transmission of heat applied to one side of the wall to the other side) is poor. However, earth walls have a good heat storage capacity, which means when warm, they will stay warm for a considerable length of time. This is called the "flywheel" effect. In a real building application the interior temperature will be an average of the high and low temperatures outside from several days earlier. This is termed the "thermal lag" effect. While the outdoor temperature may vary 30°-40° F. in a 24 hour period, the inside changes will vary only a few degrees. Thus, when it the temperature might be 90° F. in the day and 60° F. at night for several days, the inside of the building will approximate 75° F. This feels cool compared to the daytime 90°, and warm to the nighttime 60°. The comfort factor is high because this dampens the wide variations in the exterior temperatures.
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<DT VALIGN=TOP><IMG SRC="dot.jpg"> Can we coat the wall surface with something to make it waterproof?
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<DT>The most effective coating, in many respects, is a cement stucco. If the detailing of flashing for windows, eaves, scuppers, etc. is properly designed and maintained, this is a durable, long range treatment.. The major drawback to this however, in that the wall then has a waterproof coating that will not allow moisture to escape (or be seen). If serious roof leaks are not repaired, the adobe wall underneath the stucco can be damaged, sometimes to the point of failure. This cement stucco coat normally consists if a galvanized stucco hex netting, nailed or fastened to the adobe wall, an anchor (scratch) coat, a brown (leveling) coat, and a color coat of colored cement stucco. The wire net reinforcing is vital, to prevent sections of stucco from falling off, and also has a corollary effect as acting as a reinforcement that is useful in supporting the wall during seismic events. 
Lime/sand plaster is also an effective coating, but the stark white color of the lime may be a visual drawback unless colored liberally with soil. Lime is basically less stable than cement. 
An economical coating is mud plaster. A plaster is made of soil and water (the same as used for bricks and mortar), and applied in the same manner as stucco, but then may be refinished or re-finished when necessary with a wet sponge rubber float or sheepskin. The soil should be screened to remove larger particles. 
Any solid surface (or masonry wall) whether of stone, brick, adobe, or plaster will show cracks with age. With mud plaster, any internal moisture in the wall is immediately apparent, the cause corrected and the damaged surface can be readily repaired with water and a float. 
Experimental work at Fort Selden, New Mexico, by the Museum of New Mexico Monuments Division indicate that in most cases, a penetrating waterproofing spray may consolidate the surface to an adequate depth, but then the consolidated portion may separate and fall off, actually accelerating the erosion damage. Further testing of other compounds are being evaluated by the Getty Conservation Institute at Fort Selden that show promise.
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<DT VALIGN=TOP><IMG SRC="dot.jpg"> Can something be added to the mud to make it waterproof?
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<DT>In most situations, the most common and economical waterproofing additive is asphalt emulsion. It can make the bricks and plaster totally waterproof if a sufficient quantity is added to the water as the soil for bricks or plaster is mixed. It is difficult to judge the quantity of asphalt required, but usually in the range of 10% to 15% by volume of the water used. "Semi Stabilized" adobes are made with 3 to 5%, but these bricks are not truly waterproof. One drawback to asphalt stabilized bricks is that they will not melt and the scrap cannot be re-mixed in the mud pit. Another is that the asphalt does not adhere well to other surfaces containing asphalt. The bricks will also be slightly darker than the "natural" adobes. 
Portland cement and lime may also be used, but add greatly to the cost and complications. In Peru and other locations in Latin America, fermented cactus juice is sometimes added to the mixture to make it more waterproof.. Various chemicals can be effective, but may be expensive. 
Due to the relatively small erosion effects from rainfall, it is questionable if it is really necessary to use any waterproofing material in the bricks, except in certain vulnerable locations.
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<DT VALIGN=TOP><IMG SRC="dot.jpg"> Why isnt adobe used more than it is?
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<DT>There are a number of reasons, but the major one is that adobe has a poverty image throughout the world. The State of New Mexico is the prime exception. In New Mexico, it has a split image. Poverty on one hand, where a person can build with the materials under their feet, costing nothing but sweat, and luxury on the other, where one needs to be wealthy to afford an architect and contractor for an adobe home. It is a status symbol in New Mexico. A whole generation of architects, engineers, and professional has grown up that have never seen how effective it can be. 
In the past, in times of need, we have turned to adobe to provide schools, churches and homes when other resources were not available. It is ecologically sound (recycles itself) does not spend limited natural resources, and is labor intensive, providing employment. Lastly, there is an endless supply under our feet.
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<CENTER>Updated 10 March, 1997<br>&#169 1997 EACI <A HREF="mailto:mchenry at unm.edu">mchenry at unm.edu</A></CENTER>
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