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CCAT Natural Exterior Wall Plaster and Paint, part 1

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fig. a: The front of the finished wall; note the organically shaped reveal window allowing a view of substrate materials.

Also see CCAT Natural Exterior Wall Plaster and Paint, part 2; or skip to CCAT Natural Exterior Wall Plaster and Paint, Appendix.


This project was a test of natural plasters, finished with a protective lime-wash coat of natural paint, for outdoor application. The client was Campus Center for Appropriate Technology (CCAT), who was expected to fund the materials, as well as provide the test surface of a wood frame wall. The wall was constructed of two different 4' by 4' natural surfaces: one in-filled with paper-crete bricks mortared together with paper-crete slurry, and one stick framed with wood lath and holding infill of cob (also known as wattle & daub). Both sections were constructed by Myles Danforth as his class project and were in the test phase. Following research, work on the plastering and finishing half of the project took place in three phases.

  1. Test a variety of natural plaster and pigment formulations.
  2. Based on the plaster results from Phase 1, select and apply two plasters for each section of the test wall, resulting in four final test sections.
  3. Based on the pigment results from Phase 1, paint the plastered surface with a natural paint to match to the nearby official HSU signage columns.

In preparation for this project, I took workshops in Natural Plastering and Painting under Pennelys Goodshield of Sustainable Nations, and conducted book and online research. I conducted this project over the summer of 2009. With the exception of some assistance from classmates, the labor input was all my own. I would not endorse approaching such a project alone, as it was a very labor intensive. The finished product, however, achieved the goals set forth.

fig. b: The back of the finished wall.


Taking up the second part of the experiment, I researched and tested different appropriate-tech plasters with which I finished Myles' wall. After research and testing of a number of plaster formulas, I settled on two to apply to the wall so that weathering and stability could be compared over time. A clay-slip is usually applied as a binder between the wall itself and later plaster[1]; thus, the application of a clay-slip was my first step in actual construction. As we jumped through the bureaucratic hoops of campus building codes, we learned from the Building and Maintenance department that any permanent structure we built (including the experimental wall) needed to match the approved campus color scheme; as a result, part of the definition of 'appropriate' for my end of the project was to get the color right, which was purely a bureaucratic necessity but a necessity nonetheless. As it soon came to my attention that natural pigments stretched much farther in paints than in plasters, devising an appropriate natural paint was added to my to-do list; it is much more appropriate from a financial standpoint and a resource conservation standpoint to use less pigment, (usually mined minerals). In tune with CCAT's educational mission, Myles requested that a window be built into the plaster so that the paper-crete and wattle & daub would remain visible to future students and builders. My end of the project was thus broken into multiple tasks:

  1. Materials research
  2. Materials acquisition
  3. Experimentation with plaster recipes
  4. Experimentation with paint pigments
  5. Designing (with Myles' advisement) and installing the window section
  6. Applying the clay slip
  7. Applying four strips of experimental plaster (one strip of each of my two final plasters over each of Myles' two wall materials)
  8. Applying lime-wash color coat.
  9. Curing the lime.

I considered trying different paints as well, but this seemed to put too many layers of variables into the experiment. Instead, I decided on a single paint formulation during the research phase; the only experiment in the paint coat was how well I could match the school column color as per the Building and Maintenance department requirement.

Project Requirements[edit]

As opposed to mainstream construction and technological applications that are more often of a one-size-fits-all approach, the Appropriate Technology approach requires carefully thinking through your goals to find a best balance for the given circumstance. The immediate goal of this project was to design and apply
A shot of the wall after Myles finished building it; it has a gravel bed, brick foundation, post frame, and shingle roof. The left gray half is the paper-crete brick and mortar, the beige right is the cob filled wattle & daub.
materials that the good folks at CCAT would observe over the course of the following year; the long term objective being to build an entire structure using the best materials as determined by the results from this experimental wall. Since the project is for CCAT, features of Appropriate Technologies like LOCAL, NATURAL, or RECYCLED MATERIALS, COST-EFFECTIVENESS, REPEATABILITY BY NON-EXPERT BUILDERS, and WORK DURATION, were important. However, because the intention is to eventually make permanent structure(s) based on this experiment, I ranked DURABILITY (tempered by these others) as the biggest priority. Since the HSU Building and Maintenance department requires permanent structures to match approved color schemes, this bureaucratic requirement of MATCHING THE APPROVED COLOR SCHEME was also central in determining the success of this project. Finally, as a painter by trade, I consider AESTHETICS to have a high value for structures of all types.



The first task was materials preparation. Research informed me that traditional technologies for water-resistant earthen plasters are lime (the mineral) and manure [2]; another choice might have been using a natural oil such as linseed oil as an additive (here termed amendment)[2], but oils block water more completely, whereas Myles' wall materials of paper-crete and wattle & daub are often used for their vapor-permeability [3]. Vapor-permeable surfaces are considered advantageous in natural building because they allow interior and exterior humidity levels to attain equilibrium by first absorbing in, then wicking away excess moisture, not allowing it to build up within [2]. Using an oil amended plaster would eliminate this vapor-permeable quality of the wall materials, so it made sense to use the vapor-permeable, yet humidity durable, lime and manure plasters. I found a variety of formulas for mixing plaster using these key ingredients. Additional ingredients included builder's sand, lime-putty, manure, straw, clay and wheat-paste. A brief discussion of each: qualities, how I prepared them for use, where I got them, etc., follows.

Builders Sand[edit]

Sand is a component of most plasters; on a molecular level it is crystalline in structure and interlocks like a 3-D jigsaw puzzle, providing mechanical strength [4]. I could have used beach sand for this, but mixed grade builders sand is best because it has different size crystals to interlock; more uniform sized sand particles like I would find at a beach would create weaker bonds[5]. This might be okay for other projects, but as durability is one of my chief criteria, I wanted to use the most durable ingredients. Another problem with beach sand would be the difficulty of washing it, as natural salts would represent an unquantifiable (for my purposes) variable, possibly reacting in my plaster recipes. Factory sand represents a higher embedded energy cost, but allowed me to test these formulas at their strongest potential quality levels without an odd variable throwing off the results. A practical, non-experimental design might well opt for the lower embedded energy and financial cost, and added labor cost of washing local sand. My mixed grade builders sand was donated from ripped, so un-saleable, bags available at the local hardware store.

Lime Putty[edit]

Lime is a basic ingredient in earthen cements because it adds a binding quality to the mix; it is also known for high durability under adverse weather conditions [2]. Lime is available at hardware stores in the form of a powder. I used Type S Hydrated Lime. It is important to use builders lime and not agricultural lime; there are a variety of lime powders so be sure you know which one you get, as they all require different handling [2]. Lime is caustic and will burn, vinegar is recommended to be kept on hand as a chemical counter-agent for first-aid [6]. (Even with gloves and rubber coveralls I managed several chemical burns over the course of the project, thanks be to vinegar.) Lime Putty is produced by slaking (soaking) lime in water. For Type S Hydrated Lime it can be used within 1/2 hour of mixing, but all lime putties improve in quality with increased slaking time [2]. I was lucky enough to be able to slake my lime for a period of several months. I acquired it early in my spring semester and didn't start construction until the summer months. Keep in mind that the lime that went into the finished wall had slaked for several weeks longer than the test batch, so was that much improved. [Recipe under Appendix I: Important Recipes.] Lime is processed by being burned in kilns. This in turn releases CO2 into the atmosphere. However, as lime plasters, washes, etc., dry and begin curing, they react with moisture in the air, eventually pulling the same amount of CO2 back out of the air released during burning[7], and turning back into limestone around the other mix ingredients. So not counting transport, packaging, and the like, the lime itself is carbon neutral though curiously adding and removing CO2 to and from the atmosphere. My Type S Hydrated Lime was donated from ripped, so un-saleable, bags available at the local hardware store.


Manure is a traditional building material in many parts of the world[2]. It mixes well with earthen ingredients; retains vapor-permeability; contains natural enzymes which make it water-resistant, and also natural (undigested), fibers [2], [see section on Straw for discussion of tensile strength]. I opted for horse manure, as there was a stable up the road which was happy to have me reduce their manure pile. My manure was therefore not fresh. I placed the manure in a 35 gallon container and added water to bring it back to a softer, more workable state. However, because it had been sitting around for some time there were issues with both mold and bugs. Gross as it sounds, to remedy this I first carefully picked through the manure, selecting only the least bug and mold ridden parts to keep. A previous class [8], had taught me that the household natural cleanser Borax is used in earthen building as a mold inhibitor. I also knew of its usefulness in killing bugs as it works its way into their joints, destroying them. This might sound cruel to animal lovers, but a bug infested wall was not an option. I did not find clear indications of how much Borax to use, so I guesstimated, adding about ½ cup of Borax powder over about each foot of depth of manure in my bin. This was adequate for squelching the mold, but the bugs returned so I mixed in more Borax as I went in an ad hoc manner over the weeks of the project. Certainly the Borax was a small percentage of my manure, but I did not measure by weight or volume how much went in, so the more scientific minded should be aware of this loose variable in my formulas (some formulas contain it explicitly by volume).


Straw is used in earthen building to provide tensile strength; think of it as akin to metal re-bar in mainstream building[9]. Straw allows the surfaces to bend and flex without coming apart; it spreads stresses, as opposed to letting stress fracture off a given section of plaster. I had straw bales on-hand at the site that I was welcomed to work from. Straw for plaster should be well dried with no mold, about 1-3 inches in length [2]. One recommended method to cut the straw to the right length was to put it in a trash bin and use a Weedwacker on it [10]. I couldn't find anybody to loan me their Weedwacker, but this was probably for the best since such an approach would likely constitute abuse-of-power-tools, destroying the Weedwacker (which would require a replacement), and thus not be an appropriate use of technology. Instead, I got a wheel-barrel full of straw at a time and cut it down with scissors using gloves (the stuff is tougher on the hands than you might guess). Cutting straw became a welcome break during the project; I recommend not preparing it all at once, so you have an easier task to change gears as you work. With more bodies it might not matter, but working by myself, it was a welcome relaxer. One inch straw is recommended for top coats of plaster; first (Key or Scratch) coats should have longer straw to give later coats material to grip to[1]. Before I applied my finish coat of plaster I actually took scissors and trimmed the wall to ensure longer pieces would not stick out after the finish coat went on.


fig. a: Screening clay; the idea is to get the small stuff through the screen...
In addition to sand and straw, clay is a basic component of bricks and plasters, an alternative to lime, acting as the binder that holds the other elements together[2]. Although I'm focusing on lime and manure, I wanted to try a variety of raw earthen mixes to see if they had comparable durability; if they did, clay mixes certainly would have lower embedded energy costs than the lime, and lower than manure anywhere ruminant animals are not common. Clay is available for sale through potter’s suppliers and elsewhere, but can also be dug right out of the ground. However, high silt ratios in ground clay are bad for
fig. b: ...while keeping out rocks, twigs, and other debris; these clay chunks could probably be broken down smaller and be OK.

building[2]. There are several easy tests to determine clay suitability, the easiest being to roll it between your fingers to make a clay 'worm'. If the worm holds its shape, the silt content is low enough to make a decent building clay[11]. A more precise field test is available on Myles' Appropedia page [12]. I brought my resident expert, Ms. Goodshield, out to review my materials before I began. She eloquently waxed about how wonderful the quality of the clay was at the CCAT grounds where I was working, how she only wished she could get clay as good, and that I was lucky to have it right there on-hand. So I trusted her. As another pair of students were building a retaining wall and digging up a lot of clay in process, I helped them by getting it out of their way and into a trash bin. I screened this material by hand through quarter inch screens stapled to wooden frames in order to remove rocks and other debris. From there, all I needed to do was keep it hydrated so that it would retain a putty-like consistency and not turn rock-hard.

Wheat Paste[edit]

Wheat-paste is a basic natural glue[2]. It was needed to make a clay slip,[1]. [Recipe under Appendix I: Important Recipes.] Since I would be using wheat paste already, I decided to add it to my plasters in different ratios to see if it was useful as an amendment. I figured the plasters made of lime would cement up well, and hypothesized that wheat paste glue might help the more raw formulas compete. Wheat paste is easily made. [Recipe under Appendix I: Important Recipes.] Use high gluten organic (to keep out extraneous chemicals that might react with other ingredients) wheat flour; do not use whole wheat because it is low in gluten, and the gluten is the gluey part [1]. I bought my organic wheat flower from the local co-op. Wheat-paste will keep in the fridge over-night, but the fresher, the better, one shouldn’t use more than day-old wheat paste[2]. (Note: Wheat glue can be used for all sorts of things, such as to poster walls- “going wheat-pasting”).

Finding Plaster and Color Recipes[edit]

Stress Test 1 Design[edit]

Having researched some plaster formulas and gathered and prepared my ingredients, it was time to make some test plasters and determine how to administer stress tests [13]. I devised two tests that I used in conjunction. The first test was done with a wire brush like used in construction, about 1 inch wide by 5 inches long and mounted on a 1ft long handle. I used a weight on the brush to scrub the plasters, letting the weight do the work of pushing down, and trying to use my muscles just to maintain a back and forth motion. I didn’t consider the amount of weight important, just that it remain relatively constant. I taped a fist sized rock from the yard onto the back of the brush for weight, then after a few practice runs decided that thirty strokes front and back was an appropriate amount to damage the plasters while leaving enough behind to compare. I used as long as possible strokes across my test patches; they averaged about six inches square.

Stress Test 2 Design[edit]

The second test was a two part water test. I decided to shoot my garden hose full blast (not a lot of pressure actually) at the plasters where they were undamaged by the brush test. Again, the amount of water pressure wasn't important for my purposes, just that it was uniform, so full blast it was. To contain the flow and keep it aimed in full pressure against the plaster, I cut a garden hose size opening into a plastic single-serving yogurt cup, then cut about a two inch slit in the edge that would touch the plaster to allow some pressure to escape. I taped this ad hoc funnel more or less water tight onto the end of the hose with duct tape. The idea was for the blast to be concentrated where I aimed, but not allow an unquantifiable pressure build-up; I also thought that to allow moisture to escape outside the cone radius, weakening surrounding areas to a lesser extent, gave me a second surface on which to note damage, all from one test. I decided, after a little trial and error, that using a watch with a seconds hand to time two five-second blasts at the same spot, ten seconds apart, provided just the right amount of damage from which to make comparisons.

Finally, I allowed the water test to dry out over night, under heat lights and fans. Now I repeated the water test, only this time blasting about a third of the way up into brush strokes creating areas in the brush line that got full-blasted by water, others just moistened by proximity, and others not wet at all on the far end; these to compare against the first water test on undamaged plaster, and areas not abused at all except through proximity to the other tests, effectively five different areas per plaster to compare.

Preparation for Experiments[edit]

All the plasters had dried for about a week onto particle board, which had been prepared with a clay slip as an adhesion coat[2]. (See Appendix I: Important Recipes for slip recipe.) Slip can be painted or sprayed on with the proper tools; I used a cheap 3 inch brush to paint the slip onto the particle board. After the slip dried I then mixed small of amounts of plaster [See Appendix II: Experiments, for experimental plaster recipes.] and applied them to the particle board by hand [see discussion of Hurling: page 2 under Construction], allowing them to dry for several days.


I conducted two rounds of both stress tests described above. The first round was a Discovery round in which I tested five basic plaster mixes. The idea was to gain a hands on feel for the materials strengths and weakness, from which I could form hypothesis to improve the plasters. Educated by the results, I made 26 variations of my best recipes, applied these to more slipped particle board, allowing them too to dry for several days. I then tested these 26 plaster mixes. From the results of this Decision round, I selected the two best recipes for the experimental wall. (See Appendix II: Experiments, for experimental Recipes, Hypothesis, Test Results, Analysis, Conclusions, etc.)

Getting the Color Right[edit]

Part of the requirements for this project was to match the wall color to the school stucco color, a beigish pink. This I probably had the least success with, though as close as I got it really wasn't bad at all. To start it is necessary to first make a lime-wash. (Recipe under Appendix I: Important Recipes.)

The first run through I missed the pink tone. I used combination of yellow iron oxide, yellow ochre, and burnt umber mineral pigments mixed in quantities of 1/8 cup with between 1-4 cups of lime-wash. This created various beige tones, that when allowed to dry and compared against nearby columns revealed my need to move in a pink direction. That failed first batch consisted of eleven different samples. However, I did learn enough to try more precise ratios the next go, my mixes at one cup lime-wash were obviously too dark, as well as too beige.

The pigments are powdered minerals, mix them completely into water before adding to paint recipes to avoid ugly color clumping.

The second batch consisted of 15 samples- 3 came close. The first of the three consisted of 3 cups lime-wash, 1/8 cup yellow iron oxide and 1/8 cup red iron oxide. This recipe was a little too dark. The next two were very close in tone and I could have opted for either. Mix two was 2 cups lime-wash, 1/8 cup yellow iron oxide, and 1 tablespoon red iron oxide. The third mix was 2 cups lime-wash, 1/8 cup yellow ochre and 1 tablespoon red iron oxide. I chose this last mix because yellow ochre was cheaper than yellow iron oxide. I call it School Stucco Pink. [converted to 1 gallon, Recipe under Appendix I: Important Recipes.]

During the color test phase I was advised and given pigments by the professors of the campus ceramics lab, to whom I offer my gratitude. When I actually readied to make my lime-wash for the color coat, I bought pigment from the local ceramics supplier.

(Note: since I’ve been back, maintenance has now painted the columns near CCAT to the much more beige tone I’d originally gone for. I’m guessing now that my first color attempts were based on stucco closer to the ceramics shop, which colors I revised to pink when I saw the column color nearer CCAT. Maintenance have now apparently carried the more beige scheme over to the CCAT end of Campus leaving my poor wall behind and now noticeably pink.)


Thanks to Pennelys Goodshield of Sustainable Nations for invaluable training and advice, as well as to, Myles Danforth and Lonny Grafman.

  1. 1.0 1.1 1.2 1.3 Goodshield, P. (2009). Workshop Director, Sustainable Nations, ( (K. S. Perry, Interviewer) Arcata, CA. First I took I work-shop with Ms. Goodshield on natural plaster and lime-wash. Then I was able to interview her several times and she stopped by the site to consult once.
  2. 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 2.12 2.13 Guelberth, Cedar Rose & Dan Chiras. The Natural Plaster Book: Earthen, Lime, and Gypsum Plasters for Natural Homes. Gabriola Island, BC, Canada: New Society Publishers, 2003.Of all the books I read through, only one did I find myself drawn back to over and again.... This book seemed to have, compared to others, the most agreed upon and middle-of-the-road recipes; I derived all my basic formulas from it and barely scratched the surface.
  3. Danforth, Myles. (2009, Spring). (K. Scott Perry,Interviewer) Arcata, CA. Myles was knowledgeable and had plenty of helpful advice.
  4. Grafman, Lonny. (2009, Spring Semester). Engineering 305. Humboldt State University, Arcata, CA. This class provided much useful basic information for this project.
  5. Danforth, Myles. (2009, Spring). (K. Scott Perry,Interviewer) Arcata, CA. Myles was knowledgeable and had plenty of helpful advice.
  6. Mallinckrodt Baker, Inc. (2009, September 23). Material Safety Data Sheet, Calcium Oxide. (Environmental Health & Safety, Producer) Retrieved December 19, 2010, from [[1]]. Technical specifications of lime from a chemical / industrial standpoint.
  7. Lyons, Arthur. (2007). Materials for Architects and Builders (3rd ed.). San Diego, CA, USA: Butterworth Heineman. A University text designed for professional builders. Notable for its' background on lime, and chemistry equations for those interested.
  8. Engineering 114. (2006, Spring Semester). Humboldt State Univesity, Arcata, CA. This class provided a knowledge base that contributed to the project.
  9. Grafman, Lonny. (2009, Spring Semester). Engineering 305. Humboldt State University, Arcata, CA.
  10. Danforth, Myles. (2009, Spring). (K. Scott Perry,Interviewer) Arcata, CA.
  11. Grafman, Lonny. (2009, Spring Semester). Engineering 305. Humboldt State University, Arcata, CA.
  12. Danforth, M. (2009). CCAT natural wall construction. Retrieved December 19, 2010, from Appropedia: ( The Appropedia page documenting how Myles built the wall I would plaster and paint.
  13. Grafman, Lonny. (2009, Spring Semester). Engineering 305. Humboldt State University, Arcata, CA.

Also see CCAT Natural Exterior Wall Plaster and Paint, part 2; or skip to CCAT Natural Exterior Wall Plaster and Paint, Appendix.