Decoding empirical knowledge : investigation about plastering clay walls in Latin America

Kurt Rhyner   

Plastering clay walls is an ancient art in Latin America, and the technique and expertise are in the hands of rural masons, who KNOW how to ensure that plaster adheres to the wall. Grupo Sofonias thought it worthwhile to seek out scientific material to “decode” this empirical knowledge.


In an investigation that involves partners in Guatemala and Ecuador, scientists and professors at two universities joined with the practitioners to study the situation. In both countries teams of architectural and engineering students took to the field to interview local masons and gather samples of plaster from walls in a multitude of villages.
They brought back hundreds of samples to test and classify and, at the same time, discovered areas and aspects of their country they did not know before, and a great interest in their architectural and technical heritage developed among them. They are now introducing those themes into their classrooms

The experience
The experience opened up several levels of interaction that are rather uncommon to university life. As the investigation tackled a theme that is common in popular terms, but very complex to characterize scientifically, much creativity was required at all levels. Undergraduate students became real actors in the investigation, looking into themes that even the tutors did not know much about, and of which international literature revealed little. This resulted in a creative endeavor that included village masons discussing directly with university professors and students, and it became clear to everyone that the rural masons had an important say in the discussion.

After a first meeting in Nicaragua in October where the scientific team decided upon the basis of the investigation, its methodology, field work and coordination, a selection of published documentation on the subject was reviewed and several relevant French norms and standards about plastering (with lime, cement, gypsum) were translated into Spanish.

Field work and testing
In Guatemala, four students visited a total of 22 rural communities in seven provinces, conducted interviews with masons and together collected local raw materials. In the experimental area of the university two adobe walls were plastered with the sample patches, using the different raw materials and trying to replicate the different recipes that had been furnished by the local masons. Similar activities were performed in Ecuador.

All raw materials were analyzed (granulometry, limits of plasticity, limit of Atterberg, some chemical properties) and classified in five different groups.

After some 45 days of hardening (during the dry season) a simple but efficient test was performed to measure the adherence of the plaster to the wall. In an interpretation of the procedure determined in the French norms, 400 identical metal plates were cut and 5 of them glued to each sample. With a pulley, a cable and a bucket, which was slowly filled up with sand, the pull was applied to each one of them and weighed. This gives on one hand a numerical result, but equally important is to analyze the way it broke.

All the results were tabulated and the team devised ways to weigh the different information obtained. This was not an easy task, as several different parameters had to be weighed, among them the subjective assessments made by the house owners, the masons and the students. A system was designed to place a value on each parameter, based on the relative importance and reliability of the results. An exhaustive analysis showed that the “exact measurements” were not necessarily the most important factors to determine.

All students were able to use part of the investigation for assignments, and two of them actually based their graduate thesis on it.

The results

The results are interesting and confirm most of the assumptions that led to the project. As yet the major hypothesis could not be proven, as the complexity of the issue and the short time span confirmed the need for the anticipated second stage, and there is much interest in pursuing the theme.

The findings can be summarized as follows:

  • The different combinations of clays and sands give a wide array of results, however all tests were done in a dry state, and the critical route likely is in highly humid environments
  • Plasters that were applied in two or three thin layers behaved differently from one-layer applications
  • There is no conclusive evidence that granulometry is the decisive factor in the quality of a plaster. However, this is still a likely hypothesis!
  • Chemical properties seem to be important factors.
  • Sands with varying degrees of puzzolana activity are an important factor

All participants strongly recommend entering a second stage where they would concentrate on mixtures that represent the different pre-selected groups.

The most important point for this stage would be to introduce procedures to evaluate the behavior of the samples in the rainy season, whereby the plaster is exposed to rain with alternating sunshine. The “bottleneck” in the application of clay in construction is humidity; clay looses most of its structural qualities when wet.

The research has been an extremely interesting process in which investigators from different countries, with diverse backgrounds and experiences, came together to investigate a traditional art in the construction trade. The experience of scientifically characterizing something apparently well known, but at the same time surrounded by secrecy, and in many cases even with superstition, was unique.

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