The finishing touch – natural coatings for raw earth surfaces

workshop trainees applying cocciopesto, a traditional waterproof coating

Week 4 in workshop Design and Build with prefabricated rammed earth.

Once that both, the prefabricated and the locally rammed elements have been put in place the finishing process can start. In the specific case of Benevento's fixed furniture elements, this included the adjustment of all joints between assembled blocks. The edges of prefabricated rammed earth elements suffer slightly during the assembly process. This is natural and relatively easy to fix. The trainees managed to learn very quickly how to prepare an appropriate “repair-mix” to be used for this purpose. Mainly it is a mixture made with exactly the same ingredients than the rammed element yet with tow little differences: larger pieces of gravel are substituted by smaller stones or sand and more water is added than would be necessary for the ramming layers. This humid mix can be use to compensate the missing parts of rammed earth volumes. If well executed this adjustment is invisible once the mix dries. The same method can be used to repair other minor damages on the work. Generally rammed earth structures are exposed without coating in order to show their typical pattern of horizontal layers that come as a result of the progressive ramming process.

In certain circumstances, especially when exposed to strong weathering, an additional protection is necessary to avoid excessive erosion. During the workshop different transparent coatings have been tested. A specific element has been used as a sample surface to apply treatments with linseed oil, bee wax and a specific mousse that a small team of trainees decided to prepare using the gelatinous substance derived from the thick leaves of local prickly pear plants. This solution was based on a Mexican recipe where the same plant is known as “nopal” and widely cultivated. This vegetable hydro-repellent is generally used as an addition in clay based renders. In Benevento it was tested as a pure coating without hiding the supporting rammed earth surface. Empirical observations showed that raindrops leave the surface quicker and are not absorbed into the structure. However the coating is not completely rainproof. 

The challenge of coating part of the structure built for Casa Betania came up with the need for making the water basin of the element called “Acqua” waterproof. The design team decided to protect the inner surfaces and the adjacent parts of the structure with a layer of cocciopesto. This mix of hydraulic lime, volcanic sand and crushed fired bricks was the material used in ancient Rome to seal the inner surfaces of aqueducts and several built examples reached our days intact. During the workshop, AK0 decided to organise a specific seminar on this technique with the help of Mariastefania Bianco, an architect and entrepreneur in bioconstruction from Apulia in southern Italy. Mariastefania explained the working principles of pozzolan sands and fired clay in combination with lime and exposed different mix-recipes for the different layers of coating. The seminar crossed a complete application process from mix preparation, application of three distinct layers and final sanding. The designers expect the fine sanded cocciopesto finish to provide an suggestive contrast to the exposed rammed earth surfaces that will get a weathering patina during the years.