In late April, Bart Santello held a lime plaster workshop on his property in Arivaca. The goal of the workshop was to replace the existing lime plaster on the walls of his adobe out-building.  Since the original application of plaster three years ago, the coat of lime had separated from the adobe walls and thus required repair.  Possible reasons for the plaster failure included: inadequate curing of the original lime plaster coat (too rapid drying); incorrect lime to sand ratio (weak mix); use of river sand (rounded particles), as opposed to sharp particle silica sand; and/or the possibility of temperature extremes in the desert causing the lime plaster to expand and contract at a different rate than the adobe wall, thus causing the lime to pull away from the adobe.  These were the challenges facing the group as the workshop commenced!

First, the remnants of the old plaster was removed from the structure, using a hoe, an old trowel,  and stiff brooms.

The group discussed how a base coat should be formulated. It was decided to use an adobe mix of sand and clay similar to the original adobe block composition for the pumphouse, with the addition of lime to assist in the transition to the lime plaster coat to follow.

Volunteers included many of Bart's friends who are also interested in and currently working on natural construction projects. This is Peter putting the base coat over the scraped adobe blocks.

The whole crew got some serious trowel time.

Peg and Mary demonstrate their technique.

This is what the corner of the building looked like after the base coat was applied.  This transition coat was allowed to "set-up" firm (but with some moisture retained) before the lime plaster coat was applied.

As you can see, the texture is quite refined as the process nears its conclusion.

Alice is ready to mix in the next ingredient as Kyle, Arivaca's bamboo and cob specialist, discusses options for the lime plaster recipe with the group. 

 

In addition to the one part lime three parts sand mix, it was decided to add a traditional ingredient to assist the curing process by retaining water. Juice from the Nopal Cactus is a natural polymer that helps the plaster to dry more slowly.

 

The cactus was boiled for about 30 minutes, then the juice was strained through a burlap coffee bean bag. The resulting juice was very thick and slimey. It was thinned out with water at a ratio of 5 to 1 before it was added to the plaster mix.

Different combinations of lime, sand and Nopal juice were used on different parts of the building. Bart kept track of what was applied, and where, so that the results can be compared and analyzed.

It took most of the weekend to complete the transition and lime plaster coats. The pile of materials diminished as the work progressed. The photo on the right shows the application of the lime plaster coat over the base coat.

Here, Nicole "floats" the lime plaster coat with a sponge tool to bring the sand texture to the surface.

Finally, color was added to the wall using a colored clay pigment which was brushed onto the moist lime plaster.

 

Bart appreciated everyone's participation and help, and his friends enjoyed the opportunity to exchange ideas about methods for plastering an earthen building.

Fun was had by all!

 

Read more about the workshop in the June issue of the Connection ~ in Bart's "Desert Homestead" column.

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