Earthen Architecture - Glossary of Terms

Join the tradition! It's lasted thousands of years and for good reason.

Soil

Soil refers to the upper crust of the Earth's surface. It is one of the oldest and most common of building materials. It consists of clay, silt, sand, gravel, and organic matter. The word soil is preferred over dirt or mud especially when applied to a built environment. The soil near Tumacácori is rich in clay and aggregates thanks to the Santa Cruz River, and local inhabitants made ample use of the material.

The distribution of aggregate size is important in determining the durability of adobes and lime mortar. Why would the aggregates with varying size (and sharp edges) make adobes more durable? (Hint: It is the same principle that makes your brake-pads work).

Aggregates

Aggregates are particles broken down from rocks through natural erosion and sedimentation. They are categorized by size. Sand is the most common and comes in different shapes, colors and sizes. Thanks to volcanic activity in the area long ago, there are a variety of particles available for building construction. Aggregates can be held together by cementitious materials, such as clay.

Lime putty is prepared by burning limestones in a kiln such as the one shown in the image. The resulting quicklime is mixed with water to create lime putty. The lime kiln was built into a slope with an opening at the bottom (an arched opening in the photo) for easy access.

Cement

Cement is a glue that binds aggregates together. Two common types are clay cement (used for adobes) and lime cement (used for lime render). Portland cement is a type of lime cement and when mixed with aggregates and hardened becomes concrete.

Soil, complete with clay and sand, is mixed with water, often with bare feet. In Sonora, adobéros, or adobe makers, still practice this tradition.  Adobes are labor-intensive, but can be made easily by anyone who does not mind getting their hands dirty.

Adobe

Adobe is a sun-dried soil block. It is a molded (usually in rectangular shape) mixture of soil and water that is dried to harden. They are easy to store and to build walls with. Spanish-era adobes are easily recognizable by their dimensions. When adobes are fired, they become bricks. Adobes are laid in bedding mortar.

However little, these ‘islands’ of original render are crucial evidence in imagining how the Church looked like in its hey-day. The image shows the painted lime render inside the Sanctuary with at least five different colors.  Can you name all the colors?

Render

Render is a protective coat on a wall and can be applied on both interior (plaster) and exterior (stucco). Lime render is common in Tumacácori, although soil render is popular in other parts of the U.S. such as New Mexico.

A render is prepared by mixing cement and aggregates. The mixture can be applied on a wall with certain thickness, visible to the eyes. The render can be painted on, either before it dries (al fresco) or after it dries (secca). Tumacácori's painted finishes were mostly done with secca technique; as a result the paint layers tend to ‘peel’ off (see figure).

A render is applied in a plastic state and becomes solid upon curing. The gradual change in state gives rendering incredible versatility to protect and to decorate buildings. Note: Render is different from wash.

Gypsum wash, or ‘yesso’, is a common interior finishing technique (the same material as your drywall). Conservators noticed a brush-work pattern in the choir loft, shown as parallel lines from bottom-to-top. They also noticed incised sketches on the wash that would have guided the mural painters.

Wash

Wash is a very thin surface coat, and unlike render, it is liquid. Therefore, it is applied with a brush or other similar tools. Lime wash is almost translucent. It contains very fine (nano) lime particles suspended in water but invisible to the eyes. When the lime wash cures, it turns white. In Mission San José de Tumacácori, there are two gypsum wash coats on top of a preparatory lime render. In certain areas, tool marks for the wash coats are visible.

Can you distinguish between the mortar and bricks here?

Mortar

Mortar is similar to render in composition, but the application is different. While rendering shields and covers a wall, mortar bonds masonry units, such as bricks, adobes, and stones. As with render, mortar contains cementitious materials (like clay or lime putty) and aggregates. Bedding mortar goes in between the masonry units and can be recessed past the wall face. Pointing and repointing mortars are used to fill in the recessed areas between the masonry units. Although bedding mortar and repointing mortar can be of the same material and make-up, repointing mortar is meant to be sacrificial for replacement over time, while bedding mortar is meant to be more permanent.