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Missions were generally made out of local materials. Originally, there were largely reed structures; often no more than huts. As the colonization continued and more native peoples were made available for work, the missions started to be constructed out of adobe but they still had reed roofs and dirt floors. Still later, after some of the uprisings, the mission roofs were covered with tile over the reed roofing; this helped to prevent fires and better protected the adobe walls which tended to basically melt in the rains. The dirt floors were eventually replaced with adobe/clay, tile, what's called Roman Cement, or wood or stone. Of these materials, adobe was the most common as it's basically dried mud made from local dirts with added vegetable materials.
Construction of an adobe building is actually fairly straightforward; although very labor intensive. Raw materials have to be gathered; bricks, tiles and lumber have to be made or worked; and then all the processed raw materials had to be put together into the building. Of course, all of this work required labor and also had to be coordinated with all the other daily tasks (e.g., tending to the agriculture and livestock plus prayers and education). Let's look at the basic ingredients.
What is adobe?
Adobe, by name, dates from the Spanish colonization times. However, adobe as a building material can be traced as far back as 7000 BC where it was known by the Arab word "at-tub" (or Earth bricks). There are also biblical references to mixing straw and mud for construction purposes. In Spanish cultures "at-tub" became "adob" and the ending "e" was added during the combining of the Spanish and American Indian cultures during the colonization period. So, what we know as adobe has its roots in pre-history as a building material.
Adobe bricks are largely local mud, usually mixed with some sort of vegetable additive (often straw but sometimes animal manure). Not all soils are appropriate for making an adobe brick. This sometimes meant that the missions had to import their building materials from other locations if the local soil was not found to be practical for making bricks. Soil with too much clay will typically cause bricks to shrink and crack badly as they dry. The soil you want for adobe is a loamy sand material as described in this table...
Note, particularly, that in all cases the percentage of clay is always less than a third of the material and the percentage of sand is always better than half sand. That's the soil type best for making adobe bricks.
Mixing of adobe generally takes place in a pit. First, the pit is soaked so the perimeter of the pit does not dry out the materials being mixed. Then, add soil and water to make a stiff mixture. Up to 3% or so of organic materials (usually short lengths of straw or some dried manure) can be added to the mixture. Soil, water, and additives are added until the mixing pit is full of the necessary mixture.
In the meantime, a drying area close by is set up on the ground. This area should be covered with sand or straw so the mixture will not attach itself to the ground while drying. On top of this forms are placed. The forms define the size of the bricks. Some typical sizes and the weights of the dry bricks are below...
The mixture is transported from the mixing pit to the forms and the forms are filled to the top and leveled. Since the mixture should be fairly stiff you can often take the form off of the mixture quickly and reuse it for additional bricks; the form is never left on while the bricks dry. Bricks should sit and dry for at least three days or more before they areS handled in any manner. After that, they sould be turned with care to help the drying process. Many bricks are broken during this stage of construction so any building will require the making of more bricks than are eventually used in the construction.
The most common way to keep the elements away from the adobe bricks was to coat them with a plaster outer coating. The basic Spanish plaster mixture was three parts sand to one part lime mixed with water. Lime was derived from quarried limestone and/or seashells which was burned in a kiln and then crushed to make the lime for the plaster. Once rendered soft and chalky the material is slaked in water, turning it into a wet mixture the consistency of glue. The sand was mixed with this. The plaster was applied in layers and built up to not quite one inch thickness on the outside of the structure.
The first roofing materials were reeds or lengths of willow tied together by rawhide. As insurrections proved, these thule roofs were highly vulnerable to attack by fire. To counter this, and to provide better protection from water, tiles were overlayed onto the reed roofs. The mission tiles (called tejas) were largely fired or dried clay. Clay was extracted from the soil using settling ponds or direct excavation in good locales. The clay was molded over logs and dried. These tiles where then laid onto the roofing alternating one row facing up and the next row facing down so they overlapped in a way that allowed the water to run down channels on the roof and fall off to the side of the building. The tiles overlapped one another with lower tiles slid up under higher tiles to allow the water a free path off the roof. This helped keep the adobe walls dry, the occupants dry, and the tiles prevented attack by fire on the roof. The tiles looked something like this...
Note also that tiles were not just used on roofs. In the lower right part of the picture you can see tile pipes that were used to direct water to specific locations. Note that the tile pipes are shaped so that the small end of one fits into the large end of another, forming a seal to prevent leaks (water flowed in the direction from the large end to the small end). Piping was formed on a potter's wheel. Tiles were also used for flooring at times (see just below).
It is a popular legend that these tiles were molded by the Indians over their bare thighs and afterwards cut to size and kilned, but this seems unlikely; the tiles vary considerably in shape, age and are often of a size which would be impossible to mold in this manner.
Many mission floors were simply dirt. The dirt excavated for the foundations was piled inside the building so the floor was somewhat higher than than the outside. This tended to keep the floor dry during the rainy season. Applications of light sprinklings of water did tend to create a seal as the dried soil surface became more like an adobe brick. In some missions further sealing was performed using regular applications of animal blood and/or linseed oil. The clotting factors in the blood created a better seal for the floor however it also added to the smell in the building (those missions tended to smell like fresh meat). These floors tended to have a reddish color. And, of course, dirt quickly developed wear patterns with use.
A bit more durable than dirt is clay and adobe; but, not much. Indeed, it's speculated that many mission adobe floors are actually nothing more than cast-off wall bricks that did not cure properly. Both clay and adobe floor surfaces had to be frequently filled as wear patterns appeared over time.
A modified version of plaster used on the side of a mission, the next durable substance used for flooring was called Roman Cement. Understand that Roman Cement is not the cement known today; it's more of a hardened plaster. The method for making Roman Cement was documented by the Roman architect Vitruvius; thus the name. Floor cement made to Vitruvius' recipe consisted of three or two parts sand to one part lime, depending on the quality of the sand (note how similar this is to the plaster formula). Clean pit sand is preferred but riverbed or shore sand could be used. Burnt brick was sometimes also added, giving the mixture a reddish color. Again, resurfacing was periodically needed as wear patterns appeared. This floor type was also more labor intensive in its initial construction and typically only used where labor was easily available.
Where trees were available, wood made the next most durable floor. By oiling and sanding the wood floors could be kept fresh and clean for long periods of time. Unfortunately, wood flooring contributed more to problems from fire. The other problem with wood is that it's even more labor-intensive to install. Not only does the wood have to be extracted from the trees, but it has to be isolated from the under-floor to keep it from rotting due to moisture infiltration. Once installed, however, wood was a very durable option.
Tile flooring was fairly durable; but would sometimes fracture if put under stress. Like adobe, tile is made from a clay mixture, formed in a mold and dried. The big difference is in the mixture and materials. Normal adobe put into a kiln would likely explode from the heating of the moisture in the material. Tile, on the other hand, gets hard when fired. Floor tiles were called ladrillos. A skilled artisan was necessary when tile was produced. (This picture is from Mission San Juan Bautista and shows a tile brick with an animal imprint in it; animals sometimes walked across drying bricks.)
Finally, the ultimate in flooring was stone. While field/river stones or cut flagstones may have been used in some settings, they were not suitable for use in the churches or other populated areas due to the possibility of injury caused by trip and fall accidents. Formal stone floors were quarried and fitted into place. A bed of sand or mortar was used under the stone. Once the initial labor of a stone floor was expended, maintenance and upkeep were minimal.
Site preparation was the first step in any construction project. Trees and brush had to be cleared and the area leveled. Water flows had to be created to keep water away from the building site. Finally, the building would be laid out in the ground.
Once the building plans were drawn in the ground the area under the building had to be excavated in order to lay down foundation stones. Foundations usually consisted of approximately six layers of stone.
Next, adobe walls were constructed on the foundation. The walls were built up using adobe bricks and mortar. Plaster was added to the walls to seal them.
Once the walls were built up the headers and roof framing was added.
Next, reeds were lashed to the rafters to form a base for the roof.
Tiles where then laid on top of the reeds to make the structure more waterproof (note the qualifier; these roofs leaked even with the tiles, just not as much as with reeds alone).
The interior was then finished and the building would be ready to occupy/use.
Of course, the process above is described as a linear process where one thing follows another. It would be an error to assume this was the way an actual building was constructed. Labor use, weather, and many other factors would have determined what got built when and seeing a partial building with part of a roof was likely a common site.