by Michael Imwalle
The previous installment of this series briefly described the process of extracting, or mining, clay and processing it to prepare for the fabrication of roof and floor tiles. This post will briefly describe methods employed for molding, forming, and drying tiles prior to firing. In contrast to clay for making pottery, which must be kept moist and pliable during pottery production, clay for tiles needs to be relatively stiff and free of excess moisture. Clay used for the fabrication of roof tiles, or tejas, for example, needs to be moist enough to push into trapezoidal-shaped templates, but stiff enough to remove from the templates and place onto semi-cylindrical, tapered molds. Once the desired barrel shape is realized on the mold, the wet tile was removed to dry on the ground or on racks. Like the roof tiles, floor tiles, or ladrillos, were formed by pushing relatively stiff clay into square or rectangular molds then removing the tiles to air-dry before firing.
Tiles were probably made relatively close to where the clay was mined and where the tiles were ultimately fired in kilns. Kilns are typically located close to the clay source (see previous post for possible clay source location) and downwind from residences to avoid the risk of fire and excessive smoke and ash. Kilns are also located relatively close to construction sites so as to cut the cost of transportation and reduce the amount of breakage during delivery. We know from archaeological and historical evidence that ladrillos were fired for the presidio aqueduct by 1784. By 1786, thousands of tejas had been installed on the first wing of the adobe quadrangle and thousands more were ready for firing.
During the presidio era, templates, or forms, for roof tiles would have been constructed of hand-planed lumber fashioned to the desired thickness of the tile. Molds for the tiles could be fashioned in a number of ways, including forms carved from a single piece of wood and forms assembled from semicircular pieces of wood attached with wooden slats. Legend has it that roof tiles were formed on the thighs of “Indian women.” Although forms could be fashioned from a variety of materials, according to archaeological evidence, thighs do not appear to have been one of them!
During an attempt to replicate tile manufacturing and firing processes for the Smithsonian-sponsored “Ceramics Rediscovered” exhibit, SBTHP archaeologists used plaster of Paris to make a mold of an original presidio roof tile to insure accuracy in the replication process. Each 24-inch-long roof tile that tapers in width from 12 inches to 8 inches takes approximately 25-30 pounds of wet clay. Due to the fact that the wet clay shrinks about 10-12 percent when it dries, the molds need to be made slightly larger than the desired finished dimension of the tiles. Once the tiles have been dried and fired their weight is reduced to approximately 18 pounds each.
It took more than 75,000 tejas to roof the buildings and outbuildings of the original Presidio quadrangle. That required 2,250,000 pounds (1125 tons) of raw clay to be extracted and processed to make the tiles. To find out how and where all those tiles were fired, stay tuned for Part 3.
Michael Imwalle is the archaeologist at the Santa Barbara Trust for Historic Preservation.