The revitalization of terra cotta as architectural ornamentation occurred around 1900. Because of its light weight, sculptural qualities, fire resistance and wide range of colors, it was ideal to sheathe and decorate the new skyscrapers. In Portland, the revival was particularly popular from 1905 to 1930.
The process of creating terra cotta blocks begins with an architectural drawing which is used to create a clay model. If the final blocks are to be used in a specific building, each piece is numbered to show its exact placement on the building. To allow for shrinkage during the drying and firing of the block, the model must be enlarged. To do this, the artisan uses an expanded ruler called a “shrinkage rule”. Plaster molds are then made by pouring liquid plaster over the clay model.
Terra cotta, which means “baked earth”, is made from fine-grained clay mixed with grog. The grog consists of ground bits of already fired clay (such as bricks, sewer tile and fine china) and is added to give body and reduce shrinkage. Blocks are formed by hand pressing the clay into the plaster molds. After pressing, the blocks dry for several hours in the mold and are then turned out onto wooden racks for further drying.
Glaze is applied at this point. To give a mottled effect, several colors of powdered glazing material can by sprayed over the wet glaze. The terra cotta is then fired to about 2035 degrees F. in a tunnel kiln, through which the blocks move slowly for 3 1/2 days. After firing, the blocks are checked for size and trimmed when necessary. After passing inspection, they are ready for shipment directly to the building site.
Terra cotta is installed by hand. The size, therefore, is limited to what a man can lift. Hollow terra cotta blocks weigh approximately 70 pounds per cubic foot. This compares to the weight of 150 pounds per cubic foot for stone.
The greatest danger in the use of terra cotta is the possible rusting of the iron rods, which tie the blocks to the building. As they rust, they expand and can actually pop the blocks off the building. However, with careful maintenance of joints and flashings it can last indefinitely.
Today the use of architectural terra cotta is limited to restoration work. Buildings constructed after the Depression and World War II have utilized large-scale, assembly line products rather than the hand made decorative elements.