Masonry Magazine August 1985 Page. 7
1910 to 1930
Until recently, terra cotta has been used sparingly. There are still a tremendous number of terra cotta buildings in service today and many are undergoing renovation.
Multi-step Process Needed To Make Terra Cotta
After the clay mixture is aged and milled, it is processed into slugs ready for use. A mold made of plaster of Paris, a negative of the model, is packed with a nominal 1½" thickness of the clay. Figure 7 shows a mold for making a terra cotta antefix. This mold and model are constructed about 11% oversized to compensate for clay shrinkage. If the element is an ashlar block, it normally would measure 24" x 18" with a 6 depth. Webs of clay are positioned in the hollowed-out back for structural support.
After 6 to 12 hours the clay would release itself from the mold. Once out of the mold, the piece would be touched up by hand. Figure 8 shows a modeler touching up an unfired terra cotta antefix. Note the size difference between the fired and unfired elements. From this point the piece would go into a drying room maintained at 90°F. It will dry from 8 to 14 days, depending upon size and amount of detail. Figure 9 shows some unfired terra cotta on drying racks prior to firing.
The element is now ready to have a glaze applied either by spray or brush. The glaze might be a dull clay base called a slip or a gloss or semi-gloss metallic glaze. From here, the terra cotta is ready for the firing process where its temperature is gradually raised to over 2,000°F during a two-week period. The terra cotta becomes vitrified almost like a fine dinner plate. In some cases, the material must be fired again for the application of a second glaze.
The production time from shop drawing and glaze approval is typically 8 to 12 weeks. A good portion of this time goes into making the model and mold.
Restoration of Terra Cotta on Existing Buildings
Terra cotta is durable and permanent because of the excellent weathering properties of a vitrified clay body and the hard surface of the glaze. However, terra cotta on some highrise buildings has deteriorated for one or a combination of three reasons:
* Many of the details considered good practice at the time these terra cotta buildings were designed are not considered good today. Many times support angles and expansion joints were not to be found in the entire facades. See Figure 10 for good practice.
* Terra cotta was anchored with straps and supported on shelf angles. See Figures 11 and 12. In many cases, these metallic members were fabricated from carbon steel and continued on page 36
Figure 8 Figure 9
Figure 7
Figure 6