The architect's complete scale drawings and steel framing plans are furnished the manufacturer, who, following the design, makes scale shop drawings showing the jointing and construction, and full size details to the proper shrinkage dimensions. These drawings are submitted to the architect for approval before proceeding with the work.
Full size models to shrinkage scale are made of plaster tor each different shape shown on the shop drawings. Over these models sectional moulds of plaster are cast, from which later the required unrulier of pieces of Terra Cotta are produced.
From the architect's drawings or sketches, in the style and period indicated, modelled ornament is applied in clay to the face of the plaster models. Photographs of the ornamental models are submitted to the architect for approval or he may personally examine these models at the factory- the soft clay permits of such corrections or improvements which may be desired.
The mixture of clays and fusible minerals used in forming the Terra Cotta is carelully selected and proportioned to give the desired degree of plasticity and a composition which, when Bred at high temperatures, will produce a homogeneous body, amply strong to carry the required structural loads.
The foregoing processes are preparatory to actual production, the first step of which is pressing. This is a manual operation and consists of pressing the plastic clay into the mould. The walls of the pieces should not be less than one inch thick, following the contour of the mould, and the partitions should be of such thickness and so spaced as to perform their proper functions with regard to form and structure. The pressed piece remains in the mould until the clay stiffens. It is then removed from the mould and is skillfully retouched. Then it is placed in driers, where the moisture is evaporated.
From the drying process, the Terra Cotta passes into the spraying department where, by means of compressed air apparatus, the exposed surfaces are coated with the ceramic mixture which, during the firing process following, develops into the desired color or glaze.
These colors or glazes arc prepared with scrupulous care, according to exact ceramic formulae. The variety of shades and textures which may be obtained opens up an unlimited field of permanent color design in architecture.
Following the coloring process, the Terra Cotta is fired in kilns where it is subjected to a temperature rising gradually to 2,000 degrees Fahrenheit or more, depending upon the temperature of maturity of the clay and glaze. After proper firing, the kiln is allowed to cool slowly to normal temperature, an operation that causes a slow annealing of the Terra Cotta.
Terra Cotta is usually fired in periodic muffle kilns. In recent years, the tunnel kiln has been developed for the firing of Terra Cotta. In the latter type of kiln the Terra Cotta is set or loaded on cars, which travel through a long heated tunnel.
From the kiln, the Terra Cotta is removed to the lilting department, where it is laid out and marked to correspond with the piece numbers shown on the shop drawings. It is also marked to indicate the position it is to occupy in the building. Where required, the joints are squared, or cut to proper alignment and size, either by hand or grinding. Careful fitting is essential to assure satisfactory results in the erected Terra Cotta.
For rail transportation, Terra Cotta is usually shipped in bulk, securely packed in hay and braced to prevent shifting.
Upon arrival at the building site, the hay should be removed and the Terra Cotta placed in the order marked, in piles on wooden strips.
For export by vessel, the Terra Cotta is usually packed in boxes or crates, according to the special conditions encountered. Another method that has been found to be economical and entirely safis-factorv is to ship the Terra Cotta loose after it has been wrapped and tied in corrugated cardboard.
The appearance of erected Terra Cotta is greatly affected by inaccurate setting and defective pointing of the mortar joints. As the individual pieces of Terra Cotta have been carefully tilted and numbered to correspond with the erection drawings, the Pieces Must Be Erected In Accord With The Numbers Thereon if satisfactory results are to be secured.
The Terra Cotta manufacturer will contract to submit shop drawings for approval within a fixed lime after receipt of the architect's drawings and other required information. All shipping dates are computed from the dale of receipt by the manufacturer of architect's approval of shop drawings and complete data on color and texture desired. Work cannot be definitely scheduled for production until all essential information is on hand. The process of manufacture may take from six to ten weeks, depending upon the size and architectural character of the order.
A Standard Specification for the Manufacture, Furnishing and Setting of Terra Cotta and a standard form of contract have been adopted by the National Terra Cotta Society. They are recommended for general use. A copy of either may be secured by addressing the Society. The specifications are incorporated in this volume.
Terra Cotta factories arc conveniently located in the Eastern, Central and Western sections of the United States (see list in bach of this volume). All of the Society's membership will be glad to have any architect or designer interested in the processes of manufacture of Terra Cotta visit their plants.