A sharp metallic, bell-like ring and a clean, close fracture are good proof of homogeneousness, compactness and strength. Precision of the forms is in the highest degree essential, and can result only from homogeneous material and a thorough and experienced knowledge of firing.
No spalled, chipped, glazed or warped pieces of terra cotta should be accepted, and the pieces should be so hard as to resist scratching with the point of a knife. The blocks should also be of uniform color, and all mouldings should come together perfectly at the joints.
It is impracticable, though not impossible, to make terra cotta in blocks exceeding 3 feet by 4 feet by 18 inches, and when the pieces exceed this size the cost is greatly increased. The Boston Terra Cotta Works have produced a column and capital of the Corinthian order, in white terra cotta, that was 14 feet 6 inches in height, the shaft being in one piece 12 feet long; but such large pieces require great skill and care in the manufacture and burning to prevent warping, and are very expensive. As a rule it is impracticable to span an opening of any considerable length in one block, and even window sills are generally made in pieces about 18 inches long. Jamb blocks should not exceed 1 foot in height or thereabouts. Mullions, transoms and tracery should be made in as many pieces as the design will admit, and if there are several members in the depths of the mouldings they should be as much divided as possible, care being taken that each alternate course bonds well upon the other. The strings and cornices should be divided into as short lengths (18 inches to 2 feet) as convenient.
The architect should show the jointing of the terra cotta on his drawings, the joints being arranged to conform with the above requirements, and the work should also be designed so as to form a part of the construction and to adapt itself as far as possible to being divided into small pieces. When used for trimmings in connection with brickwork it is very essential that the pieces shall be of the exact height to bond in with the courses of brick, and a small piece of brickwork should be built up, to get the exact heights, before the final drawings for terra cotta are sent to the manufacturers. All horizontal joints should be proportioned so as to be equal to about one-fourth the height of the joints in the adjoining face brickwork. For elaborate work it is generally necessary to consult with the manufacturers in regard to the best disposition of the joints.
Window sills, when made of several pieces, should have roll joints as shown in Fig. 171, which should terminate under the wood sills rather than against the edge.
Cornices. - Where buildings are trimmed with terra cotta the cornice is generally made of the same material. For cornices having considerable projection terra cotta possesses the advantages over stone of being much lighter, thus permitting of lighter walls, and in most cases much cheaper. With stone cornices it is necessary that the various pieces be of sufficient depth to balance on the wall. With terra cotta cornices, however, this is not customary, the various pieces being made to build into the wall only from 8 to 12 inches and being supported by ironwork. When modillions are used they may generally be made to support the construction, as shown in Figs. 172 and 173.
Generally small steel I or T-beams are used for supporting the projecting members, and where the projection is so great as to overbalance the weight of the masonry on the built-in end, allowing for the weight of snow on the projection, the inner end of the beam must be anchored down by rods, carried down into the wall until the weight of the masonry above the anchor is ample to counteract the leverage of the projection. Unless the wall is very heavy it is also advisable to anchor the top of the wall to the roof timbers to prevent its inclining outward.
Figs. 172 and 173* show sections of terra cotta cornices that have actually been erected and the manner in which they are supported.
Fig. 174 shows a section of the cornice on the Equitable Life Insurance Co.'s Building, in Denver, Col.
These sections may be taken as models of good and economical construction in terra cotta where a heavy projection is required.
When a cornice is to be supported by ironwork the method of anchoring must be decided on before the work is made, as provision must be made in making the blocks for inserting the beams or anchors. Generally the beams are placed in the joints in a slot made for the purpose. A copy of the detail drawings should be furnished the contractor for the ironwork, to enable him to get out his part of the work correctly.