This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
Blocks of terracotta are now being frequently used in place of bricks, specially for the facing of buildings. The blocks should be so shaped as to form a good ad with the brickwork, or whatever material is used for the backing. They are usually de 12-18 in. long, 6-15 in. high, and 4 1/2-9 in. thick on the bed. These dimensions; suitable for bonding into brick backing. When the blocks are of the thicknesses have mentioned, the joints are made square and flush as in ordinary ashlar work. In ne cases, however, the blocks are made 6 in. and 1 1/2 in. thick alternately. A "lip nt," as shown in Fig. 1300, is then employed. This plan, however, is not often adopted, does it afford such substantial work as the other. The mortar joints may be celled as in Fig. 1301. Such joints throw off the water, prevent the terracotta from shing, and relieve the face of the work better than if the joints were full and flush wiih the surface of the blocks.
The advantages of terracotta are as follows : - (1) If properly burnt, it is unaffected the atmosphere, or by acid fumes of any description.
(2) If solid, it weighs 1-2 lb. cr ft. cube; but if hollow, as generally used, it weighs only 60-70 lb. per ft. cube, or If the weight of the lightest building stones.
(3) Its resistance, when solid, to co-mission is nearly 1/3 greater than that of Portland stone.
(4) Page found by experiment at it lost 1/16 in. in thickness, while York stone lost 1/4 in. with the same amount of tion. It is, therefore, well adapted for floors.
(5) It is cheaper in London than the better criptions of building stone. It is so easily moulded into any shape, that for intricate rk, such as carvings, etc, it is only half the cost of stone. On the other hand, terra-ta is subject to unequal shrinkage in burning, which sometimes causes the pieces to twisted. When this is the case, great care must be taken in fixing the blocks, otherwise the long lines of a building, such as those of the string-courses or cornices, which are intended to be straight, are apt to be uneven, and the faces of blocks are often " in winding." Twisted and warped blocks are sometimes set right by chiselling, but this should be avoided, for if the vitrified skin on the surface be removed, the material will not be able to withstand the attacks of the atmosphere, etc. Terracotta is made in several colours, depending chiefly upon the amount of heat it has gone through. White, pale-grey, pale-yellow, or straw-colour, indicate a want of firing. Rich yellow, pink, and buff varieties are generally well burnt. A green hue is a sign of absorption of moisture, and of bad material. A glazed surface can be given to terracotta if required.
Inferior terracotta is sometimes made by overlaying a coarsely-prepared common body with a thin coating of a finer and more expensive clay. Unless these bodies have been most carefully tested and assimilated in their contraction and expansion, they are sure in course of time to destroy one another; that is, the inequality in their shrinkage will cause hair cracks in the fine outer skin, which will inevitably retain moisture, and cause the surface layer to drop off in scales after winter frosts. Another very reprehensible custom is that of coating over the clay, just before it goes into the kiln, with a thin wash of some ochreish paint, mixed with finely ground clay, which produces a sort of artificial bloom, very pretty looking for the first year or two after the work is executed, but sure to wear off before long.