(b) Terra-cotta (unglazed),
(c) Bricks and unglazed tiles devoid of decoration.
In this sense, terra-cotta occupies an intermediate position between pottery and bricks, but no satisfactory definition has yet been found for it. Thus, bricks with a modelled or moulded ornament are, strictly, terra-cotta, yet are not so named, and some pottery is unglazed and yet is never classed as terra-cotta, whilst glazed bricks are never regarded as pottery. Again during the past few years, what is termed 'glazed terra-cotta' has been largely used for architectural purposes, yet this is really 'faience.'
Although this overlapping of terms may appear confusing to the reader, it does not cause any appreciable amount of inconvenience to the manufacturers or users, as it is not difficult for a practical clayworker to decide in which of the three classes mentioned a given article should be placed.
Partly on account of the lesser weight, but chiefly in order to reduce the tendency to crack and to facilitate drying and burning, terra-cotta articles are usually made hollow.
It is necessary that clays used in the manufacture of terra-cotta should be of so fine a texture that the finest modelling can be executed. Such clays occur naturally in several geological formations, and some may be prepared from coarser materials by careful washing, whereby the larger grains of sand, stones, etc., are removed. Some shales, when finely ground, make excellent clays for architectural terra-cotta, portions of all the better known fireclay deposits being used for this purpose. It is, however, necessary to use only those shales which are naturally of fine texture, as mechanical grinding cannot effect a sufficient sub-division of the particles of some of the coarser shales.
The finer Triassic 'marls' are also admirable for terra-cotta work, the most famous deposit being the Etruria Marl Series in the Upper Coal Measures near Ruabon.
The most important characteristics required in terra-cotta clays are (a) fine texture, or at any rate the ability to yield a fine, dense surface, (b) small shrinkage with little tendency to twist, warp or crack in firing, (c) pleasing and uniform colour when fired, and (d) a sufficient proportion of fluxes to make it resistant to weather without giving a glossy appearance to the finished product.
In large pieces of terra-cotta some irregularity of shape is almost unavoidable, but, if care is taken in the selection and manipulation of the material, this need not be unsightly.
The durability of terra-cotta is largely dependent on the nature of the surface. The most suitable clays, when fired, have a thin 'skin' of vitrified material which is very resistant to climatic influences, and so long as this remains intact the ware will continue in perfect condition. If this 'skin' is removed, rain will penetrate the material and under the influence of frost may cause rapid disintegration.