All kinds of clays are used, varying with the different regions in which the pottery was made, and ranging in hue from black to grey, drab, yellow, brown, and red. In quality, too, the clay varies to a considerable extent, some being of a coarse, pebbly character. The red clay of the Allier district in France, where most of the Gaulish pottery was manufactured, is of a ferruginous nature; its natural colour is modified by baking, though it never becomes white.[[3083]] The pottery of St. Rémy-en-Rollat in that neighbourhood is made of the same white clay as the terracotta figures (p. [382]).[[3084]] In Italy, as a rule, careful attention seems to have been paid to the preparing and mixing of the clay, and in the glazed red wares it is uniformly good. In fact, the remarkable similarity in technique and appearance of this ware throughout the Roman Empire has led to the view that there can only have been one centre from which it was exported. Against this, however, must be urged the undeniably provincial and almost barbarous character of the decoration on much of the pottery found in Central and Northern Europe; and therefore, without denying that exportation went on, as it undoubtedly did, we should prefer to suppose that this red glaze was produced in some special artificial manner, such as by using red ochre or iron oxide (see below), the knowledge of which became common property. As Semper said forty years ago[[3085]]: “Not only did barbarians, Gauls, Britons, and Germans, learn to know and use Roman technique, but also Egypt, Asia, and the Greeks, already immortalised by their own pottery, dropped their local processes, and voluntarily adopted Roman forms and technique.” Clay and glaze, form and technical method, are in all parts the same; it is only the decoration that varies and reflects the spirit and taste of the locality.

Formerly it was thought that the red glaze was obtained in the baking, after careful polishing of the surface, and that special means were adopted to this end. In the kilns of Castor (see below) Artis thought that he detected contrivances for this purpose; but it is now generally agreed that the glaze is artificial, not natural. In ordinary wares and in the lamps a red glaze is produced by a mere polishing of the surface, and this varies in tone and lustre with the proportion of oxide of iron in the paste, and the degree of heat employed in the baking. But in the terra sigillata the red glaze reaches a high and uniform state of perfection. This seems to have been produced by a kind of varnish, the elements of which are not absolutely certain; but it would appear that the substance added to produce the effect was of an essentially alkaloid nature. This has been deduced by Dragendorff[[3086]] from a series of analyses made from fragments of different wares, both without and with the glaze; in the latter case the alkaloid constituents show a marked increase in quantity, whereas the proportion of the iron oxide and other elements remain constant. These investigations were made by Dr. Lilienthal, of Dorpat, on five fragments: (1) from a vase of the Republican period found at Corneto; (2) from a bowl of fine terra sigillata of the first century after Christ; (3) from a deep cup of the same style; (4) from late provincial ware of the second or third century; (5) from a degenerate fabric with rough clay and inferior glaze, the results being as follows:—

1. Without glaze[[3087]]:

(1)(2)(3)(4)(5)
Silica55·0852·8752·05454·7566·70
Clay earth23·1023·95—18·8221·01
Iron oxide14·134·7813·96614·485·89
Carbonate of lime5·2213·80—5·303·20
Magnesia0·752·351·8503·381·26
Potash0·790·891·8521·551·02
Carbonate of sodium0·280·450·5230·530·57

2. With glaze[[3088]]:

(1)(2)(3)(4)(5)
Silica54·18—51·92453·70—
Clay21·31——16·93—
Iron15·00—12·16814·70—
Carbonate of lime6·01——5·82—
Magnesia1·943·122·2015·722·05
Potash0·951·062·2101·821·27
Carbonate of sodium0·370·490·9210·620·69

It must be borne in mind that, although the final effect is due to the alkaloids, the red colour of the vases is produced by the iron oxide which was inherent in the composition of the clay, none being added with the varnish, as the quantities show. All the fragments also showed traces of manganese and sulphuric acid. Previously analyses had been given by Brongniart and Blümner,[[3089]] with results approximately similar, but not so definite. Fabroni had thought that the iron oxide was combined with a vitreous paste,[[3090]] and Keller, by practical experiments, essayed to show that borax was employed to provide the required appearance,[[3091]] and further maintained that the furnace at Castor already alluded to was used for dissolving that substance. He was not far from the truth, but the results obtained by Dragendorff seem to militate against his conclusions.

In any case the glaze is very perfect, of so bright a red as to resemble coral, and serving, as Blümner says, to enhance the ground colour where a modern glaze would only conceal its imperfect tone. It is so fine and so carefully laid on that it does not interfere with any outlines or details, in this again evincing its superiority to modern glaze. It seems to have been applied not with the brush, but by dipping the vase into the liquid.[[3092]] Black glaze, such as occurs on the earlier Italian fabrics (p. [481]), was produced from an alkaline silicate.[[3093]]

The ordinary unglazed wares were classified by Brongniart under four heads[[3094]]: (1) pale yellow; (2) red (dark red to red-brown; first century of Empire); (3) grey or ash-coloured (down to the end of the Western Empire); (4) black (mainly provincial). This distribution was in its general lines adopted by subsequent writers, such as Buckman[[3095]] and Birch, but was felt to be inadequate, and some slight modifications were adopted. For practical purposes, however, it will be found to work fairly well as a convenient method of grouping the commoner wares. None of them as a rule have any decoration. They will be considered in fuller detail in a subsequent chapter.