1 8 ROMAN POTTERY FOUND IN BRITAIN. 
These sub-divisions frequently overlap, since two or three kinds 
of ornament are often found upon the sqme vessel. 
Plain clay vessels, especially those of early date, or with Late 
Celtic affinities, are ruddy ornamented with cordons, thumb- 
markings or indentations, grooves (horizontal, vertical or oblique), 
patterns scored with a blunt point or combed, stabbing or 
punctures, roulette notching or hatching, and they are sometimes 
burnished on the upper portion of the body, but these incidental 
peculiarities are not of a distinctive character, or capable of 
being used to deignate a class. 
The following is a brief explanation of the technique of the 
processes of colour-coating, compiled from the works of the 
authorities above quoted, and others of recent date, on which the 
present classification is based :— 
1. —Alkaline ov true glaze. Frit composed of clay vitrified by 
melting with alkali is ground to impalpable powder and mixed 
with finely ground red or black iron oxide in water. " With this 
solution, the vessels in biscuit state (after first firing), are coated 
by dipping, steeping, painting, or otherwise, and then re-fired at 
a temperature only sufficient to vitrify the glaze. 
2. —Red terra sigillata glaze. The vessels are generally composed 
of well levigated clay containing naturally 5 or 6 per cent, of red 
iron oxide in a free state, and after first firing, are dipped in a 
slip of similar clay containing naturally or by addition 7 to 11 per 
cent, of haematite. (The addition of an alkali, such as salt or 
soda, to the dip, though probable, has not been proved). The 
vessels are then re-fired. 
* The colouring ingredients are the same, viz., ferric oxide or peroxide and 
ferro-ferric or magnetic oxide, whether added in the form of haematite, ochre, 
or ‘ mine’ for red or brown ; or ‘ bull dog ’ (impure silicate of iron), scale, or 
ground iron for black or grey. 
This was the glaze applied by the Greeks on their famous red and black 
vases, and later by the Romans on the so-called Rhenish ware, described in the 
preceding Report, 1910, plate VIII., No. 7, a.b.c. 
An American archaeologist, Oliver S. Tonks, who has published the result of 
his experiments in the Journal of American Archeology, 2 Ser., vol. XII., 190S, 
part 4, p. 420, states that ‘‘ S parts of nitrate of soda and 1 part of clay fritted 
together and mixed in the proportion of 2 parts of frit and 1 of ferrous oxide, 
produced a glaze identical with that on the Greek vases.” 
Its percentage composition from an average of Salvetat’s analyses in 
Brongniart’s Traito des Arts Ceramiques is as follows : silica 48*15, alumina 1190, 
oxide of iron 11*85, chalk 5*70, magnesia 2*30, alkali 17*10. 
