ROMAN POTTERY FOUND IN BRITAIN. 19 
This is stated to be the process by which the glaze or lustre is 
produced on red terra sigillata or ‘ Samian.’ 
3. — Black varnish or lustre. Vessels of ordinary clay in biscuit 
state are dipped in a watery solution of ‘ ground iron ’ or ‘ bull¬ 
dog ' (impure silicate of iron from the smelting furnaces), and re¬ 
tired ; or haematite is used in the dip as the colouring ingredient 
and reduced to black iron oxide by ‘ fuming ’ at the end of the 
firing process." 
4. —Facing with I hematite. A process going back, in Egypt, to 
the earliest pre-historic period, of baking clay vessels in an open 
lire, mouth downwards, produces a glossy black on the inside and 
upper portion of the outside nearest the rim, and a soft red on 
the lower portion of the outside nearest the base, after they have 
been coated with haematite.! 
* Welcker, dealing with the finds of ordinary pottery from the ovens before 
the north gate at Heddernheim, where the potteries were at work from about 
A.D. 133 to A.D. 200, infers that the black colour-coating on large and small 
urns was obtained by entire or partial dipping previous to the second baking, 
the vessel being held upside down between the fingers in the solution, as shown 
by the marks of the latter, and the streaks which have run down on the 
uncoated portion of the base. 
A black-brown epidermis was found on indented goblets, which had 
apparently been first coated with white slip and then fumed dark. 
Folded goblets, unguent pots, and other vessels with bands of roulette 
hatching, and a metallic glistening, slightly iridescent glaze-like coating, had-a 
rough matt surface in places where the fine coating had been driven in by 
excessive heat, while the glistening surface was preserved in the protected 
portions. 
On all the examples of varnishing or slip-.coating that occurred, there was 
the appearance of a colouring ingredient being added differing from that of the 
vessel. Hedd. Mitteil, IV., p. 118 ff. 
It may be added that, in the opinion of expert chemists, the iron oxide 
combines with the surface of the clay body under the influence of carbonyl gas, 
or inappreciable quantities of salt, potash, or borax present in the fuel, to form 
ferro-silicate, which is a very stable compound—a sort of iron glaze or impure 
opaque glass. The slight iridescence observed by Welcker on some of the 
wasters at Heddernheim confirms this opinion. 
f This is described by Prof. Flinders Petrie, The Arts and Crafts of Ancient 
Egypt, 1909, p. 130, as one of the methods employed by the ancient Egyptians 
of baking and colouring their pottery. The same authority furnishes the 
following explanation of the chemistry of the black and grey body colours, and 
black lustre or varnish on ordinary clay vases of the Roman period. 
He says it is “ probable that all black pottery is due to black oxide of iron 
produced by imperfect combustion. Smoke—or fine carbon dust—could not 
possibly penetrate through close grained pottery, and the black extends all 
