20 
ROMAN POTTERY FOUND IN BRITAIN. 
5. — Matt-black colour coating. The vessels in biscuit state are 
dipped into a slip or engobe consisting of impure silicate of iron 
in water, without any inter-mixture of clay or other ingredient, 
and ‘ fumed ’ or re-fired at a low heat, as in the previous instance. 
The matt-black coating on vessels of so-called Castor ware 
appears to have been so produced/'" 
6. Clay slip coating. The vessels after drying are steeped in a 
thin solution (slip or engobe) of the same or a different kind of 
clay, and then fired in the usual manner. 
Examples of handled pitchers of ordinary brick-red clay which 
have been coated with white clay slip of soft texture and easily 
rubbed off, are often met with. 
7. —‘ Fuming.' Vessels of ordinary clay containing the usual 
proportion, 4 to 5 per cent., of red iron oxide in a free state, 
which gives the clay its brown-red colour, are fumed, or heated 
in dense black smoke at the end of the firing process, and the 
clay-body thereby reduced to a uniform blue-grey colour, f 
The terra nigra (Belgic), Upchurch, and other black, grey- 
black, and blue-grey coarse unglazed wares are fumed wares. 
The practice goes back to the Early Iron Age or La Tene period, 
and continued to be employed to the end of the II. Century A.D. 
through the mass, naturally owing to the action of reducing gases to which the 
pottery is quite pervious ” Carbonyl gas (carbon monoxide), which is the 
result of imperfect combustion, has reduced the red per-oxide of iron to black 
magnetic oxide, such as is familiar to us in the black scale on sheet steel; 
carbonyl is a solvent of magnetic oxide of iron, and so dissolves and re¬ 
composes the surface facing. The iron on the haematite faced portions of the 
vessels is reduced to a brilliant mirror-like coat of black all over, except on 
those portions where the heat has lasted long enough for the oxygen to pass 
through the pottery, and the red colour of the haematite has been retained. 
* In modern practice ‘ ground iron ’ mixed with water is used as a dip for 
producing grey or half grey and half red roofing tiles :—the former, after firing, 
are dipped completely, and the latter half-way in the solution, and are then 
re-fired at a lower temperature. 
flhe mode of production of the blue grey bricks and tiles in common use 
at the present day is thus explained by Mr. W. Burton, F.C.S., in a paper 
entitled, “ The Palette of the Potter, 1 ' published in The Journal of the Society of 
Arts , 28th Feby., 1896, p. 323 :—“ In this case a clay which would in the 
ordinary course burn to a full red colour, is fired in such a way that dense 
volumes of black smoke are turned into the oven during the last hour or two of 
firing, with the result that the red oxide of iron in the clay is deprived of part 
of its oxygen, and is converted into another oxide of iron, the magnetic oxide, 
which is of a purplish black colour.” 
