62 THE ESSEX NATURALIST.
Group D. Five pieces, soft, uniform red, glaze defective or
absent.
Q. Made from different materials that have reacted
imperfectly under firing conditions that would
otherwise be suitable. App. Dens. 1.85
% Figure 100
Group E. Four pieces, hard, uniform black.
Q. Overtired in a reducing atmosphere.
App. Dens. 2.12
% Figure 100
The differences in apparent density are difficult to interpret.
As regards the black colour of certain of the fragments, Dr. R.
Lessing is very confident that the black colour of the brick,
referred to in the Table, is due to iron compounds, probably
ferrous silicates. On the other hand, Dr. A. B. Searle, who
has been so kind as to afford me valued advice and help in drafting
my remarks, is of opinion that the high apparent density of
"black ware" is probably due to more of the pores being filled
with molten material (since solidified), this material being formed
in larger quantities by firing in a reducing atmosphere than when
ware is burned in an oxidising atmosphere. He differs from
Dr. Lessing in suggesting that the black colour is chiefly due to
carbon and only to a very small extent to ferrous silicates.
The presence of a reducing atmosphere was common in some
parts of Roman Kilns, whilst oxidising conditions prevailed in
other parts. In firing pottery "in the open" (i.e. among the
fuel, but covered with inverted vessels) reducing conditions are
much stronger and complete; oxidising conditions are much
more difficult to secure (Cf. L. Franchet, "Cerantique Primitive"
(Paris, 1911) (20 ffr.) (see also footnote, "Hand-made Pottery
in Jutland" ante).
APPENDIX.
I am indebted to Dr. A. B. Searle for the following statement
on the characters of the raw material used in the manufacture
of the crude red ware found on the Canvey Red-Hill mounds,
and the technique employed in the manufacture of the Crude
Ware (b)—"briquetage":—
The deposits vary in character from an alluvium containing only a
very small proportion of sand and a large proportion of silt to a typical