Agriculture Reference
In-Depth Information
built to a height of up to eight courses. When half-dry the rows of bricks would
then be 'skintled' (scattered), set slantwise and further apart to complete dry-
ing. Hacking took between three to six weeks in which the brick lost about one
quarter of its weight (Hammond, 1981, 19).
Firing bricks in clamps still dominated where demand was large, but occa-
sional. Improvements in permanent kilns had led to the introduction of the
roofless intermittent up-draught 'scotch' kiln, which was essentially based on
the principle of the clamp (Woodforde, 1976, 60). This was more controlla-
ble and less expensive on fuel (wood and coal), than its less sophisticated pre-
decessors. As a general procedure the kiln-fired bricks would be lightly fired
for several days to prevent warping, then the heat was increased for two or
more days. The fire holes, which also acted as vents were then blocked and the
fire was allowed to burn out (Hammond, 1981, 22). The kiln had to be left to
cool down between seven and ten days before finally removing and grading
the bricks for use.
In the firing phase, however, more care might be exercised, such as placing
bricks to be used as rubbers within a protected area, what brickmakers term a
'box', within the top third of the clamp, to help ensure the desired temperature
(R Ireland, 2003). Alternatively, they might be reserved for a more controlled
firing in a kiln; still possibly set within a box. This special treatment allowed for
the extra price charged for rubbers, which could return a handsome profit over
their standard bricks (Bolton and Hendry, 1940, Volume XVII, 54).
The rubbing bricks may be considered as those baked to a point just short of
vitrification, within the kiln or clamp, either deliberately or naturally protected
from the more intense heat by their proximity to the other bricks that them-
selves went on to partial or full vitrification. A visit to a clamp that was in the
process of being unloaded in Boom, near Antwerp, in which over 650,000 bricks
had been fired showed the bricks to be carefully graded in terms of colour,
hardness, and other criteria for quality and loaded on to pallets for dispatch.
Amongst this wide variation of fired bricks it was possible to select bricks that
were capable of being easily cut and rubbed (Fig. 59).
Preparation of a rubber has been described by Lamb and Shepherd (1996,
68-70). The brickearth was wash-milled and pumped into a 'washback' lined
with sand, in which the material is allowed to settle and mature for several
months. More sand was added when moulding the bricks, and, after drying
they were kiln-fired at a temperature of 1,140ºC. This, however, is far too gen-
eralised a description, some of which is better suited to the second half of the
nineteenth century. Also the temperature of 1,140ºC is too high for producing
rubbers.
During this period the over-wintered brickearth, or clay, would have been
tempered to a soft consistency for moulding, utilising on-site sand to aid release
from the timber mould/form. The bricks, after drying sufficiently, would then
