Agriculture Reference
In-Depth Information
below ground level. Machine excavation is seasonal and enough clay is won to
weather outside and produce a full year's brickmaking.
The matured clay is ground down in a Craven wet pan, at which point water
is added to produce a slop that is stockpiled and soured for three days prior
to use. When required, it is loaded in to a mixer/feeder to be conveyed to a
pug mill in to which grogging sand is added to aid plasticity and open-up the
body to facilitate improved drying. The total sand content, including the natur-
ally occurring sand/silt is between 48% and 51%. Water is added to maintain
moisture contents of between 23.5% and 24.8%. Finally, in order to obtain the
colour range required varying percentages of fine coal dust (breeze) is added.
This breeze, it is considered by the author, should not be used as it causes
internal hardening within the rubber and so effects its quality.
The pugged clay is fed to the hand-moulders by belt feed, which they remove
as required. The clot of clay is rolled on the pre-sanded bench and then
thrown in to the pre-sanded timber mould with the excess being removed with
the wire bow and the brick released from the turned-over mould on to a tray.
All the handmade products, including the rubbers intended for the gauged
arches, are dried in computer-operated chamber dryers, with full temperature
and humidity control. The cycle time is 45 hours, with moisture levels reduced
from 24% to less than 2%. The gas-fired dryers can reach 90ºC but generally
operate at 65ºC, with a fully controlled airflow to monitor critical shrinkage of
about 8% on length, 10% on width, and 5% on height of a typical brick.
Firing can be carried out in any one of three kilns to produce the desired
product. For example, for a light orange, they might place the brick in the top
part of the intermittent or moving-hood kiln, yet for a redder hue bricks would
be placed to the centre of the beehive kiln.
The intermittent up-draught kiln is gas-fired with a capacity for 10,500
bricks. The downdraught moving-hood kiln is oil-fired with a 45,000 brick
capacity, and the downdraught, oil-fired beehive kiln has a capacity of 38,000.
In any of these kilns, the rubbers make up about 5% of the brick total. The
stacking position of the rubbers is chosen only to achieve a required colour,
rather than protection from over-firing. The firing temperature and time var-
ies with the kiln. For the beehive and moving-hood, a temperature of 1,050ºC
for two days is normal, yet, for the intermittent kiln, 1,020ºC for 12 hours are
standard. The expected loss also varies with kiln type, 3% with the intermittent
kiln, 8% for the moving-hood and 10% the beehive.
Michelmersh can supply oversized rubbers but generally offer machine-
cut gauged arches for on-site assembly. Their service extends to assisting the
designer by sending out a representative to establish a colour/texture match
and then determine the exact architectural requirements; with drawings utilis-
ing their on-site CAD program. All arch sets are numbered, dry-assembled in
delivery boxes for despatch to the customer, protected by shrink-wrapping.
