Agriculture Reference
In-Depth Information
depending on the size of the clamp and the strength and direction of the wind
etc. Such a clamp was generally used for firing only standard bricks; special
shapes (and roof tiles) needed to be fired in permanent kilns where open-
stacking and a more controlled burning was possible. Providing, of course,
that the correct raw material was suitable for making rubbers in the first place,
due to the lower overall firing temperatures, bricks capable of being cut and
rubbed were naturally found within certain zones of the clamp, or kiln. Once
the clamp was disassembled, or the kiln emptied, these were simply graded-
out and reserved for cut and rubbed enrichments. It was possible to burn
some particular shapes of green-moulded specials, such as plinths, in a clamp
by interlocking them so their arrangement provided minimal voids with a flat
upper surface for stacking above them; but this was not a common practice
(R Ireland, 2003). A kiln, depending on type and size, could vary in capacity,
typically from 8,000 to 30,000 open-stacked bricks; and, compared to a clamp,
a kiln fired the bricks more quickly, five or six days rather than weeks.
Bricks, during this period and for several centuries afterwards, were fired
longer and at much lower temperatures than their modern counterparts, from
750 to 950ºC (1382 to 1742ºF) being a typical range, as opposed to 1,000ºC
(1832ºF
) common today. This is why, in contrast to other bricks, rubbing
quality bricks are said to be 'baked' rather than burnt to a state short of com-
plete vitrification. By baking the bricks, they are heated sufficiently to remove
the plasticity from the material; hence when struck together they give a dull
thud as opposed to the ring that is normal to well-fired bricks. The lower his-
torical temperatures were due to primitive firing conditions and use of the
above fuels, particularly wood. Wood, such as hornbeam, silver birch, willow,
alder, sycamore, sweet chestnut and Scots pine, was the original fuel used for
firing of bricks. The advantage of timber over other fuels was, and remains,
that it burned slower with long even flames. This produced a gradual build-up
of heat that penetrated the stack of bricks, across a broad burning zone and,
as it burned away, gave an ash that was easily removed from the firing channels
and thus did not choke-up, cutting-off the vital airflow.
In addition, steam was released during burning from the mainly green
wood, as well as any remaining moisture within the bricks. This helped to lower
the temperature necessary for a good overall firing and promoted a beneficial
open-pored structure within all the bricks. This meant it was almost impossible
to fire bricks too hard and that the lower overall ambient firing temperature
achieved was ideal for the production of rubbing bricks. This is as opposed to
the use of volatile fuels like coal, and particularly fuel-oil, and liquid petroleum
gas (LPG) that are in common use by brickmakers today; that burn drier and
reach higher overall temperatures more quickly.
All of the above factors involved in traditional brick burning led to a wide
range of fired bricks, which, upon emptying the kiln or disassembling the
