Lime Kiln Charging Floor.
The Lime Kilns.
The first stage in the softening
process took place in the lime kilns, where chalk was heated to a high temperature,
the 5 kilns at Twyford have identical internal dimensions although the exteriors
appear different. The kilns are of a design known as "flare" kilns
and are of the intermittent type, that means the kilns are not burning continuously
but are loaded, heated up, cooled down and emptied.
This results in an inefficient process both in the use
of the kilns because they are not producing lime continuously and in fuel,
because of the heating and cooling of the kiln structure, the alternate heating
and cooling of the kilns caused much stressing of the brickwork and to strengthen
the kilns, ironwork was introduced.
In several places there are wall-plates
which in conjunction with tie rods passing completely through the structure,
hold it together. Iron hoops surround the domes and chimneys to the kilns
for the same purpose.
The kilns themselves are built of ordinary bricks on
the outside but lined on the inside with firebricks, there is a fire grate
at the bottom with an ash pit beneath, then above it the part of the kiln where
the lime burning took place and at the top, a dome with a chimney. The kilns
were charged (loaded) through the doorway at charging floor level, the doorway
being bricked up during the charging.
Flare kilns are usually fired from the
stokehole for the whole burn of the kiln, however it seems that at Twyford,
layers of fuel were intermixed with the chalk to assist in the burning, an
unusual method of working with this type of kiln. The chalk for the kilns had
to be in lumps of at least 3" (80mm) across because it is very important
to have a free flow of air through the kiln while it is burning, to carry gases
away.
When charging a kiln, first of all a tunnel of chalk was built over the
firebars at the bottom of the kiln. Once the tunnel had been constructed, layers
of chalk and coke were placed alternately, which continued until the kiln was
full, When charging was complete, a coal fire was lit on the firebars which
caught the the layers of coke alight and the fire travelled up through the
kiln. The kiln had to reach a temperature of at least 900°Centigrade [1650°Fahrenheit]
and be kept at that temperature for one or two days to-allow chalk in the
centres of the lumps to change to quick lime.
The temperature was controlled
by the air-flow through the kiln and the quantity and position of the fuel,
for efficient burning it was also important to maintain an even temperature
across the kiln.
Once the limeburner judged that the burning was complete, the kiln was allowed
to cool down and the lime removed. Kilns were emptied both through the charging
door and the stokeholes, the lime being removed to the lime store either in tipping
wagons or manually in baskets. Loading the kiln, firing, cooling and emptying
the kiln took about one week.
