Reasons for Designation
Glass has been produced in England since the Roman period, although field
evidence is scarce until the late medieval period. Wood was the main
manufacturing fuel up to the early 17th century, so the industry was located
in woodland areas, particularly the Weald. From about 1610, production shifted
to the coalfields.
Glass production requires three major components: silica, alkali and lime,
together with colouring material for certain products and decolourisers for
clear glass. Lead was also used in the production of certain types of glass
during the Roman period and after the 17th century. The manufacturing process
involves three stages, fritting, melting and annealing. Fritting was a common
practice before the 19th century involving heating the main glass constituents
to produce an unmolten material for grinding, melting and annealing. Melting
involved the remelting of previously formed glass, and the production of new
glass from raw materials. Until the late 19th century, glass was normally
melted in pre-fired crucibles of refractory clay, on stone benches called
sieges, within the melting furnace. Use of coal as the preferred fuel and
automatic bottle-making machinery in the 1880s led to changes to the melting
furnaces and the use of larger furnaces, hitherto conical structures over
circular furnaces. Regenerative furnaces were developed in the 1860s, and tank
furnaces for bulk melting quickly followed. Flat-glass production methods were
made obsolete by the Pilkington float-glass system of 1959. The third process
is annealing. Because the rapid cooling of molten glass can give rise to
internal stresses, glass was treated in furnaces designed to heat the glass to
a point where deformation begins, then cooled gradually. In the 19th century
conveyors were introduced to take glass through a hot zone into cool air.
Features on glass manufacturing sites include various types of furnaces,
producer-gas plants for the making of gas from coke at 19th century
glassworks, bottle-making machinery, blowing irons or pipes for blowing glass,
glass residues and various buildings used as stores or warehouses. A total of
135 glass production sites (representing about 25% of the estimated national
archaeological resource for the industry) have been identified as being of
national importance. This selection, compiled and assessed though a
comprehensive national survey of the glass industry, is designed to represent
the industry's chronological depth, technological breadth and regional
diversity, and to include all the better preserved glass sites, together with
rare individual component features.
The Number nine Tank House is the best surviving example of a late 19th
century glassmaking tank furnace building in England, comprising the
building, its cone and furnace. In addition there are the well-preserved
remains of the system of flues to bring in gas from the supplier. The
building now forms the centrepiece of a museum devoted to the glass
industry. It has been extensively excavated under modern conditions,
repaired and the outside features consolidated. In addition, many features
have been added to the display to give a comprehensive picture of the
glassmaking process in operation at the time of its foundation. The
glassworks is of prime importance in the historical development of the
technology of glassmaking and its status as a museum gives it a very high
value for education and recreational enjoyment at the heart of a
regeneration scheme for the town centre.
Details
The monument includes a brick-built glassmaking factory known as a tank
house, with a conical flue, containing the remains of a Siemen's
regenerative tank furnace dating from the late 19th century. It was set up
in 1889 by Pilkingtons the family of glassmakers, and continued in use
until 1920. In the history of glassmaking in Britain, this furnace
represents a radical departure from traditional glassmaking techniques.
The glass was formerly melted in large pots which were built inside a
furnace and as each pot became exhausted, another was built in its place.
In a Siemen's furnace the glass was melted in a brick-lined tank and was
continuously fed with material and continuously drawn off as required. A
second innovation in this design was the reuse of the heat used in the
melt with a resulting saving of fuel.
The tank was fuelled by a mixture of gas and air, heated before
introduction into the furnace, and this heat was reused to warm firebricks
in an underground system of flues before re-entry to the furnace. The
glass which was produced in this process was in the form of large (up to
2.5m long) blown cylinders which were then cut, flattened and polished to
produce large glass sheets.
The building which housed this process is known locally as the `Hotties'.
From the outside the building is rectangular, with a truncated cone
protruding through the centre of the roof. The walls are buttressed on the
long sides. It measures approximately 23m by 11m and stands 8m high.
Inside it is the cone, made of brick, on steel girders, supported by cast
iron pillars. The brick cone was constructed so that the base is square,
rising to an oval at the top. This cone is a survival of a traditional
form and has the benefit of deflecting some of the waste heat from the
furnace back down to the metal (molten glass) below. Beneath the cone are
the remains of the tank furnace, and on either side was a swing chamber
for the glass to be gathered (tapped on a blowpipe) and blown into shape
before being cut and flattened into plates.
Only the eastern chamber survives, since the area on the western side was
modified for later use as storerooms. Arches on each of the short walls
provided ventilation for the workers in the chamber and were bricked up or
unbricked according to the temperature outside. Beneath this tank there
are brick-vaulted tunnels to bring in gas and air from the southern side
of the building.
Outside the building, on the southern side, archaeologists have exposed
the tunnels through which coal gas was brought from the supplier to the
furnace. Immediately adjacent are the remains of a coal mine pit head and
cage shaft which were in use before the glass works were built. This pit
was used to provide coal for firing the earlier conventional furnaces
which were the mainstay of the glass industry on this site before the
adoption of the Siemen's furnaces in 1883. The Tank House is Listed Grade
II*.
The abutments of a modern footbridge and the modern portico leading into
the building, the surface of all footpaths around the site and the cast-
iron water pipes and their fittings on the canal side are excluded from
the scheduling, although the ground beneath these features is included.
Fixtures and fittings not original to the Tank House or its period of use
are also excluded.
MAP EXTRACT
The site of the monument is shown on the attached map extract.
