104 / 208
1 0 2
T H E B U I L D I N G C O N S E R VAT I O N D I R E C T O R Y 2 0 1 5
T W E N T Y S E C O N D E D I T I O N
3.2
STRUCTURE & FABR I C :
MASONRY
moulds, allowed to air dry, then fired.In the
Tudor period purpose-built kilns were too
small for the vast number of bricks required
for the large, prestigious buildings then being
constructed in brick. So, bricks were usually
fired in simple wood-fired ‘clamps’, with the
bricks stacked around fire tunnels and the
whole structure daubed in clay or covered
with turfs. Firing took several days and the
clamps were then allowed to cool slowly.
Temperatures in the clamp varied,
affecting the degree of vitrification – the
fusion of silicate particles which occurs when
making ceramics or glass. In ideal conditions
the degree of vitrification would be sufficient
to form a well-bound matrix. Over-firing
could lead to excessive vitrification in the
bricks closest to the heat, causing them to
slump, while under-firing could result in some
being too soft for external use.
The nature of local brickmaking in
clamps made brick quality less reliable
than that of later mass-produced bricks,
and slight variations in size and shape
required Tudor builders to use deep joints
to accommodate the irregularities. In this
way they were able to take the use of bricks
to new heights with great palaces such
as Hampton Court, Lambeth Palace and
Oxburgh Hall. Brick became an exciting
medium which allowed a new and dynamic
evolution in design and embellishment.
Some of the finest historic brickwork
is demonstrated in 15th-century chimney
building, first in larger buildings but later
in simpler vernacular buildings too. Timber
and plaster fire hoods and smoke bays, which
caught fire very easily, were replaced by
purpose-built chimney breasts and stacks
in brick or stone. Early examples were often
added as external projections. In areas where
good building stone was plentiful, brick was
often preferred for chimneys for its ability to
resist heat.
While stone was used throughout the
country especially for large and high status
buildings, domestic building in in much of
southern England was still very much geared
to timber framed buildings. However, after the
Great Fire of London in 1666 a ban on timber
buildings in the city promoted the use of more
fire-resistant materials and bricks came into
regular use. Aided by a growing shortage of
timber for building, attitudes to brick began to
change, first in the capital and then beyond.
Bell Hall at Naburn near York epitomises
the growing trend for building in brick in
England in the late-17th century. Built on a
stone undercroft, this Grade I listed house is a
striking gentleman’s residence which was built
for the then MP of York, Sir John Hewley in
1680. He had no doubt seen good brick houses
in London and went to Hull to find builders
in brick for this house. By then the quality
of production had improved substantially,
resulting in more uniform dimensions and
allowing thinner joints to be used. An indent
or ‘frog’ was introduced, primarily to improve
the moulding of the clay, but it also had a
secondary effect of very much improving the
grip of the mortar to the brick .
Regulations to control brick sizes were
introduced during the 18th and 19th centuries,
but the big changes came with an improvement
in quality in late Georgian times, perhaps
as a result of canal building and the need
for stronger bricks for engineering work. In
the industrial revolution that followed, the
massive movement of people from country
to towns and cities spurred vast building
programmes. Without this step-change in brick
manufacturing it is uncertain whether towns
would have been able to expand so quickly.
By then clamp firing was less common,
with the major brick manufacturers producing
large numbers of bricks in down-draft kilns,
with the bricks separated from the burning
fuel by a low perimeter wall. This resulted in a
product of more uniform quality and colour.
Production became more efficient
following the introduction of continuous firing
kilns in the mid-19th century, such as the
Hoffman kiln, Firing occurred in successive
chambers in rotation, with the heat from one
firing being used to preheat the next one. The
development coincided with the removal of a
brick tax which had been introduced almost
100 years earlier to help pay for the wars in
America, as well as rapid expansion in the
railway network. The use of brick proliferated.
The colour of brick is primarily a product
of the clay used and the amount of air allowed
into the clamp or kiln during firing. A brick
that appears red on the surface may have
a core of yellow or deep grey where little
oxidisation has occurred, and the colour
varies across the face too, as a result of the
movement of air in the kiln or clamp and the
way the bricks were stacked.
Regional variations range from pale buff
yellow in the South East, such as London’s
stock bricks, to the bright reds found from the
West Midlands to the great northern cities of
Liverpool and Manchester, such as Accrington
Nori bricks. ‘Staffordshire blue bricks’, on the
other hand, are almost black due to a firing
process which achieved high temperatures
and oxygen deficiency, reducing the iron
oxides that would otherwise have coloured the
bricks. Performance and density varies too.
Staffordshire blues, for example, are strong
and densely made and were favoured for
engineering works.
CONSTRUCTION
Before repairs are designed, it is essential to
develop an understanding of the building’s
construction or how the element that is failing
or causing problems was made.
From the beginning of their use in late
and post-medieval times, bricks were laid in
lime mortar, which is relatively soft and highly
Tudor brickwork in the cloisters at Queens’ College,
Cambridge
A chamber in a continuous firing kiln at Northcot
Brick, now fired using gas and a small quantity
of coal to replicate the effects of late 19th century
technology. Firing progress is checked by inserting
a rod through a firing hole above the chamber to
measure variations in the size of the stacked bricks.
Brick making at Coleford Brick and Tile: the mould
for the frog can be seen fixed to the bench, and the
wooden stock (here being cleaned) is then placed over
it and filled with clay