History of windows
History of windows
Throughout the early medieval period, the great
majority of windows were unglazed. In timberframed
buildings they were simple openings in
the structural frame. Wider openings were often
sub-divided into two or more ‘lights’ with plain or
moulded mullions. Vertical wood or iron bars were
inserted to keep out intruders. Taller windows
might be sub-divided horizontally with transoms.
Glass was extremely expensive and rare and was
not considered a fixture. Timber shutters were
widely used for security, privacy and to reduce
draughts. In England, they were often internal and
either hinged or slid in runners. Although these
early shutters have rarely survived, the runners
sometimes remain.
Windows were also often
covered with oiled fabric, nailed directly to the
frame or stretched over a thin timber lattice.
Much of the plain glass and most if not all of
the coloured glass used in England during the
medieval period was imported from the continent
and thus prohibitively expensive for widespread
domestic use. By the late medieval period and
into the 17th century, windows became more
sophisticated with wooden tracery, moulded
mullions and deep projecting cills. As glass was
no longer quite as expensive it started to be used
for ordinary domestic buildings.
Timber quality
Timber quality
Many 18th and 19th centuries sash windows
continue to provide excellent service thanks
largely to the high quality timber used in their
manufacture. Most were made from heartwood
of imported Scots pine (Pinus sylvestris) grown
slowly in natural forests. However, by the early
20th century, trees cultivated on plantations
were an increasingly important source of
timber. Plantation grown trees are encouraged
to grow to a marketable size in the shortest
possible time. As a result, they contain a larger
proportion of sapwood than slow-grown trees.
Sapwood is more permeable than heartwood
and contains sugars and starches that provide
an excellent food source for fungi; this makes
it susceptible to decay and unsuitable for
external joinery. Nevertheless, in the postwar
years, it became common practice to
use timber containing a high proportion of
sapwood for many joinery tasks. The results of
this can be seen in the large number of timber
windows, dating from the 1960s and 70s, which
now require replacement. Therefore, it makes
good sense to retain old joinery wherever
it is sound. When repair or replacement is
required, heartwood of one of the more durable
softwood species, such as Scots pine/ European
redwood (Pinus sylvestris) or imported Douglas
fir (Pseudotsuga menziesi), should be used.
As it is very difficult to ensure that timber is
entirely free of sapwood, pre-treatment with
preservative is generally recommended. An
alternative would be to use chemically modified
‘acetylated’ softwood which is exceptionally
durable, and dimensionally stable.