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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

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.

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