Page 113 - The Building Conservation Directory 2013

t w e n t i e t h a n n i v e r s a r y e d i t i o n 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 3 1 1 3
3.3
Structure & Fabric :
Metal,
Wood & Glass
Lime Tilia vulgaris
Lime, with its ability to take detail, is the finest
of all the carving woods. However, it lacks the
stability needed for the structural elements of
joinery and is slower to carve than pine. From
the third quarter of the 17th century lime was
used extensively for carved decoration on oak
or deal joinery.
Walnut Juglans regia
Walnut was used extensively for furniture
in the first quarter of the 18th century and
occasionally for joinery.
Mahogany Swietenia macrophylla
From the second half of the 18th century
mahogany from Central America was
occasionally used for carved joinery. It looks
good, works easily and is extremely stable.
It may be hard to obtain suitable varieties to
match early work.
Timber processing
Native hardwoods such as oak would be riven
(split using wedges). Timber was then cut
using a two-man saw with the tree trunk set
on trestles or over a saw-pit with one sawyer
above and one below the piece being sawn.
Deal was machine sawn using water-powered
sawmills before importation.
After seasoning, air-dried timber has a
moisture content of about 17 per cent. The
wood retains its ability to lose and regain
moisture as the relative humidity in its
environment varies. As it dries it shrinks or
‘moves’. The greatest shrinkage takes place
along the circumference of the annular
growth rings of the tree. The thickness of
the rings is slightly reduced and the length
of the timber along the grain moves very
little. This differential shrinkage is the
source of cupping and bowing in timber.
The traditional way to minimise these
problems is to ‘quarter-saw’ the timber to
create even shrinkage within the plank.
REPAIR MATERIALS
Timber selection
When choosing timber for a repair it is
important to try to match the species of wood,
the moisture content, the grain width and the
grain direction. This matching will make the
repair look less obvious and will ensure the
minimum of differential movement between
the repair and the original piece. If replacing
an entire component it is best to select timber
as near quarter-sawn as possible, and essential
to reject any timber which has hairline cracks
and compression shakes, sap wood and large
or dead knots.
Glue selection
If the relative humidity in the room is within
reasonable limits and good contact surfaces
have been achieved, a traditional protein glue
such as pearl glue will prove as strong as any
modern alternative and is easily reversible. It
should be considered before other options.
Fresh glue should be made regularly in small
batches and should be heated in a traditional
glue pot. If the glue is old or if it is overheated
the collagen links on which its strength
depends will break down. Microballoons (a
lightweight filler) can be added to improve the
glue’s gap-filling properties but will cause a
small reduction in strength.
Epoxy resins give a strong bond and
can prove useful if gap-filling properties are
required. In some cases there may be a trade-
off between poor reversibility and the need to
avoid the loss of original material at the glued
joint. A small amount of pigment can be added
Carving repair on an early 16th-century dais screen
Repaired 16th-century panelling with new panel in
background, and (below) the back of the panelling
showing its repair.
15th-century screen showing constructional layers