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T W E N T Y F I R S T E D I T I O N
3.3
STRUCTURE & FABRIC :
ME TAL,
WOOD & GLASS
The recent renaissance in the use of the UK’s waterways is generating a host of new timber conservation
and repair challenges, especially as environmental conservation legislation precludes the use of many of the
imported tropical hardwoods that would otherwise be the replacement materials of choice.
Investigations are ongoing in this area.
For example, Edinburgh Napier University’s
Forest Products Research Institute is
working with Historic Scotland to combine
dendrochronological data from the latter’s
considerable store of timbers retrieved from
older buildings with acoustic testing in order
to build a detailed catalogue of available and
appropriate replacement or repair material.
Given that hand-held acoustic devices are
relatively inexpensive when measured against
the specialist nature of conservation work,
other historic buildings agencies may also
start to apply these techniques to gather
accurate information about structural and
non-structural timbers, allowing them to
make better-informed conservation decisions.
When it comes to the best repair timber,
it is not always a case of like-for-like. If,
for example, the existing timber is pine
the prevailing wisdom has long been that
the repair should be of the same species.
But is this, arguably ‘purist’ conservation
approach, always the best way to repair
historic timber? Research has shown that the
simple introduction of a matching species can
introduce new stresses on an older timber
with physical and mechanical properties
that have changed significantly over time
and which, although of the same biological
species, has an entirely different provenance.
Again, there is much work to be done in this
area of investigation, but with research being
carried out in different institutions around
Europe, the scientific information increasingly
available to historic building specialists allows
for more evidence-based decisions to be made.
PERFORMANCE ISSUES AFFECTING
THE SELECTION OF REPAIR TIMBER
Buildings are not the only type of historic
timber structure that require continuing
maintenance, occasional alteration and
eventual repair or replacement. Britain’s
industrial revolution generated many new
types of structure that were often constructed
from extremely hard woods only available
from overseas. Recent years have seen a
renaissance in the use of the country’s
waterways, for example, and with this the
need to regenerate our canals and related
infrastructure. This has thrown up many new
conservation and repair challenges because
many of the timber elements involved are part
of structures that have been listed as having
significant heritage value and require careful
conservation. While this may seem a relatively
straightforward exercise – simple like-for-
like species replacement –environmental
conservation legislation precludes the use of
many of the imported tropical hardwoods that
would otherwise be the replacement materials
of choice.
Lock gates are particularly susceptible
to attack from specialised fungi and as such
fall within Hazard Class 4 or 5 (depending on
fresh or salt water conditions) and Durability
Class 5 and were traditionally built using
European oak. By way of contrast, the balance
beams to the lock gates are not themselves
in continuous contact with the canal water
but their structural design means that
elements of their construction are in ground
contact and likely to be almost continuously
subject to wetting (Hazard Class 4). These
and other on-land elements have over the
years been replaced using species such
as ekki that, with its interlocked grain, is
rated as very durable, with good resistance
to insect attacks and good weathering
characteristics. Unfortunately, despite its
excellent performance in marine situations,
this slow-grown example is now also on
the International Union for Conservation
of Nature’s red list. Over-exploitation has
resulted in large scale destruction of wet
evergreen forest throughout its range (across
west Africa), causing a population reduction of
over 20 per cent in the past three generations,
and ekki is now deemed to be vulnerable.
Given that our waterways are managed
by quasi-public bodies, finding replacement
timber in these circumstances can create real
headaches. Current UK timber procurement
policy requires that all timber and wood-
derived products for use on the government
estate must come from independently
verifiable legal and sustainable sources
(including licensed Forest Law Enforcement,
Governance and Trade partners) with
appropriate documentation being required
to prove this. The policy is mandatory for all
central government departments, executive
agencies and non-departmental public bodies.
Local authorities, other public bodies and the
private sector are also encouraged to adopt
sustainable timber procurement policies –
hence the procurement challenge for our, and
indeed other European, canal networks.
In such circumstances, radical
alternatives are being explored for situations
where listing criteria either do not apply
or do not specify the nature of the species
required in a historic timber structure. In
the Netherlands, for example, chemically
modified timbers are now being used to line
canals. One such modified timber is Accoya,
a product made by submitting radiata pine
to an acetylation process. First developed in
the 1920s, acetylation increases the wood’s
natural acetyl content through impregnation
of acetic anhydride at high temperature and
pressure. This process produces significant
improvements to dimensional stability
(up to 75%) and durability, effectively
imparting the physical properties of
valuable hardwoods to relatively inexpensive
softwoods. With its ease of machining,
non-toxicity, resistance to UV degradation
and perhaps (as the manufacturer’s claim)
enhanced mould and insect resistance, the
product is likely to be of interest to those
specifying replacement timber in non-listed
historic structures due to its longevity,
future reversibility and after-life use.
The process is now being applied to other
species and in due course a broader palette
of acetylated timber products may well
appear on the market. In the meantime, and
although they would be inappropriate in many
conservation contexts, sustainable modified
timbers of this sort are clearly worth more
considered investigation.
REVERSIBILITY
The testing and conservation of historic
timber structures is a constantly evolving
area of research and practice and there is an
ongoing need to acquire deeper insights into
the factors (physical, mechanical, biological,
chemical and environmental) that affect the
ageing process and their interactions. From
this knowledge, methods for studying the
various forms of deterioration that take place
over long periods of time can be established,
together with criteria for evaluating the
long-term compatibility of interventions,
treatments and products designed to improve
the conservation of wooden structures and
objects. To achieve this, however, techniques
capable of predicting future behaviour are
required to ensure that ‘retreatability’ is
considered at an early stage, thus avoiding the
use of any conservation and repair method
that might impede future interventions.
A good example of this is the timber
bridge, a structural type that has been
universally employed for hundreds, if not
thousands of years. The UK has relatively few
timber bridges (although there are remarkable
