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REMEDIAL TREATMENT:
IN SITU TREATMENTS AND TREATMENTS OF HISTORIC STRUCTURES
By
D.J. Dickinson Senior University Lecturer, Imperial College of Science,
Technology, and Medicine, London, England.
Using Borates
to treat Historical Structures In
recent years the use of borates for remedial treatments has become more
important. Although in-situ treatments do not necessarily use large amounts
of chemical in comparison to pretreatment, the associated technology can
represent significant developments. The use of different forms of borates
especially designed for in-situ treatment are reviewed and illustrated
by specific case histories.
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Borates have been used as active ingredients
in most areas of wood preservation for many years. Early work by Bateman
and Baechler, and Cummins pointed the way to the exploitation of borates
as wood preservatives (2,12). Several reviews on the use of borates have
been published, by Bunn, Carr, and Cockroft and Levy (8,10,11). In recent
years interest in borates has gathered momentum in the United Kingdom and
the United States and the situation has recently been reviewed by Barnes
et al. and Dickinson and Murphy (1,16)
The main areas of interest and the biggest
use of borates in wood preservation has been and probably always will be,
the pretreatment of timber. The scope and breadth of papers at this meeting
demonstrate this point. There is little doubt that the future of borates
as wood preservatives is in the area of pretreatment. The development of
application technology and modifications of formulation will be the key
to an almost certain greater success in the future.
However, in recent years several interesting
applications and forms of borate preservatives, not necessarily for pretreatment,
have been studied and reported. Although many of these examples represent
a limited specialized market, they serve well to illustrate the special
properties of this active ingredient. Borates have time and again proved
ideal for specialized preservation requirements, even though the total chemical
usage may be limited. In this respect borates can be considered as a tool
of the conservation wood technologist, principally involved in remedial
and in-situ treatment or in the conservation of wooden structures and artifacts.
These specialized uses of borates often
take advantage of several valuable properties of the preservatives – diffusibility,
spectrum of biological activity and ability to formulate borates in a variety
of forms – making its application suitable for a range of environments and
structures. In addition, the low mammalian toxicity of the inorganic borates
is important when the chemical is used in situ and where total chemical
containment is not certain.
The examples contained in this paper
are not intended to form a comprehensive list of the specialized uses of
borates, but they have been chosen to illustrate how the various properties
of borates can be utilized. It is hoped that this will stimulate further
development and use of borates for specialist wood protection. The author
has been personally involved in several of the examples discussed but other
examples, which are reported in the literature, are included to illustrate
the specific uses of borates in the preservation and conservation of timber
and wooden artifacts.
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Developments with in-situ treatments Fused borate rods.
Probably
the most significant advance in the use of borates for in-situ
treatments has been the development and extended use of fused borate rods.
In the mid-1970’s a major project was conducted in Sweden on the in-situ
treatment of railway sleepers with borate pastes (5). This work showed that
while injection of the pastes was effective, what was really needed was
a highly concentrated borate in a soluble form, but where solubility was
controlled. This led to the development of the fused borate rod. Ironically,
their use in railway sleepers was not adopted in Sweden but has since been
used in the United States.
At about the same time in the United
Kingdom important work was being carried out on the early stages of decay
in window joinery (9). The progress of colonization of painted joinery out
of ground contact was characterized by large increases in permeability,
progressing along the grain from the joint. These regions become much wetter
after rain. The area most at risk from decay was susceptible to wetting
up to high levels prior to decay; water was entering the end grains and
at some stage at least, moving along the grain (14).
Initial trials in the United Kingdom
and major field and laboratory experiments in Sweden clearly showed that
the use of fused borate rods in joinery had great potential for the control
of decay and successful remedial treatments have been carried out in Sweden
since that time (13). The concept has been adopted in many other countries
and similar work in the United States is reported at this meeting.
Probably the most important concept that
was established in this early work was the principle of establishing the
pathways of water entry and intercepting these when inserting the borate
rod. Recent work in the United Kingdom has shown that in steady state situations
at low moisture contents (MC’s) the diffusion of boron from rods is limited,
emphasizing the need to establish where water is entering the system (70
and placing the rods to intercept the water and therefore distribute the
borates into the timber.
The use of borates will undoubtedly
increase in wood preservation. Although the bulk chemical use will principally
be in pretreatment of timber, specialist use of borates will also continue
to gain in importance, particularly for the treatment of old wooden artifacts.
At present borates are used in several forms and the user needs to carefully
design the treatment protocol to suit the individual situation. Fused borate
rods are ideally suited for the localized treatment of very wet areas and
for the strategic placement at points of water entry. They can also be inserted
into potential danger areas where water ingress is possible and likely to
cause decay problems. Such areas are near end grains and joints in structures,
or where checks are likely to develop.
Many wooden artifacts are waterlogged
and structures like wooden boats often contain large volumes of very wet
wood. In such situations the use of simple water solutions, often with repeated
applications, can give very effective in-situ diffusion treatments. Research
is still in progress to study the use of borates alone, and in combination
with other chemicals to treat drier regions of timber, but solutions of
borates I monoethylene glycol can give good distribution in drier wood.
The choice and use of borate systems for treatment of artifacts and for
in-situ treatments requires care and must always be part of an overall strategy.
Scope for other novel formulations exist, e.g., systems to deliver borates
to large surface areas in inaccessible situations. Such systems will be
developed when the need arises as was the case in the past with the development
of existing systems. |
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