do? Or what to do? That is the question.
stonemason has to be entrusted to make frequent decisions
on how much original stone can be safely retained (Photo:
material be replaced? Why? Who decides? What to replace it with?
Are there any alternatives? Questions such as these, sooner or
later, are faced by all those who own, work on or are responsible
for historic buildings, particularly where historic or original
stones are concerned. They are questions that I had to face time
and time again in my role as Regional Superintendent of Works
for Cadw. Although the buildings for which I was responsible were
scheduled ancient monuments (mostly roofless ruins), the principles
are easily transferable to most historic buildings. This article
is not prescriptive but endeavours to highlight the issues involved
and stimulate the thought processes.
first question to be asked is: why are we contemplating stone
replacement? Is it for purely aesthetic reasons? If so, we should
seriously question our motives. Remember, we are dealing with fabric
that is of historic and/or architectural importance, and therefore
a vital element of the building's record. With replacement, the
patina of age will be lost, leaving a sometimes stark reminder,
maybe for years to come, that this is new stone.
only reason stone should be replaced is because it has failed,
compromising the structural integrity of the building.The cause of failure should be investigated as it may be the nature of the replacement. Is it due to the environment? Weather erosion,
salt damage, or location, such as sandstone placed below limestone,
can be a major factor in stone decay.
Tintern Abbey the unique circumstances favoured a principle
of replacing stone which had a life expectancy of less than
Bad maintenance or poor
repairs in the past may be the culprit; much stone has been lost
due to hard, impervious cement pointing. Was the stone of poor
quality to begin with, was it 'wrong-bedded' or, more likely,
was it rendered over to protect it?
It follows that the environs
of the stone may also need to be modified or corrected so that
the new stone does not suffer the same fate. This becomes particularly
important when the stone has failed structurally due to movement
in the building, causing undue, and undesigned for, pressure on
it. If this is the case, then specialist advice must be sought,
usually by employing a structural engineer who has experience
of historic buildings.
It might be argued that replacement is
acceptable when the stone is at high level or is otherwise inaccessible
without a scaffold, as the expense of reinstalling a scaffold
might prohibit further work for many years. The time delay,
fifty years plus, could mean that a moulded or carved stone might
be in perilous condition before it could be inspected again, and
that the 'on-site' record of those mouldings might be lost for
Tintern Abbey was just such a case and here the project
team decided on the following strategy:
- that the best preserved
components of each window type would be retained and conserved
for the future
- that those components whose future life was
thought to be less than 75 years would be replaced by stones dressed
to the original profile
- that for new work, the most compatible
source of commercially available stone would be used
- that none
of the missing components of the windows would be restored so
that the pattern of fenestration accurately reflects the earliest
representations of the abbey church.
replacement stonework at the bottom and right, designed to
match the original
It is worth
mentioning that, at Tintern Abbey, previous works had been carried
out in the early 20th century using the 'conserve as found' philosophy.
This meant that the decay to the dressed stonework was addressed
using cement mortar repairs, copper strips and selective piecing
of new stonework. However, 75 years hence, the windows were in
a perilous condition and in danger of collapse.
At Tintern, a
project team made the decisions based on a site philosophy they
had developed which produced a range of options. This is the ideal
and should be a collaboration between a conservation architect,
an archaeologist and the mason who is to carry out the work. It
is appreciated that this may not always be possible, especially
in the case of the less prestigious buildings, but if decisions
are made unilaterally then key factors may be missed.
there are alternatives to replacing stone which can offer ways
of retaining the material with minimal intervention. Rubble stonework
may have been rendered in the past and, if there is evidence of
this, then re-rendering with lime mortar would be an obvious solution
to the problem. Rendering may also confer other benefits such
as acting as a 'poultice' to draw out salts from the stonework.
Limestone can be repaired using a palette of techniques including
plastic lime repairs, micro-pinning, and lime shelter coats. The
beauty of these methods is that they are largely reversible. They
can also be used successfully on sandstones but consideration
must be given to the possible effects of salt migration (due to
acid rain) into the sandstone. Then there are the more invasive
resin-based approaches that may be more acceptable than losing
a potentially important piece of history. Consolidants also come
into this category and are particularly useful for statuary.
pointing used to distinguish old work from new
it is decided that stone replacement is the preferred course of
action to be taken, there are still a number of issues that must
be resolved prior to starting the works.
How much stone is to
be replaced? Always aim for minimal intervention. Adequate recording
should be undertaken both to preserve a record of the old and
to instruct the formation of the new. This can take
the form of photographs, drawings (both scale and freehand) and
templates (essential when replacing moulded stones) or a combination
Are there adequately skilled personnel available to undertake
the work? Stonemasonry is a highly skilled profession and should
only be undertaken by suitably qualified or experienced people. Replacing
dressed stonework in-situ is particularly demanding due, for example,
to the tolerances involved.
With what should the stone be replaced?
There are a number of options, the most obvious being to replace
like with like. If the original stone type is up to the job, and
not the cause of the problem, then the geology of the existing
stone must be determined. This may be easy, as in the case of
a Pennant or Bathstone (although even with these attention must
be paid to the differences between different quarry locations),
or very complicated, requiring the employment of a geologist.
Once the stone type is identified then begins the task of finding
a suitable match. This is often extremely difficult due to the
fact that the original sources of the stone will, most likely,
be no longer available and so alternatives have to be sought.
Here again the services of a geologist can prove invaluable. The
stonemason should be involved in this process to ensure that a
usable stone is chosen (and not one which suffers from bed problems,
poor quality, or inferior workability, for example). Durability
and weathering are also important - will the stone last and how
long will it take to 'marry' with the old visually?
wheel window conserved with a combination of renewal and mortar
It may be
that an exact or 'near enough' match is not obtainable and a compromise
of either 'geologically correct' or a visual match will have to
be accepted. Such a compromise may be acceptable for another reason:
that it delineates the 'old' from the 'new' and so respects the
historical record of the building; it may even be decided to use
an obviously 'alien' stone for the same reason. This philosophy
of 'honest repair' can also involve the use of other material
including artificial (cast) stone (although this tends not to
weather very sympathetically and can look quite different to surrounding
stone in wet weather) and 'tile repairs' (as promoted by the SPAB).
'Honest repair' brings me onto a subject which is peculiar to
(though probably not exclusive to) ancient monuments and is not
really about stone replacement but about the addition of stone.
Stone is not generally added to ancient monuments as they are
almost always 'conserved as found'. However, sometimes it becomes
necessary to do so in order to preserve structural integrity,
or to protect the fabric of the existing masonry. When this is
carried out, it is usual to delineate the original from the new.
been, and still are, various methods of accomplishing this, including
the use of 'alien' stone (as above), 'hungry' or 'inch-back' pointing
at the juncture of the two builds, clay tile or slate 'slips'
at the juncture, judicious dating of the new and use of masons
marks, copper pins at the juncture, use of a contrasting colour or texture
mortar in the new build, or drilling holes in the stones of the
new build adjacent to the juncture (used in drystone walling). Each one should
be decided on a site-by-site basis and should depend on the preferences
of the personnel involved in the decision making process. One
should be aware of these when engaged on ancient monument work.
One last thought. Consider what is to be done with the original
stone after it is removed, assuming it can be taken out intact
and is of significant historic or architectural value. Possible
options include displaying it in a protected environment on site,
putting it in a museum, or burying it on site (and record the
location). To conclude, there is no single answer to what can
and cannot be replaced. The quality of the conservation work depends
on the quality of the decision-making process, and in particular
on the stonemason knowing when to stop and consult the rest of
the conservation team.
article is reproduced from The Building Conservation Directory, 2007
has been employed by Cadw for the last 22 years in various roles
including Regional Superintendent of Works for West Wales,
Planning and Resource Manager and most lately Finance and
Training Manager. He has been at the forefront of the 'Lime
Revival' in Wales, is a founder member of the Welsh Stone
Forum and editor of 'Stone in Wales'. Currently he is Chair
of the HLF Traditional Skills Bursary Scheme Management
Group, a member of the Traditional Skills Survey for Wales
Steering Group and Welsh representative to the NHTG .
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