informing the conservation of the built env ironment - Historic Scotland

Old cement repairs being removed prior to consolidation and repair.
both faces to be properly examined and recorded. Work
then commenced to remove the ferrous bars from either
face. The method adopted was to initially drill the heads
off the eight rivets or screws securing the bars through
the stone. Having provided support for the head of the
cross from an overhead crane the bars were carefully
removed from the face of the stone. Considerable ferrous
residue and staining of the stone was revealed where
extensive corrosion of the bars had taken place. Areas
of cement mortar repair were revealed around fractures
between the head and shaft which had to be cut back to
allow the two sections to be separated.
The sections of the cross were then laid flat to allow
work to commence on the ferrous residue initially using
hand tools to carefully pluck the heaviest deposits away.
A YAG laser was then used to remove the staining that
had been absorbed into the pore structure of the stone
as this would not respond to other treatment. Included
in this work was the removal of biological growth to
allow a detailed examination of the surface prior to
consolidation work.
Removal of the ferrous staining using a YAG laser.
As part of the more detailed examination it was noticed
that the head of the cross and the top of the shaft did
not line up well. This discrepancy was down to the fact
that a significant portion of the shaft had been removed
as part of 19th century reconstruction. The cross was
then re-erected in the studio with support to the head
which was not permanently attached to the base at this
stage. It was thought it would make placing the cross in
its new location easier if the head were attached insitu
on its return to Kilmartin.
Laser cleaning in progress (above).
Post conservation with new collar stone (below).
Repairs were undertaken to secure friable fragments
and to replace areas where cement had been previously
removed. The holes where the metal pins had been
removed were also filled. In both cases an acrylic mortar
was prepared using fine chlorite schist aggregate
derived from specially crushed stone of a type found
in Argyll. Areas of hairline fracturing were injected
with acrylic resin in solvents to stabilise the surface.
A decision was made not to infill and recreate the areas
where the ornament had been removed. In discussion
with the director of the museum it was decided that
this should be left to show the earlier intervention and
not to cover up what was at the time thought to be
acceptable intervention.
A schist collar stone was prepared for the base of the cross
to a design commonly found with this type and period
of monument. The purpose of this stone to conceal the
concrete foundation and give a suitable finish around its
base. The stone was sourced from a farm near Kilmichael
and kindly donated by the owner for this purpose.
28 FOCUS | Historic Scotland 2011