Page 106 - The Building Conservation Directory 2013

1 0 6 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 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
3.2
Structure & Fabric :
Masonry
SandstoneConservation and Repair
Jamie Coath
“The time has come,” the Walrus said,
“To talk of many crimes:
Of cramps and cracks and sealing-wax
Of cement matrix and limes
And why the steam is boiling hot
And what are the dwell times?”
Graham Lott in his article on Britain’s
Building Sandstones (page 94) has
highlighted the importance of selecting
compatible replacement stone when repairing
sandstone masonry and how this is becoming
more difficult as suitable supplies diminish.
This makes the effective conservation of
our existing sandstone structures more and
more significant. This article attempts to
provide some brief practical guidance on the
common aspects of the conservation and
repair of sandstone, including for example
cramp repairs and other indenting, the need
for cleaning, friability and fissibility issues,
porosity and the use of consolidants.
The first step where any masonry is
found to be deteriorating is to develop a clear
understanding of the principal issues affecting
the stonework and the mortar joints, including
inherent defects, construction defects, and
problems resulting from inappropriate
maintenance and repair.
Typical defects
In sandstones the purity and strength of the
constituent minerals and the matrix that
bonds them together vary greatly, as does grain
size or texture, not only from one geological
source to another, but also sometimes between
beds in the same quarry. This has a significant
effect upon the causes and mechanisms of
decay and consequently upon the conservation
issues that arise. There are pure siliceous
sandstones in which both the grains and
cementitious matrix are silica based. These
Severe salt crystallisation decay to sandstone features at Wentworth Castle, Barnsley
could be more susceptible to alkaline chemical
attack, making them vulnerable to materials
used in their conservation and repair, such as
pure reactive limes or particular cleaning and
‘degreasing’ chemicals. There are also the more
borderline calcareous sandstones which may
have siliceous grains but a partially lime based
cement matrix, such as some of the regional
‘ragstones’, which may be more vulnerable to
acid attack. There are also sandstones with
significant clay content either distributed
throughout the cement matrix which
consequently makes them generally weaker
in resisting decay, or in concentrated clay
beds which differentially erode, sometimes
in alarming fashion due to the action of salt
crystallisation and wind abrasion. The top
illustration (at Wentworth Castle) shows
a particularly extreme example of salt
crystallisation decay caused by over saturation
of local sandstone that has high clay content.