BCD Special Report on
Historic Churches
19
th annual edition
25
Like the results of analyses, the specific
circumstances and requirements of the mortar
and the structure as a whole (circumstantial
factors) must also be fully understood and
holistically considered when specifying a
new mortar. As well as looking at how the
original mortar has performed (and any
causal factors), considerations must include
the extent of the specific work, for instance
whether an area of pointing is being repaired
or replaced. The priority for a repair mortar
is on achieving chemical and aesthetic match
with the existing and presumably successful
mortar, but where an area of missing or failed
mortar is being entirely replaced, the emphasis
is on remedying the failure. All elements and
information must be borne in mind, especially
where there is conflict between historical
accuracy and performance requirements.
The performance of a mortar is of course
the priority, if the rest of the fabric is to
be saved. However, this need not be at the
expense of a pleasing aesthetic match. With
a good grasp of mortar technology and the
specific requirements of the mortar, and a
wide library of aggregates, there should be no
need to compromise on either aspect. There
may be various approaches to be considered
in order to replicate the many visual aspects
of a mortar. Any number of suitable ‘recipes’
can then be made up in small quantities as
samples or ‘biscuits’ to see exactly how the
components work and look together. Tended
and cured properly, they can give a good idea
of the size and distribution of aggregate in the
binder and the overall workability, tone and
texture of the mortar. The best of these can
then be trialled in situ to see how each mortar
will work with the surrounding material and
in the specific joint sizes. These sample areas
also provide an opportunity for further trials
in recreating the profile, texture and character
of the pointing style; those elements which
alongside its ultimate protective function, make
pointing mortars such an important element
of our historic buildings in their own right.
Claire Davies
BA BSc Cons is a freelance
lime mortars specialist and architectural
fabric conservator with extensive experience
of working on prestigious heritage sites and
vernacular buildings. She established Claire Davies
Conservation (www.clairedaviesconservation.
co.uk), based in Uckfield, East Sussex, which
carries out specialist conservation works
throughout the South East of England.
Mortar
Analyses
Suitability
General appearance
Aggregate colour
Aggregate shape
Aggregate grading range
Aggregate type/mineralogy
Binder type
Binder colour
Mix proportions
Presence of organic additives
Visual (low level magnification/hand lens)
✔ ✔ ✔
✔
Chemical disaggregation
✔ ✔ ✔
✔
✔
Sectioning/petrographic analysis
✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔
Scanning electron microscopy
✔ ✔ ✔ ✔
✔
X-ray microanalysis
✔ ✔
Thermal analysis
✔
✔
Gas chromatography
✔
‘
Biscuits’ of mortar made up from various possible
specifications provide a good indication of what
a finished mortar will look like. The best match is
altered further if necessary, or used for trials in situ
to be viewed alongside the host materials.
A mortar ‘biscuit’ is compared to existing mortar. The correct size and distribution of aggregates is achieved by
blending a number of sands and other aggregate inclusions.
on binder/aggregate proportions by isolating
acid-insoluble aggregates. (However, any
lime-based material in the aggregate, such as
limestone, chalk or shell, will also be lost in
the process.) Sieving the residue using a set of
British Standard mesh sieves will also give an
idea of the particle size distribution. The level
of solubility, the nature of the residue and the
effervescent reaction seen during dissolution
can also help to indicate the binder type.
More sophisticated laboratory testing
can identify the age, composition, strength,
moisture content, salinity and extent of
carbonation in a sample of mortar; helping
to understanding complex defects in mortars
which have failed or deteriorated. These
include more detailed visual analyses (such
as more intricate chemical disaggregation
analysis, and ‘sectioning’; the examination of
slices of mortar which have been impregnated
with resin and polished back to allow detailed
analysis of physical structure) and instrumental
techniques such as scanning electronic
microscopy (SEM), X-ray microanalysis,
and various forms of thermal analysis.
Although vital in the research of lime
mortar technology, the results of such analyses
taken in isolation are of limited practical
use when formulating replacement mortars
unless interpreted into accurate and practical
suggestions by an experienced and well-
informed conservation scientist. Historic
mortars may be poorly mixed (meaning that
samples are not always reliably representative)
and may also have undergone chemical changes
which are largely impossible to differentiate
from what may have been present at the point
of its formulation. Such chemical changes may
include environmental effects on the mortar
itself, such as the introduction of salts and
atmospheric pollutants, or changes occurring
in individual elements. It is particularly
difficult to differentiate between mortar
used as an aggregate and the lime added
specifically as the binder. Analysis cannot
provide details about the quality or preparation
of the original lime, which may also have
performed differently to a modern substitute.
Broadly speaking, simple analysis can often
be more practically and financially valuable,
allowing resources to be better focussed on
other equally important aspects, such as
high quality workmanship and materials.
