24
BCD Special Report on
Historic Churches
19
th annual edition
An example of galleting in Chichester, East Sussex: Slivers of flint have been inserted into the joints, decreasing
the difference between the durability of the flint masonry and surrounding mortar. (Photo: Colin Arnott)
son in the mid 19th century, became a popular
binder in the late 19th century, although it
was not until the early 20th century that it
became the predominant mortar for all new
buildings. Prior to this, common binders
included lime, clay, gypsum and vegetable
oils, waxes and resins. Of these, lime is by far
the most common binder found in historic
mortars and the most widely used in building
conservation. Lime is extremely compatible
with the vast majority of historical building
materials. Sharing many chemical properties, it
adheres well to the host material and its vapour
permeability does not restrict water movement
within the fabric. It is also flexible and has some
limited ability to self-repair. However, the use
and specification of lime does not guarantee
success and a great deal of consideration
should be given to the type and strength
of lime included. In conjunction with the
information derived from analysis, the binder
type and ratio should ultimately be determined
by the requirements expected of the mortar
and the condition of the surrounding
materials. Some modification of the original
specification may therefore be necessary.
The most common types of lime used in
conservation today are non-hydraulic lime
(
most often as a putty lime), which hardens
by carbonation when exposed to atmospheric
carbon dioxide, and natural hydraulic lime
(
NHL), powdered lime which initially sets with
water and develops strength as it carbonates.
Non-hydraulic lime is categorised as a
‘
feeble’ strength lime and is compatible with
very soft and weathered masonry. It cures
slowly and requires extensive protection
from frost and weather. It is best used in
internal and sheltered areas but can also be
used as sacrificial protection for very soft
material in exposed locations. Its versatility
can be increased with the inclusion of
pozzolanic materials, discussed below.
Natural hydraulic lime is available in
three classes; NHL 2, NHL 3.5 and NHL 5,
which are commonly believed to equate to the
classes of ‘feeble’, ‘moderately’ and ‘eminently’
hydraulic but in fact refer to their minimum
compressive strength in N/mm
2
at 28 days. It
should also be borne in mind that the hydraulic
set continues to strengthen for up to two
years, making explicit rating difficult. There
is also sufficient overlap in the boundaries
of classification for some limes to fall into
two classes, and the classified strengths can
vary between manufacturers. It is advisable
to check the strength of the mortar as
recorded in the manufacturer’s data sheets.
NHL 2, which may be considered to
be a moderately hydraulic lime, has many
applications. It is compatible with moderately
sound masonry and can survive in more
exposed situations than a lime putty. NHL 3.5
can be used effectively in more challenging
situations such as chimneys and copings and is
compatible only with sound masonry. NHL 5
should only be used in the most exposed
areas as its hardness is comparable to that
of cement, although it is more permeable.
ADDITIVES
Other materials can be added to change the
appearance, performance and properties of
a mortar. Set-additives or pozzolans, (finely
ground materials containing silica or alumina
which have been heated to high temperatures,
such as volcanic ash or by-products of
manufacturing and burning of coal and clays)
can be introduced to increase the versatility
of a non-hydraulic lime as they react with the
lime in the presence of water and produce a
hydraulic set, which is both quicker and harder.
Many other organic materials are known
to have been included in the past as they are
mentioned in historic texts, but have left no
physical trace. These include cheese, eggs,
blood, urine and beer. Animal hair, although
usually found in plaster, is less commonly found
in historic pointing mortars. As its use has
proved beneficial, the practice has not died out.
MORTAR ANALYSES
The process of matching a mortar begins with
simple visual analysis of a sample of original
mortar, if possible, in situ. There is of course a
great deal more to matching historic pointing
than simply formulating a compatible mortar,
and any character elements such as joint
profile, pointing style and tending finish are
fundamental to the appearance and nature of
pointing and should be carefully examined and
recorded so that they too can be recreated.
With a good sample of unweathered
mortar one can glean basic information
through very simple tests. Surface scratching
with a thumbnail and crumbling between
the fingers gives an indication of the strength
of the binder. Magnification with a hand
lens (or electronic microscope for greater
magnification) allows better examination of
the distribution and types of aggregate.
To help with finding a good visual match,
on-site analysis can be furthered with a
chemical disaggregation test, dissolving the
mortar in a weak solution of hydrochloric acid.
This can usually provide basic information
A selection of sands and other aggregates in a range of sizes and colours
