Page 158 - The Building Conservation Directory 2013

1 5 8 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
4.1
Services & Treatment : Protection & Remedial Treatment
trowel’ to take up its initial stiffening. Prior
to hardening, the mortar is compressed back
with a trowel to ensure a tight bond with the
adjacent stone. Suction control as previously
described is critical in achieving a good
bond between stone and mortar. Finally, the
dubbing out coat can be scraped back with the
edge of a trowel to open up the surface to aid
the curing process, and also to provide a good
key for the pricking-up coat. The dubbing out
should bring the wall face to a suitable level for
application of a thin harling coat. In certain
circumstances, where the wall face is relatively
sound and flat, a single application of harling
will be sufficient, omitting the pricking-up and
straightening coat.
Pricking-up coats These are applied in
a similar manner to the harling, either by
the traditional method of hand casting the
mortar using a specially shaped trowel, or
with mechanical spray equipment (where
appropriate). Pricking-up coats generally
comprise a thin, binder-rich and gritty coating
which acts to even out the background suction
levels. This is particularly important where
high capillarity lime mortars exist alongside
impervious stones. It also provides a good
mechanical ‘key’ for the subsequent harling,
hence the requirement for this mix to contain
a sharp, gritty sand. No attempt is made to
‘straighten’ the wall faces with harling at
this stage (this should have been done as
part of the dubbing out process) as variable
thicknesses can lead to an inconsistent
appearance in the finished work and can
impact on its performance.
Straightening coat This may be required
where the background masonry is particularly
uneven and/or where a flatter finish is
required. It is important to bear in mind that
the final finish coat will be a relatively thin
coating of consistent thickness, so will not
mask any unevenness in the background.
Typically this coat is cast or sprayed on, for
the best adhesion, scraping the mortar from
the high points (a plasterer’s straight edge can
be used) and filling the low points as work
progresses – it is important not to overwork
or ‘move’ the mortar around too much on
the wall as this can weaken its bond with the
background.
Finish coat Finally, the harl is applied to
a consistent thickness and texture. Careful
planning is required for large elevations
without natural breaks such as internal or
external returns or string courses. Visible
day joints can be avoided by ‘fairing’ out the
harling (gradually reducing its thickness)
in a wave like shape (not a straight line) and
blended in the following day. It is also essential
that the scaffolding is designed specifically
for the purposes of harling with removable
inner boards to avoid the risk of ‘lift lines’ in
the finish. Fairing into vertical faces of returns
and reveals is a typical, traditional detail, and
harling is normally faired out at the base of
the wall to discourage moisture retention.
The number of coats and relative
thicknesses of harling will vary from building
to building depending on the type and
condition of the substrate, and the type of
finish and appearance desired. Similarly the
timing between coats for curing and drying
will vary depending on the materials selection,
number of coats and thickness of coats.
Individual coats of harl should generally be no
thicker than about 10mm – often traditional
Scottish lime harling is less than this in
overall thickness. Once the lime harling is
complete, it should be allowed additional
drying time, after curing. Any patchiness in
the moisture levels across the harling may
lead to an inconsistent appearance when
limewashing. This will be more pronounced
when using stronger pigmented limewash.
The state of the art
Reinstatement of previously stripped harling
finishes to traditional and historic buildings
is now becoming more prevalent. One of the
challenges over the last two decades has been
re-learning the almost lost traditions in terms
of both materials and methods. Much of the
early ‘lime-revival’ harling work carried out in
the early 1990s was founded on the practices
employed in the application of cement harling/
pebbledash or plasterwork, where float applied
base and straightening coats were commonly
used. Today, much of the harling work carried
out utilises excessively-strong modern
hydraulic limes to over-compensate for some
of these earlier failures, as the mechanisms
involved in the application of lime mortars,
along with the properties of the range of
binders now available are still to be fully
disseminated and applied across the industry.
Mechanical spray application of
lime harling is also becoming more
Inchnadamph Church, Assynt: mechanical spray
applied lime harling and plain limewash
Hand-cast harling – note the flat trowel used to throw the harl. (Photo: LimeRich Stonemasonry & Plastering)
Applying harling by mechanical spray: in some circumstances this method may be appropriately used for
the pricking up or straightening coat, prior to the application of a hand-cast finishing coat. (Photo: LimeRich
Stonemasonry & Plastering)