Myths, Preconceptions and the Relevance of Good Practice
recent years, investigations carried out during repair and restoration
work have led to a significant increase in our knowledge of the techniques
used in early plasterwork, but little is documented. Ian Constantinides
examines the use of traditional lime plaster and explodes some popular
myths and misconceptions.
Park: The chapel gallery ceiling
the stylistic development of plasterwork through the ages is well documented,
the history of pre-Victorian plastering technology is relatively little
researched. Sources lack real detail on the craftsman's materials and
methods of working. Millar's authoritative textbook, 'Plastering Plain
and Decorative' is a bible to most, but is limited in that it describes
state of the art lime plastering relevant only to Victorian and Edwardian
recent years, our understanding of this earlier work has increased considerably.
While the conservation, repair and renewal of historic plasterwork has
lagged behind that of stone, fewer horror stories are heard. It doesn't
require a particularly sensitive eye to see how uncomfortably the skilled,
but uniformly repetitious repair work of the fibrous plasterers compares
with the original, in situ, hand work of an early Jacobean ceiling, and
the wholesale replacement of solid plasterwork by fibrous plaster is now
less common. But it is less evident how changing materials and techniques
can effect the structural integrity of historic plasterwork.
addition of gypsum, for example, which sets rapidly on addition of water,
radically affects the working time and consequently the decorative treatment,
as well as the set itself. Gypsum is sometimes found in humble work of
early date and is often not apparent where most expected. An exemplar
was made for Uppark House in which a stiff lime putty-hair mix was rammed
into solid moulds and released while still green. If there is any gypsum
in the cast elements it is so small that it does not materially affect
the setting. Once cast, the mouldings could be further worked or tweaked
by hand to add definition, enrich details, or accentuate undercuts, and
consequently each cast is slightly different. Runs of egg and dart or
waterleaf made in this manner were often fixed whilst still green, and
could deform to follow irregularities. The result is a liveliness and
individuality which is entirely different from the disciplined craftsmanship
evident at Prior Park also of the mid 1700s, where the cast elements were
in poured plaster of Paris and each cast is identical. The work is of
the highest quality, every bit as good, but very different in character.
Thirlestane and a number of other Scottish castles with ceilings by the
same band of itinerant craftsmen, the frieze and cast enrichments are
all in a well haired, lime rich mix (approximately one part of lime to
a fifth part aggregate). They are gypsum free and so flexible that at
Craigievar a cast is wrapped around a corbel with a radius of some 200mm.
the importance of doing so, only recently have real attempts been made
to understand the nature of the material that is being repaired or renewed,
and a number of significant changes in perceptions and attitudes have
increased our understanding of the trade. Most significantly, it is now
generally recognised that a mortar complying with British Standards has
little in common with the materials used in our building heritage, and
it is clear that currently accepted good practice in workmanship is often
far removed from that which produced all the idiosyncrasies of the work
we are trying to conserve. Mortar analysis is increasingly recognised
as a key factor in the conservation programme, particularly at the pre-contract
of the original is best carried out as a separate pre-contract, prior
to the development of a specification for the work, as the programme and
financial implications of the main contract often prohibit thorough research
later on. A few building conservation firms carry out their analysis in-house
and the interaction between analyst, plasterer and historian is crucial.
is an increasing tendency to award pre-contract investigatory works to
specialist firms, and as a consequence specifiers are increasingly confident
and abandon many of the standard clauses and BS references in the specification
of workmanship and materials that often result in bland and inappropriate
work, however technically accomplished. Multiple pre-contract analysis
has contributed significantly to understanding and dating building chronology.
analyst not only looks at the material's constituents, but is trained
to interpret clues on techniques. Material types fall into contexts of
geology, chronology and social change. The analyst will typically look
for type and quantity of hair, its presence or absence in all coats including
the setting, how the coats were scratched, whether they were scoured,
how much they shrank on drying, whether shrinkage cracks were transmitted
through the coats, and whether the coats were applied green on green,
or green on dry.
information is ancillary to the analyst's prime interest in identifying
and quantifying aggregate type, grading, and origin, lime binder to aggregate
ratio, the presence of gypsum gauging, hydraulic additives and organic
constituents such as oils, fats or glues.
is only one way to do it and that is the right way' is an often heard
cry from craftsmen and specifiers alike. However as our knowledge increases
through analysis and investigation, it is apparent that there are as many
'right ways' as there are buildings with historic plasterwork,
particularly in pre-Victorian times.
is widely believed that proportions are ideally 1 part of lime to 3 parts
of sharp washed pit sand for base coats and approximately equal parts
of lime and silver sand for setting coats. However, analysis has shown
that every permutation exists from pure lime plasters to pure clays. Aggregates
vary from the coarse to the fine and from sharp to soft. Sands, as we
understand them, are often not found at all. Binder aggregate ratios vary
from lime rich to lime lean. Hair is sometimes not found at all, whereas
elsewhere it may be present in such an abundance that sheets of plaster
can literally be rolled up like a carpet without significant damage. Hair
may even be found in setting coats. Surprisingly often analysts find fine,
white (or coloured) hair well trowelled into the surface and almost invisible
even when paint-stripped.
it is often held that one should always try to make lime mortars hard.
Indeed lime mortars can be scoured and polished with limewater to take
an almost marble like finish, if required. However, it is often the weaker
softer materials that survive longest. The soft, hairy clay or lime plasters
can accommodate an almost extraordinary amount of structural movement,
whilst their harder counterparts would have broken off in sheets.
cast work, screeds for the work and setting coats were not always gauged
with gypsum. The use of gypsum seems to vary with locality, the client's
wealth, the proximity of gypsum mines, and local practice, although of
course its use became more prevalent in the 19th century as gypsum became
more easily available.
techniques used also vary considerably. Popular misconceptions include;
joints should be raked out in brick and stonework to give a good key.
This is a common fallacy and derives from modern practice with cementitious
renders. In fact by doing so with lime renders one ends up by applying
a coat of uneven thickness, thin at the high points of the stone and
perhaps 50mm thick or more over the joints. This leads to differential
rates of drying within the same coat and an undue amount of tending
to reduce cracking
base coats are scratched using a three pronged lath scratcher. Whilst
this is certainly good traditional practice, it by no means prevails.
At Woodchester Mansion the ceilings were cross scratched with a lath
scratcher but the walls were economically keyed with the flat blade
of a gauging trowel. At Thirlestane Castle the pricking up has been
scratched using a coarse comb of lathing nails with the heads rather
than the points in contact with the plaster
cracking in base coats is unacceptable. At Woodchester Mansion a Victorian
house, unfinished and abandoned, we see plasterwork in all its stages
from the bare substrate to the completed setting coat. It is evident
that the base coats were applied, and tended minimally, allowed to crack
under control, but most importantly, the setting coat was not applied
until the undercoats had dried completely, so that no further cracking
would show through the setting
aggregates were finer in top coats than in undercoats. Again this is
often true especially in interior work, but in one example at Prior
Park the aggregates in the top coat are up to 20mm in diameter whilst
those in the base coat rarely exceed 9mm
coats are progressively thinner the nearer the surface. Whilst this
is often true, there are exceptions. The Roman Baths in Bath dating
from the early centuries AD have plasterwork of a total thickness of
some 30mm, of which the top coat is 20mm thick. At Prior Park work
of the early 19th century which appears to follow 'state of the art'
good practice consists of a coring-out (first) coat averaging some 5-10mm thick whilst the finish varies between 20-35mm in one coat.
conclusion, plasterwork is probably the least defined of the trades. Understanding
the materials, 'unlearning' the many preconceptions, using traditional
tools and minute examination of the original that survives, are the keys
to preserving this country's plastering heritage. Modern good practice
is only a stepping stone in a trade rich in the unexpected.
- William Millar, Plastering: Plain and Decorative, (1897), Donhead, Shaftesbury 1998
- Alec Clifton-Taylor, The Pattern of English Buildings, Faber, London, 1987
- AR Powys, The Repair of Ancient Buildings, (1929), SPAB, London, 1981
- See also SPAB pamphlets on plaster and its repair
This article is reproduced from The Building Conservation Directory, 1995
IAN CONSTANTINIDES founded St Blaise Ltd in 1977 after working as a materials scientist.
Lime Mortars and Renders
PRODUCTS AND SERVICES
Lime: hair & fibre reinforcements
Lime, non-hydraulic (lime putty)
Lime, pointing tools
Lime, pozzolanic additives
Communications Limited 2010