Originally, hair was added to historic plaster to act as an interlocking
agent, tying the other loose, brittle components together into an integrated
whole. Without this hair, the plaster elements would crumble with relatively
minor pressure. In plaster surfaces that maintain their integrity, this
reinforcing hair creates a single sheet of plaster, much like a giant
cross-linked molecule, capable of remaining in place an indefinite length of
time. However, a crack indicates that the strength of the interlocking hair has
been exceeded, and the hair has broken, resulting in an integrity loss to the
sheet of plaster.
Historic plaster on wooden lath has a subtle undulation and waviness that
corresponds to the shape of the lath beneath it. This subtle ripple effect, that
can be see easily in raking light or felt by running a hand over the surface, is
the defining characteristic of original plaster from the 18th and 19th
centuries. Replastering does not achieve this effect, so the delicate nuance of
the wall and ceiling surfaces is lost forever with the removal of original
plaster. Similarly, treatments that result in skim coating the plaster also
destroy this defining characteristic. For these reasons, it is desirable to
develop a treatment system for cracked plaster surfaces that maintains the
nature of the surface yet reintroduces integrity to the plaster.
The traditional method of repair for plaster cracks is to open up the cracks
in an undercut manner, fill the cracks with a gypsum/perlite plaster, and level
the fills. This process does not re-create the fibrous bridge of the cracked
area that was originally provided by the hair, and the crack may re-open with
movement of the wall due to seasonal changes or subtle shifts of the building.
If the network of existing cracks is sufficiently large, this traditional repair
method would result in excessive disruption of the plaster. For this reason,
often the historic plaster is removed and the area is re-plastered with gypsum/perlite
plaster. These types of treatment processes result at best in a significant
degree of invasiveness to the plaster, and at worst, in complete loss of the
original plaster.
American Conservation Consortium has developed a system that is based upon
the common conservation practice of facing painted surfaces. A thin, long-fiber
membrane is adhered to the plaster surface, effectively bridging the cracks and
returning the integrity of a single sheet of plaster. This imitates the original
inclusion of hair in the plaster mix. By choosing both the adhesive and the
membrane to be flexible, a certain amount of movement of the cracks is possible
without them reappearing. Additionally, the flexibility allows the membrane to
conform to irregular surfaces. This system has been applied to 18th
and 19th century plaster with excellent results. In general, the
plaster surfaces in the Dwelling House are relatively smooth and are excellent
candidates for this treatment system.
Each plaster surface is different, and therefore the specific combination of
membrane and adhesive will vary. However, typically, all surfaces in a room, or
even a building, will receive the same type of treatment, although there can be
certain rooms that require different approaches in different areas. Typically,
membrane thicknesses are around 1 mil, so that the membrane will be fully
incorporated within the equivalent of several coats of paint. Properly applied,
the membrane is completely invisible once the treatment has been completed. All
of the subtle undulations of the plaster are maintained. And, most importantly,
the original plaster has been preserved.
The combination of adhesive and membrane depends upon several factors. First
and most important is how the membrane reacts to the adhesive. The second factor
is the nature of the plaster surface. Bare plaster will allow the use of most
adhesives, as will older oil paint. However, certain paints such as whitewash or
calcimine, can create adhesion problems and require special preparation as part
of the treatment. Thus, a treatment of a specific plaster surface is a
balance between the nature of the plaster/paint combination, the amount of time
to be spent on surface preparation, the desired membrane, and the appropriate
adhesives. And, of course, the skill and experience of the treatment crew
is of paramount importance.
This system is not designed for plaster that is heavily detached from the
lath, and is not a substitute for injection stabilization processes.
However, it will stabilize a surprising amount of plaster detached from the lath
if surrounding plaster is still well attached. Envision the process as
similar to a stretched head skin on a drum. The tension created will
support a large amount of weight.
American Conservation Consortium has done extensive testing of various
adhesives, membranes and procedures to develop a system of plaster stabilization
that is highly effective. We would be glad to provide treatment services
with our staff for smaller projects, or we will train and manage your staff for
larger projects.
