Consolidating pencil on cement plaster

Orit Soffer <orit-s [at] actcom__net__il> writes

>We have 60 year old pencil marks on a wall plastered with modern
>cement-based plaster, that we want to preserve. The building
>containing this wall is in very bad conditions and needs a thorough
>structural intervention to prevent it from falling apart. There is
>no way to preserve the plaster during this intervention, so the only
>way to preserve the pencil marks would be by detaching the pieces of
>plaster bearing them. In order to do that we are going first to
>consolidate the pencil marks and then cover them with some sort of
>facing in order to protect them during the process of detachment.
>
>We are going to conduct a small research first (on another wall
>where we will create similar pencil marks) in order to test various
>methods of consolidation and facing and their resistance to large
>quantities of water, such as will be applied during the detachment
>process. We will appreciate greatly any assistance and advice, such
>as:

>If I understand the situation, you are dealing graphite pencil
marks on outer surface of a cementitious plaster that is presumably
applied to a masonry wall. You intend to remove some of the plaster
that carries the pencil marks. You intend to use a great deal of
water to accomplish this--presumably to erode or dissolve the base
coats of plaster. You are worried that in the process, the water
will erode or otherwise destroy the pencil marks. You give no
indication as to the size of the subject area.  My suggestion if the
above is the case is that you consider consolidating the plaster
itself and after consolidation, removing it by cutting it off the
wall.  This approach would have you not touching the actual pencil
marks until the piece is safely removed to a lab.

A modern plaster will have a surface putty coat that could be from
1/16 inch thick to as much as 1/4 inch thick. If it is on the
thicker end of the scale, the putty coat could be removed without
any consolidation, simply be carefully sawing it off the base coats
with the right hand tools.  If it is very thin, then ensure its bond
to its base coat by consolidation and execute the sawing operation
at a deeper level say one inch into the plaster, taking the putty
coat and some or all of the next coat(s) of plaster.

As for what material to use, we have had great success doing this
kind of thing with Rohm and Haas Rhoplex MC 76 diluted (up to
25/75%) with methyl hydrate. The acrylic resin is put in a pail a
few feet above the subject and siphoned through 3/8 inch plastic
tubes connected to hypodermic needles that are placed in fine
drilled holes on a 45 degree angle downwards along the upper edge of
the subject. The needles are secured in place and sealed against
back washing of resin with a bit of moulding clay. I expect
Plasticine or plaster of Paris would also work. Depending on the
porosity of the plaster, the resin seeps into the matrix, will not
exit through the gypsum surface, so saturates the plaster. More or
fewer holes regulate the rate of absorption. Vibration with an
electric palm sander helps move the penetration along. Warming the
surface gently with a hair dryer also encourages the spread of the
resin. Depending again on porosity, it will take from minutes to
several hours to get transmission of the resin through 12" of
vertical wall surface.

Once you have saturated the plaster, stop the feeding and allow the
alcohol to evaporate. Gentle heat and good ventilation will promote
this. This leaves the resin in place within the plaster matrix where
it will coalesce and give the plaster a great increase in
strength--sufficient to withstand the sawing action. Using a
variation on this method we have handled sections of a fresco two
meters by one meter, but only 1/4" thick. At this size, a specially
designed face frame to support the sample was also incorporated.

There are safety precautions required when working with methyl
hydrate (methanol), but the resin itself is non-toxic, water soluble
and can be reversed out of the plaster in the lab if that was
thought to be necessary. Good luck with your experiment,

Rod. Stewart
Historic Plaster Conservation Services Limited
26 Barrett Street
Port Hope, Ontario
Canada L1A1M7


                                  ***
                  Conservation DistList Instance 18:23
                 Distributed: Monday, November 22, 2004
                       Message Id: cdl-18-23-001
                                  ***