Poster Session,
1998 AIC meeting
Published in "ConservationNews" UKIC, July 2000
French Polish
from a Conservators Point of View:
Some Ideas for
a Better Coating.
This poster
brings some new ideas in maintaining the tradition of French polishing while introducing
modern materials that will hopefully prolong the life of the film forming materials and
improve it’s workability.
The technique
of French polish has been around since the very late 18th century. Over the years, this
traditional application of shellac has changed little. Various recipes can be found in
many finishing books that call for a variety of different ingredients that were believed
to enhance the qualities of the finish. Studying
old recipe books, it became clear that most were concoctions based mostly on hearsay and
unfounded boastings of the different materials. Even
today, woodworking magazines are publishing articles based on these old recipes and flawed
concepts.
As furniture
conservation matures, a more detailed look has been taken at the individual resins used in
early varnish making. The resin of coniferous
trees (pines) typically known as rosin, was often listed as a key ingredient. Under a variety of different names, (Venice
Turpentine, Burgundy Pitch, Colophony, and Elemi,) rosin is now known to degrade rapidly
and severely. Linseed oil and other drying
and non-drying oils are often listed as ingredients. With its crosslinking capabilities,
linseed oil can cause the film to become tenacious and darken. Plus, such incompatible materials as oils, rosins
and even gum Arabic, can lead to an internal breakdown within the film itself.
Shellac, the
key ingredient in French polish is a very versatile and desirable natural resin. It is the addition of all the cheaper incompatible
resins in the recipes that caused concern.
I attempted to
create a more stable, and acceptable coating for historic furniture, plus keeping the
visual beauty of traditional French polish. I
began to eliminate those cheaper resins prone to rapid degradation that have always been
listed in the traditional literature as important ingredients. I also wished to create a clear polish,
which meant I needed to eliminate the naturally occurring wax in the shellac. (The wax
tends to cloud the polish). While there are commercially available shellacs that have the
wax already removed, they tend to be chemically processed to the point of instability,
most notable the white, bleached or “blond” shellacs. I prefer to use button shellac primarily due
to the limited amount of processing it has gone through.
But the wax serves as a natural plactisizer allowing the brittle shellac
resin to remain somewhat flexible. With the wax eliminated, I needed to replace the
wax’s placticizing effects with another material.
Among the softer plant resins I have looked into are Manila
Copal, Benzoin, sandarac, damar, mastic and others. I
have chosen benzoin and the Manila copal based on traditional successes of these resins
and the pleasant odor the benzoin imparts.
However, they
are added in small quantities in relation to the shellac.
I added .5 oz
of B-72 and immediately noticed an improved workability of the polish. The grain filling properties and lack of shrinkage
caused by the presence of the B-72 made the polish quicker to use. Plus the added benefit
of such a stable material as the ethyl methacrylate copolymer, makes this polish warrant
further investigation. I also
experimented with B-67 (isobutyl methacrylate polymer), often listed a varnish resin, but
I found that B-67 made the polish drag and stick as it was being padded out. Plus B-67 has
a tendency to crosslink.