Paper Conservation: Decisions & Compromises

Verdigris 2:
Wet Chemical Treatments which are not Easy to Decide and Apply
Kyujin Ahn | Andreas Hartl | Christa Hofmann | Ute Henniges | Antje Potthast
University of Natural Resources and Life Sciences, Vienna, Department of Chemistry, Vienna, Austria;
Austrian National Library, Conservation Department, Vienna, Austria
Introduction
Paper objects with copper pigments are threatened
as they may exhibit an accelerated degradation
of cellulose and heavy discoloration of the
paper depending on the condition of the object.
In some cases, the paper eventually becomes very
fragile and even handling of the object becomes
critical. Paper conservators and scientists have
been seeking solutions in many directions – reinforcement,
environmental controls, solution
treatments, etc. As a part of the forMuse programme
of the Austrian Ministry of Science and
Research, various solution treatments of paper
with verdigris were tested simulating a practical
situation that conservators might face in their
workshop. In the present study, various chemical
treatments of a handmade rag paper containing
verdigris bound in gum arabic, will be discussed
in comparison to the results obtained from the
same rag paper impregnated with copper. The
copper impregnation represents a more homogeneous
situation which can be better controlled
and monitored.
Methods and Materials
The solutions applied to pre-aged test specimen
varied from calcium bicarbonate in water to metal
ion complexing agents or antioxidants in ethanol.
Spraying on a suction table and brushing on
the verso were mainly applied to samples with
partially soluble verdigris (Type 1). Only immersion
treatment was carried out for the samples
that underwent oxidation and impregnation of a
fixed content of copper (Type 2) prior to the treatment.
After applying accelerated aging, measurements
of molar mass and carbonyl group content
of samples were performed by GPC (Gel Permeation
Chromatography)-Fluorescence-MALLS
(Multi Angle Laser Light Scattering)-RI (Refractive
Index) system after selective chemical labelling
of carbonyl groups.
Results
None of the alkaline treatments in water nor in a
mixture of water and ethanol reduced cellulose
degradation significantly for samples of type 1
regardless of the application methods employed.
Especially 100% aqueous alkaline treatments
degraded the sample much more compared to
the untreated control. On the other hand, when
those solutions were applied to samples of type
2 different results are obtained: Alkaline treatments
with calcium bicarbonate or magnesium
propylate were both effective to reduce Mw loss
and formation of carbonyl groups. Benzotriazole
in ethanol which has been used in the field of
metal conservation worked as the most efficient
inhibitor of copper-catalyzed degradation of cellulose
among the applied chemical solutions,
more importantly for both types of the samples
although it causes significant discoloration of
paper depending on its concentration. It was also
found to be effective under photo-oxidative as
well as under hot and humid aging conditions. A
concentration of 0.5% was high enough to form
a benzotriazole-Cu complex by immersion of
samples of type 2 in solution while the same concentration
of benzotriazole was not yet promising
for sample type 1 with a spraying application
method.
Calcium phytate treatment followed by a deacidfication
treatment did not lead to any retardation
of cellulose degradation in the presence
of copper ions for both types of samples unlike
for paper containing iron gall inks ((Neevel 1995;
Reißland 1999). Both sample types treated with
tetrabutylammonium bromide, known as an antioxidant
scavenging hydroxyl radicals, exhibited
significant reduction of degradation compared
to the untreated control. A single application of
ethyl-p-hydroxybenzoate which decreased the
degradation rate when it was combined with a
non-aqueous deacidfication treatment (Henniges
et al. 2006) did not show any beneficial effect in
terms of molar mass protection of cellulose for
both sample types.
ICOM-CC Graphic Documents Working Group Interim Meeting | Vienna 17 – 19 April 2013
36