Interventive conservation of black-dyed organic materials: the
problem of metal-polyphenol complexes
Project leader: Marei Hacke, Helen
Wilson
Department: Conservation and Scientific
Research
Project start: October
2008
End date: September 2011
Other British Museum
staff:
Vincent Daniels
Pippa Cruickshank
Monique Pullan
External
partners:
University of Manchester, http://www.manchester.ac.uk/
Professor Chris Carr, http://www.materials.manchester.ac.uk/aboutus/staff/chriscarr/
Project funded by:
Arts and Humanities Research Council/
EPSRC (Collaborative Doctoral Award), organised
through the Science and Heritage Programme
The British Museum
Description:
Black iron-tannate dyes have been used for
thousands of years on nearly every continent. Their usage on a vast
array of
materials to produce objects
of ceremonial, domestic and/or aesthetic value has resulted in the
presence of important iron-tannate dyed objects in museum
collections worldwide.
The dye is formed through the combination of
tannins and iron ions. Tannins are extracted from leaves, bark,
fruits and galls, while iron ions are sourced from iron sulphate
(vitriol), iron-rich mud and metal iron filings. The wide range of
sources and recipes leads to an equally wide range of dyes which
can vary in colour from black to grey and brown. The underlying
chemistry is the same for all these dyes and also for iron-gall ink
which was a very popular ink in the West from medieval times
through to the nineteenth century.
Unfortunately, the excess iron often present
in these dyes and inks can accelerate the natural fibre degradation
processes of oxidation and acid hydrolysis. This results in weak,
brittle fibres which lead to eventual loss of material in an often
very short time.
Consolidation, environmental controls and
minimal handling all reduce risks to these objects and to some
extent inhibit degradation. However, a chemical treatment is needed
to slow down the rate of degradation which is accelerated by the
iron (and sometimes copper) ions. A successful aqueous treatment is
available for iron-gall ink on paper but its requirement for water
makes it unsuitable for the dyed objects in the British Museum.
This project aims to develop a
non-aqueous treatment to inhibit the accelerated degradation.
Possible treatments to be investigated could include the use of
antioxidants, chelating agents or deacidifiers.
Objectives:
The key objective is to develop a non-aqueous
treatment to inhibit the acceleration of natural degradation
processes catalysed by iron-tannate dyes. A basic outline to
achieve this is as fo
llows:
- Research the history, chemistry and usage of
iron-tannate dyes including the presence of iron-tannate dyed
objects within the British Museum collection
- Produce iron-tannate dyed model materials for
future testing. The main chosen materials are cotton, abaca, wool
and silk
- Develop an accelerated aging procedure and
apply to the model materials
- Apply the chosen treatments developed through
research and experimentation, to the model materials
- Artificially age the model materials
- Analysis throughout both stages of artificial
aging and comparison with reference materials will provide rates of
degradation for comparison
- Research and experiment with application
methods for the treatments
- If a treatment is successful, it may be
possible to apply to a Museum object selected by conservators.
News:
Pursuing problematic black dyes - A gallery
talk by Helen Wilson and Marei Hacke, Tuesday 19
May, 13.15.
Images (from top):
- Helen Wilson preparing a modern textile for analysis with X-ray
fluorescence (XRF). (Note object is not from the British Museum
collection)
- A banana leaf or hibiscus fibre loin cloth from the Caroline
Islands, Micronesia (Oc1904-282) - the damaged sections caused
by iron-tannate dye
- Detail from a polynesian tiputa (poncho) made from barkcloth
decorated with iron-tannate dye/pigment (Oc.4253/Oc1886C1.42).
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