Interventive conservation of black-dyed organic materials: the problem of metal-polyphenol complexes
- Marei Hacke, project leader
- Helen Wilson
- Vincent Daniels
- Pippa Cruickshank
- Monique Pullan
- An Arts and Humanities Research Council Collaborative Doctoral Award, organised through the Science and Heritage Programme,
The British Museum
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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.
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 follows:
- 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.