Consolidation of degraded iron-tannate dyed Phormium tenax fibres using zinc alginate

Abstract

From pre-European contact times Maori have produced finely woven garments and cloaks woven from fibres extracted from the leaves of Phormium tenax (New Zealand flax). One of the dyes traditionally used to decorate this weaving was the black iron-tannate dye. However, this complex causes the underlying substrate fibres to deteriorate with a loss of mechanical strength and embrittlement. Also the dye fades and ultimately becomes a tan colour. To preserve badly degraded fibres and retain their physical integrity it is necessary to treat them with a binding agent known as a consolidant. However, many of these polymeric materials do not prevent the fading of the fibres with further ageing. In this research consolidants based on zinc alginates-a seaweed derivative, were found to be very much more effective in retaining colour than the consolidants previously used by museum conservators. The alginate, the polysaccharide part of the consolidant, and the zinc ions were found to act independently of each other to slow the chemistry responsible for the fading of the black iron-tannate dye. Zinc ions were found to be more effective in retaining the colour than the other divalent metal ions tested; calcium, magnesium and barium. The accelerated thermal ageing process is accompanied by a decarboxylation reaction sensitized by ferric and other metal ions present to produce carbon dioxide with the simultaneous consumption of oxygen. These reactions were substantially increased in fibres that had been treated with zinc alginate and zinc hydroxyalginate. From this it is clear that the decarboxylation reaction is not responsible for the fading of the dye.