Overview
Herbarium collections are generally very stable as long as they have been pressed and dried correctly and stored at a moderate temperature and humidity, free from pests.
During 1997, as Curator Botany at Te Papa, Dr Wendy Nelson (now with NIWA) discovered a disturbing phenomenon affecting some of the Agardh Herbarium red algae specimens in the Natural History Museum Herbarium. Back in New Zealand she also discovered affected specimens in Te Papa's collection. These specimens can be seen below.
Wendy Nelson and a colleague, Ruth Falshaw, wrote about this phenomenon in Taxon, 1999.
Affected specimens develop brown-black patches and became slightly sticky. The herbarium sheet and flimsy itself also become discoloured and holes may appear. Not all the plant material of the affected Te Papa specimens deteriorated. There is a distinct line between affected and non-affected material. The Ph was 0 to 1 for affected material and 5 or 6 for unaffected material.
They found this form of deterioration, affected specimens of two genera: Gigartina and Callophyllis. The blackening was found to be the result of break-down of carrageenan cell wall material, found in these genera. Concentrated sulphuric acid turns sugars into a black tar.
Other examples have since been found in other herbaria, such as the State Herbarium of South Australia. In 2009, specimens of Gloiocladia, Gigartina and Hymenocladia were found to have developed the familiar brown/black patches which turned sticky and affected the paper.
Once triggered, the reaction slowly spreads to unaffected thalli surfaces. The recommended action for affected specimens is: image, separate and remount unaffected material (isolate the original material) and monitor the condition of the newly mounted material.
Reference: Nelson, W.A., Falshaw, R., 1999. Irreversible deterioration of some carrageenophytes (Rhodophyta) in herbaria. Taxon 48: 325-329.