Thursday, 6 December 2012

Pesticides on Museum Objects

Part of the Cook-Voyage project at the Pitt Rivers is to look at the question of pesticide residues on objects from the collections.  Chemicals have always been applied to objects in museum collections to protect them from pests, mainly insects, which can cause a lot of damage. 

As the Forster Collection has been in Oxford since 1776, and the Banks collection was at Christchurch even before that, the Cook-voyage collections potentially span virtually the whole history of pesticide use in museums.  Even though we know that chemicals such as arsenic, mercury and DDT, were routinely applied to museum collections, finding evidence of what was actually done is difficult.  

Label from a poncho from the Tahitian Mourner's costume, giving information on the condition of the feather cape.

A label on the Tahitian Mourner's costume, written just before the Ashmolean's ethnographic holdings were transferred to the Pitt Rivers in 1886, records that the cloak was 'now in a very bad condition' and an entry in the Accession Registers goes on to say that this was 'through the moth, and (it) has been removed to the Museum in the Parks (Oxford University Natural History Museum) and placed under the charge of Professor Moseley to be fumigated.'  We don't know, however, what fumigation involved. 

Sometimes, the Pitt Rivers annual reports give us more of a clue.  In 1946-47, the Curator, T.K. Penniman, wrote that 'we have tried the Clymax spray on all of our material, and have found it thoroughly effective in destroying every sort of pest that attacks every sort of material that human beings use.'  Powerful stuff indeed!  There is also a mention of a fumigation chamber being built so that methyl bromide gas could be used to kill pest infestations on museum objects, but it is not clear whether this was ever actually used.  

Jeremy Uden and Kloe Rumsey, conservation intern, taking samples of surface deposits from a Tongan mat.
The analysis of pesticide residues is being done for us by Andrew Charlton, an analytical chemist at the Wildlife Incident Unit, part of the Food and Environment Research Agency.  He usually works on investigating pesticide uptake by wildlife, especially honey bees.  The samples he works on are usually destroyed during the analysis - not something that is practical for museum objects.  We are using cotton wool swabs dampened with distilled water to take samples of the surface dirt and residues from objects - we know that the object is not damaged in the process, which is of paramount importance, but we have to accept that we may not be able to get large amounts of any residues remaining on the surface onto the swabs. 

Once Andrew receives the swabs, any residues on them are extracted with an organic solvent for analysis using gas chromatography - mass spectrometry (GC-MS), a highly sensitive technique which can detect parts per billion (ppb) levels of target pesticides.  Each target pesticide is detected in the mass spectrometer by the presence of characteristic molecular fragments providing a unique ‘fingerprint’.  To detect the presence of arsenic or mercury compounds, swabs are extracted with dilute acid for elemental analysis by inductively coupled plasma - mass spectrometry (ICP-MS).  Argon plasma at a very high temperature (approximately 6000 to 10000°C) is used to turn the sample solution into ionised atoms (ions). Each element produces ions with a characteristic mass and charge, which are detected by the mass spectrometer. ICP-MS can detect residues of arsenic or mercury pesticides in the swabs at ppb levels.

Cotton wool swabs used to remove surface deposits from a fragment of feather from the Tahitian Mourner's costume cloak.

The results of the analysis will help us to understand how pesticides were used in the museum, and may help answer questions we are starting to ask about whether the pesticides have contributed to damage, such as staining and deterioration in certain areas of some of the objects.  The results will also help us to make sure that the precautions we routinely take when we handle objects, such as wearing gloves and lab coats, are effective.