Carbonyl pollutants are of concern to the National Museum of Scotland (NMS) as corrosion of metals and glass have been observed and attributed to these gases. The aldehydes and carboxylic acids result from out-gassing of construction and dressing materials such as wood, wood products, paints and varnish. Corrosion of metals in old show cases results in growth of white lead formate and some glass in old storage cabinets has developed crystalline formates.
Pollutant monitoring is undertaken to :
A number of points must be considered during such studies:
A study of pollutant levels, corrosion and mitigation strategies in the Islamic and Oriental galleries of the NMS indicate that
Further studies are required and include:
Appropriate solutions must be inexpensive and simple to implement. Such further research can only occur with close collaboration between museums and universities.
The relative humidity in galleries with affected objects is normally between 20 - 45 %, i.e. very low. The levels of formaldehyde are known to vary over the year so there does appear to be a seasonal affect. The levels of acetaldehyde were much lower than formaldehyde and the levels also show less variability. The corrosion of lead tokens was found to correlate with the formaldehyde concentration. Cases were observed with high acetic acid concentrations but low formaldehyde show little corrosion. Thus conversely to the BM, it is suggested that formaldehyde and formic acid are just as, or even more, deleterious than acetic acid.
Survey of corrosion and artefact composition :
Lead formate and lead carbonate were detected - the lead carbonate is probably derived from lead acetate. Some artefacts showed variable corrosion depending on the composition. Higher lead levels gave high corrosion. Solder alloys did not corrode (i.e. when the tin levels were high). If the tin (or zinc) levels increased the corrosion was reduced. The case which had the most corrosion had the highest concentration of formaldehyde. The case with the highest concentration of acetic acid had the least corrosion of the artefacts.
Activated charcoal cloth was used to see if the levels could be reduced. The corrosion rates did reduce, and so mitigation was partially successful.
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© April 25th, 2000