Dr. KATH EREMIN

The Royal Museum of Scotland

CARBONYL POLLUTANTS : A MUSEUM PERSPECTIVE

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 :

• ascertain the specific gas present
• ascertain the level of each gas
• predict the effect of the organic gases on different artefacts
• define the minimum acceptable levels
• test mitigation strategies

A number of points must be considered during such studies:

• whether gas levels are constant or fluctuate over time ?
• whether different mixtures of gases are present and what effect does this have ?
• what influences the effect of organic gases on artefacts ?
• what is the effect of fluctuations in temperature and humidity ?

A study of pollutant levels, corrosion and mitigation strategies in the Islamic and Oriental galleries of the NMS indicate that

• formaldehyde, acetaldehyde, formic acid and acetic acid are all present in these show cases
• formaldehyde levels fluctuate and may be seasonal, so long term monitoring is required
• the proportions of the different gases vary between cases
• formaldehyde appears to be the most corrosive gas in these cases
• use of activated charcoal cloth reduces gas levels and corrosion, but does not prevent corrosion
• an increase in lead in the copper alloys increases corrosion
• an increase in tin in the copper alloys reduces corrosion in some examples

Further studies are required and include:

• development of monitors for continuous monitoring of gas levels
• development of more effective mitigation strategies
• investigation of the effects of different combination of gases
• investigation of the effects on different materials and alloys

Appropriate solutions must be inexpensive and simple to implement. Such further research can only occur with close collaboration between museums and universities.

ADDITIONAL NOTES

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.