James Crawford
Destructive microclimates |
Corrosion of lead by vapours from wood
| Lead is a metal humankind has used since antiquity. It is as an example of a material which is vulnerable in some types of environments found in cultural heritage contexts. When present with certain cabinetry and carpentry construction materials, and ventilation is restricted, lead can be attacked by emitted volatile organic compounds (VOCs). Implicated VOCs include formaldehyde, formic and acetic acids coming from some timbers (most notoriously oak), polyvinyl acetate (PVA) wood-glue and medium density fibreboard (MDF) binders. |  |
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Corrosion-induced collapse of a pipe from the Stellwagen organ (1467 AD) (images courtesy of University of Gothenburg/GOArt ) |
Corrosion of a lead seal on a Roman Catholic bull stored in oak furniture (image courtesy of C. Degrigny) |
Simulating corrosive atmospheres with laboratory experiments
When a microclimate cannot be ventilated or when materials are inseparable from their deteriogens, protection of susceptible surfaces can be made. New protection materials need laboratory testing in aggressive atmospheres prior to field-testing. In the timelapse video below, the recorded corrosion of unprotected lead over time has been accelerated 2 million times.
| Atmospheric corrosion of uncoated lead by oak: 2 years in 25 seconds.
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| Laboratory simulation of a humidified microclimate with oak VOCs, and X-ray diffraction confirmation of the formed lead acetate corrosion products | |
A proposed corrosion mechanism for lead exposed to acetic acid atmospheres
Degrigny & Le Gall (1999) in Studies in Conservation, Vol. 44, No. 3, 157-169 after Turgoose (1985), in Lead and Tin: Studies in Conservation and Technology, UKIC Occasional Papers No. 3, 15-26