Carbonyl compounds are of concern in museums because of their potential to damage artefacts. Lead coupons were exposed to gaseous formaldehyde and formic acid in various humid oxidizing atmospheres. The resulting corrosion products were weighed and identified using X-ray diffraction analysis after as much as fifty-six days exposure. When formaldehyde was in more oxidizing environments (hydrogen peroxide containing atmospheres) it caused heavy corrosion, but in the dark there was the merest tarnish even at high humidities. This suggests that oxidation is an important control on corrosion by formaldehyde. Thermodynamic calculations indicate at low formic acid concentrations, (below about 1 ppb), the corrosion products should be (carbonates) plumbonacrite and hydrocerussite, while at higher concentrations lead formate would be expected.
This work has two significant findings.
1. Formaldehyde is not damaging towards museum artefacts, but clearly has the potential to cause damage if it is oxidized to formic acid. Realistic estimations of the risk presented by formaldehyde in any museum can only be assessed in terms of whether the particular environment is well controlled in both relative humidity and light levels and the presence of oxidants such as ozone or hydrogen peroxide.
2. The type of efflorescence found on artefacts needs to be re-examined in the light of this work. The nature of the substrate does affect the corrosion found but the level of acid also affects the type of corrosion found. Conservation scientists need to be clear about whether it is the substrate or the acid levels that are causing a particular corrosion.
The results are significant in the establishment of threshold levels for formaldehyde. Further, the effects of the substrate cannot be clearly defined until the chemistry of all pollutants has been fully investigated and understood.
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© May 11th, 2000