The formation of formates: a review of metal formates on heritage objects

Abstract

Metal formates sometimes occur as degradation products on heritage objects due to the use of wood products or other sources of formic acid and formaldehyde. They are often related to alkaline surfaces which transform formaldehyde directly into formate. The dominant role of formate on alkaline surfaces, for example in glass-induced metal corrosion (GIM) or calcium carbonate degradation, was explored in the Stuttgart research on rare heritage corrosion products. This review discusses these findings together with those from the literature: ocurrences of sodium and potassium formate on glass, calcium formate and calcium acetate-formate phases on calcareous materials, magnesium formate on Sorel cement and dolomitic sandstone, lead formate on metal and pigments, cadmium formate on coated objects, as well as various copper and zinc formates on copper alloys. In the latter cases, formates dominate as glass-induced metal corrosion products. The formation of formates constitutes irreversible damage, degrading the material of heritage objects. Therefore, preventive conservation needs to remove all sources of carbonyl pollutants in order to avoid such corrosion.

Influence of volatile organic compounds (VOCs) in the alteration of historical unstable glasses

“…However, with aldehydes this process is not so clear, although the following scenario is most likely to occur: (a) the aldehyde (formaldehyde) generated from the cabinet's materials contaminates the atmosphere, (b) it is transformed into an organic acid in the presence of an oxidant such as hydrogen peroxide, and (c) the acid attacks the material, losing hydrogen in the process and forming the corresponding anion (Reaction ). What is not yet known for sure is whether the aldehyde oxidizes to its corresponding organic acid in the air or on the glass surface 8,11 . A Cannizzaro reaction could also occur (Reaction ), which involves the base‐induced disproportionation of two molecules of a non‐enolizable aldehyde to give a primary alcohol and a carboxylic acid 12 …”

“…What is not yet known for sure is whether the aldehyde oxidizes to its corresponding organic acid in the air or on the glass surface. 8,11 A Cannizzaro reaction could also occur (Reaction 5), which involves the base-induced disproportionation of two molecules of a non-enolizable aldehyde to give a primary alcohol and a carboxylic acid. 12 (Reaction 4) (Reaction 5) The VOCs are particularly dangerous to unstable glasses, which are glasses with low former and/or stabilizer oxides.…”

Water Sorption on Formic Acid-Modified Hydroxyapatite as a Function of Relative Humidity: Simulated Environmental Aging Effects on Hygroscopicity and Nutrient Release

“…This suggests that this area corresponds to Ca(HCOO) 2 formation as previous studies have shown that FA adsorption on HAP (010) occurs through the transfer of the acidic proton on FA to the surface and HCO − bonding to the surface Ca site. 19,25,26 Since the confocal Raman analysis depth is several hundred nanometers into the crystal, HAP parent structure peaks were detected in the areas where Ca(HCOO) 2 was detected, as shown in the combined images. Raman mapping indicates the formation of Ca(HCOO) 2 on the HAP particles but localized to distinct parts of the HAP particles.…”

“…These peaks are attributed to the formation of Ca(HCOO) 2 as the FA vapor reacts with HAP and exhibits new C−H and C−O stretching modes. 25,26 While Raman spectra acquired at select spots for the FAmodified HAP suggested the formation of Ca(HCOO) 2 , to investigate the spatial distribution of new Raman signals over the particles, Raman mapping was used. Figure 2a,b shows the optical confocal images of HAP particles exposed to FA for 24 and 48 h, respectively.…”

“…In the case of FA-modified HAP, additional peaks at 1356, 1392, 2880, and 2900 cm –1 were observed. These peaks are attributed to the formation of Ca­(HCOO) 2 as the FA vapor reacts with HAP and exhibits new C–H and C–O stretching modes. , …”