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Pollutants can cause or significantly affect the oxidation or hydrolysis of skins and leathers. The risk category is ranked in general as moderate to low for most skins and leathers, but will increase the higher the pollution levels. Vegetable-tanned leathers are most vulnerable, more so than alum-tawed skins or rawhide for example, while chrome-tanned leathers are the least vulnerable (Haines 1991). Sulfur dioxide (SO2) is considered the main source of the acidic environment that causes hydrolytic breakdown in leather, while ozone, sulfur dioxide, nitrogen oxides and peroxides are key causes of oxidative breakdown processes. Visible light and ultraviolet (UV) radiation, which can create high-energy free radicals, are also significant factors in initiating oxidation processes. Heat, moisture and the presence of acids or acidic gases also play roles in increasing the rates of both oxidative or hydrolytic breakdown, as well as the presence of metal ions, in particular iron or copper ions (Florian 2006). Sources of metal ion contaminants include iron-based black dyes or mordants, iron-gall ink, some pigments (such as verdigris), blood or rust stains, dust, soiling, metal attachments, or machining and processing impurities.
“Red rot” is a term that refers to the condition whereby vegetable-tanned leather has undergone severe chemical degradation (and not rot, per se, which implies a biological origin) due to inherent tanning compounds as well as atmospheric pollutants [usually sulfur dioxide (SO2)]. Red-rotted leather (Figure 30) is more acidic (typical pH in the range of 3.0-3.5). It is often damaged and worn and has a powdery surface, and physically, it is quite weak and vulnerable to abrasion damage or tearing, especially if improperly supported during handling, storage or display. In the more advanced states, it is noticeably red.
© Government of Canada, Canadian Conservation Institute. CCI 124703-0004
Figure 30. Red rot on leather bookbindings. The leather is quite red, powdery, weak and easily torn.
In the 1930s, when coal was more widely used than today and was the main cause of SO2 pollution in the atmosphere, it was observed in the United Kingdom that leather bookbindings in urban locations were suffering more from red rot than those in cleaner rural settings (Brimblecombe 1996). In Canada, SO2 levels were high in industrialized areas of the country from the start of industrialization up to the 1970s when acid-rain legislations greatly reduced SO2 levels in the air. Many examples of historic vegetable-tanned leather have suffered from SO2 deterioration over time and now are weak and degraded.
Degradation due to metal parts
Stains and chemical weakening of leather, skins and fur-skins can be caused by their oils or by leather dressing applications in intimate contact with metal parts, usually iron or copper or their alloys (Figures 31a and b). The oils attack the metals, and the resulting rust or corrosion products in turn can stain and attack or weaken the skin and leather around it. In the case of copper and its alloys, these corrosion products are typically waxy and green (copper stearate).
© 2013 President and Fellows of Harvard College, Peabody Museum of Archaeology and Ethnology, 90-17-10/49318
Figure 31a. The iron tinklers on the black-tanned pouch are rusting and causing damage.
© Government of Canada, Canadian Conservation Institute. CCI 124703-0050
Figure 31b. Brass rivets corroding due to the leather’s oils. The reaction produced a green corrosion product (copper stearate).
Recommendations
- Protect from dust, grime and pollutants using display cases, dust covers, etc. (Figure 32).
- Wear gloves and avoid direct handling to avoid transferring oils or grime from the hands. In particular, take care with parchment, alum-tawed skins and semi-tanned skins as they are easily soiled.
- Measure pollutants levels, assess risks and, if necessary, reduce airborne pollutants.
- Identify all sensitive leather and skin objects, including items suffering from red rot. Provide enhanced pollution protection for these items. For storage, this can include wrapping red rot–affected leather objects in acid-free tissue paper, padded out as required and enclosing them in acid-free, alkaline-buffered boxes.
- Avoid any contact between metals (e.g. shelving, tags, clips) and skins and leathers. Where metal is part of the object and the metal is in contact with the skin or leather, insert a Melinex interleaf or other barrier, if possible. If the metal is actively corroding, consult a conservator for further advice.
