The Staining Effect of Iron (II) Sulfate on Nine Different Wooden Substrates

Abstract

Leaving wooden façades uncoated has become popular in modern architecture, especially for large buildings like multi-story houses, in order to circumvent frequent maintenance, particularly repainting. To obtain a quick and even artificial graying of the entire façade that gradually turns into natural graying, a one-off treatment with iron (II) sulfate may be applied. Its mode of action is commonly ascribed to a reaction with phenolic wood extractives, especially hydrolyzable tannins. This does not however sufficiently explain iron (II) sulfate’s ability to color wood species containing only marginal amounts of phenolic extractives; moreover, little is known about the influence of the wooden substrate and light conditions on the color development of façades treated with iron (II) sulfate. In the present study, we investigated the influence of wood extractives, exposure conditions, and nine different wooden substrates on iron (II) sulfate’s staining effect. Spruce specimens with and without extractives were treated with a 4% iron (II) sulfate solution and exposed to sunlight behind window glass. Both wood types darkened slowly but significantly during 51 weeks of exposure. This shows that artificial graying with iron (II) sulfate (1) does not require precipitation unlike natural graying, (2) takes place without initial wood extractives, and (3) proceeds at a slow rate. Specimens protected from sunlight changed their color only slightly, suggesting that photo-induced phenoxyl and ketyl radicals from photolysis of lignin’s ether bonds oxidize iron (II) to iron (III). Specimens made of spruce, pine, larch, and western red cedar (WRC) and exposed outdoors decreased strongly in lightness during the first two months of exposure. In contrast, a staining effect of iron (II) sulfate in terms of artificial graying was not seen on acetylated radiata pine, possibly because iron ions are hindered from entering the cell wall. Specimens partly protected by a roof overhang showed an uneven color development; this is due to the protection from radiation and not from precipitation as is known for natural graying.

Electrospinning of Softwood Organosolv Lignin without Polymer Addition

“…A difference of color could be observed between lignin fibers produced by electrospinning of lignin solutions in DMF with and without FeCl 3 addition. It has been demonstrated that a complexation between cations (including Fe 3+ ) and phenolics (from wood extractives) can change the wood surface color. − Phenolic compounds are known to easily chelate the transition metal cations, as it is the case with iron or copper, which have been well-studied. − This is why we have used colorimetry to document and quantify the color difference observed between the fibers produced with and without FeCl 3 . According to ASTM D2244-16, the CIELab technique was used to quantify color changes .…”

Effectiveness of European oak wood staining with iron (II) sulphate during natural weathering

“…The iron-induced stains in oak are gray or blue-black coloured, occurring more frequently in heartwood than in sapwood (Kreber 1994), the most probably due to difference in extractives content in sapwood and heartwood. On the other hand, according to Hundhausen et al 2020, the dark colour development of the extracted spruce confirmed that any reaction with wood extractives is not necessary for wood colour changes due to treatment with iron (II) sulphate. Discolouration occurs during each contact of wet wood with iron, but certain wood species show particular predisposition.…”

“…The oak samples treated with iron (II) sulphate were intensively darker right after the treatment than the untreated reference samples (Figure 1). Wood colour turned immediately dark grey-blue-black after the treatment with iron (II) sulfate as the iron ions reacted with gallic acid to ferrous gallate, which subsequently oxidized to a dark ferric pyrogallate complex (Krekel 1999, Hundhausen et al 2020). The differences in colour between the applied variants were below the visibility limit.…”

“…According to the contemporary trends it is recommended to use untreated wood that turns grey after exposure to weather under aboveground conditions (Oberhofnerová et al 2017). Leaving uncoated wood during external exposition has become popular in modern architecture in order to circumvent frequent maintenance such as repainting (Hundhausen et al 2020). The decisive element for using different wood species in the harsh outside environment is their durability, in accordance with EN 350 (2016).…”

“…In Scandinavia, a popular artificial graying method is treatment with iron sulphate that gradually turns the wood surface colour from artificial into natural graying. In this scope mainly coniferous wood species (such as spruce) were tested (Hundhausen et al 2020). Darkening wood surfaces is based on the reaction of extractives with polyphenolic structure and metal ions (Canevari et al 2016).…”