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Open AccessArticle

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

1
Department of R&D and Consultancy, NTI (Norwegian Institute of Wood Technology), P.O. Box 113 Blindern, 0314 Oslo, Norway
2
Department of Wood Biology and Wood Products, University of Göttingen, 37073 Göttingen, Germany
3
Faculty of Wood Technology and Construction, Rosenheim Technical University of Applied Sciences, 83024 Rosenheim, Germany
4
Department of Wood Technology and Construction, Salzburg University of Applied Sciences, 5412 Puch bei Hallein, Salzburg, Austria
*
Author to whom correspondence should be addressed.
Forests 2020, 11(6), 658; https://doi.org/10.3390/f11060658
Received: 7 May 2020 / Revised: 2 June 2020 / Accepted: 3 June 2020 / Published: 9 June 2020
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. View Full-Text
Keywords: artificial graying; iron sulfate; sunlight; phenolic extractives; photodegradation; weathering; wooden façades artificial graying; iron sulfate; sunlight; phenolic extractives; photodegradation; weathering; wooden façades
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Hundhausen, U.; Mai, C.; Slabohm, M.; Gschweidl, F.; Schwarzenbrunner, R. The Staining Effect of Iron (II) Sulfate on Nine Different Wooden Substrates. Forests 2020, 11, 658.

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