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.

Experiments on finger-jointing acetylated beech (Fagus sylvatica L.) laminated veneer lumber (LVL) have been made. The specimens were examined on its wet tensile shear strength (TSS) using three adhesives, including phenol resorcinol formaldehyde (PRF), one-component polyurethane (PUR) (without primer), and melamine urea formaldehyde (MUF). Contact angles (CA) of uncured and drop-applied MUF, PRF, and PUR adhesives on freshly cut finger-joints were evaluated. Surface roughness was measured using a laser-scanning-microscope (LSM). Results showed that PRF bonded acetylated specimens had highest wet TSS, followed by specimens with PUR bonding. MUF performed poorly, which was most likely caused by its inadequate water resistance and changes in chemical reactions due to remaining acetic acid. Acetylated finger-joints had a topography similar to untreated joints. Moreover, CA were just barely lower for MUF and PRF on acetylated wood than on untreated references.

Acetylation with acetic anhydride is well known to improve the dimensional stability and durability of wood. Veneer is appealing for acetylation because of its thin thickness, which supports a complete and even impregnation of difficult-to-treat wood species, such as beech (Fagus sylvatica L.). Unlike resin-based veneer impregnation, acetylated veneer does not require any additional curing. As a result, veneer properties are already altered prior to bonding. The compression thickness reduction in acetylated beech veneer during the manufacturing of laminated veneer lumber (LVL) utilizing 1, 3, and 6 MPa at 150 °C for 30 min is investigated in this study. The results show that acetylated beech veneer is considerably less compressible than the references. Moreover, the density of acetylated LVL at low pressure (1 MPa) is similar to the one of references, even though the compressibility is much lower. This is due to the added acetyl groups after acetylation. The reduction in compressibility is most likely caused due to a decrease in moisture content (MC) and its accompanied mechanisms.

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The overall aim of this study was to investigate the durability of acetylated beech (Fagus sylvatica L.) laminated veneer lumber (LVL) against wood-destroying basidiomycetes. The secondary objective was to test whether the specimen size affects the mass loss and durability assessment of wood-material under test. The durability test was based on the pre-standard prEN 113-3. Six materials (acetylated beech LVL, untreated beech LVL, beech solid wood, pine sapwood (Pinus sylvestris L.), larch (Larix decidua Mill.) 3-layer slab, larch solid wood) were tested using three specimen geometry designs (50 × 25 × 15 mm3 as well as 50 × 50 × 19 mm3 with and without sealed edges) against Coniophora puteana, Rhodonia placenta, Gloeophyllum trabeum, Trametes versicolor, and Pleurotus ostreatus. The durability assessment was made using the arithmetic mean and median percentage mass loss (ML), the relative ML (x-values), and the decay susceptibility index (DSI). It was found that mass loss was affected by the test fungus, the material, and the specimen size and design, with the latter being the most essential factor in this study. In addition, the assessment parameter had a significant effect on the durability classification. Furthermore, small differences in ML resulted in different durability classes (DC) in some cases, whereas large differences in ML did not. However, acetylated beech LVL was always considerably durable (DC 1) against all tested fungi independent of the specimen design and durability assessment method.

The purpose of this review is to put previous research findings on acetylated wood and the fabrication of veneer-based products in a common context. The first research on wood acetylation was already conducted in the 1920s using wood meal, whereas relevant research on veneer acetylation was published nearly two decades later, during the 1940s. In the years that followed, a great deal of research has been done on both solid wood and composite acetylation. Developments in the 1990s and early 2000s resulted in the creation of commercial products. Nowadays, wood is becoming increasingly popular in construction. Therefore, high-performance materials with high dimensional stability and durability are required. Veneers are thereby of particular relevance because of their propensity to absorb chemicals into even tough-to-treat wood species. However, acetylation alters the bonding properties of wood, which is important for the manufacture of engineered veneer products, especially in load-bearing construction. A large amount of research is now being conducted on the acetylation of veneer, and acetylated veneer products are anticipated in the near future. This study covers the fundamentals of bonding but focuses specifically on veneer acetylation and its fabrication to engineered veneer-based products. The influencing factors of acetylation on bonding are also discussed.