Clear wood

Clear wood is the facade material of today. Its use, however, asks much more of the architect’s knowledge of materials. Modern preservation and production methods make aesthetic options better manageable.

Modifying proves promising preservation method
Clear wood is the facade material of today. Its use, however, asks much more of the architect’s knowledge of materials. Modern preservation and production methods make aesthetic options better manageable. An overview.

Clear Wood Without a Care
The natural look is a powerful presence in present day architecture. Visual aspects like natural texture, structure and color of concrete, stone, metal and wood are shown in all their raw power. With its natural appeal, favorable price and manageability, clear wood is increasingly used as facade material. Current fire safety, durability and environmental regulations make application of clear wood facades far from easy. Plus, much stricter environment standards completely banning facades made of tropical hardwood lead to the development and use of wood types traditionally never considered. Designers’ ethical views are changing too. The widely encountered natural graying, under certain conditions bringing out a wonderful silvery sheen, no longer is always the desired result. On the contrary: increasingly, designers want to contrast the warm, brown orange glow of the original wood with the gray tones of concrete or the brown of basalt. Which in turn evolves into new uses. The pertinent question therefore is what currently are the technical / chemical options open to designers for prolonged safeguarding of the chosen facade. This article addresses preservation by means of impregnating or a transparent finish, and the latest development of modifying wood.

To fire-proof and preserve wood it can be impregnated with a preservative. Drawback of such a preservation method is that it causes darkening and fading of the wood. Special fire retardant coatings (heat applied foam) are hygroscopic and hence not suitable for outdoor applications. Impregnating the wood with fire retardant agents increases the risk of leaching requiring varnishing of the treated surface and resulting in additional maintenance. TNO tested products are: Hickson Garantor Non Com exterior Method: impregnating under vacuum. Properties: fire retardant up to fire class I, no discoloration, no leaching. Extra protection against leaching not required.
Flame Guard HCA 40, in combination with Olympic stain cedar. Use: specifically for western red cedar. Property: fire class I.

Transparent Finish
Finishes with a transparent protective coating are delivered by five methods and/or products:
Olielazuur. Binding agent: boiled linseed oil.
Solvent: citrus dilution. Pigment: ochre and other natural dyes and titan white.
Properties: no increased durability. Service cycle: 2 years (producer: UULA).
Impregnating wood tar: Binding agent: combination of fired wood tar and boiled linseed oil.
Solvent: gum turpentine. Pigment: ochre and other natural dyes and titan white.
Properties: slight increase in preservation period; can be painted.
Service cycle: 3 years (producer: UULA).
Olympic Stain Formula based on linseed oil with a water repellant component and an agent providing protection against damage caused by sunlight.
Property: high percentage of pigments.
Service cycle: 4 to 6 years (supplier: Leegwater).
Impregnating Stain. Transparent finish, e.g. Sigmalife.
Service cycle: approx. 4 years.
Prodema. HPL board laminated with eyong, impregnated in thermal cured phenol resin.
Property: once the phenol resin has polymerized, the result is less than 2 percent  water absorption.

In the near future, wood modification (using acetyl) will make non durable wood types suitable for use as facade material. This is a promising development as modifying allows each problem to be tackled separately, without impairing the wood’s aesthetic qualities. In recent years, this method has been tested by Stichting Hout Research in Wageningen (see box, page 24). Such developments are almost always initiated following new technical possibilities. Whereby acetyl is especially interesting in terms of architecture and aesthetics. If wood types like pine, deal and birch can be modified to such a degree that they meet technical requirements then the typical marking, structure, color and sheen can be used in the architectural presentation. This means that, provided it is made of modified wood, even a board material like OSB may be used as facade material.

Properties enhanced as desired through the use of acetyl
Modifying the molecules that constitute the wood also alters the wood’s properties. This results in drier, more durable, color fast, form fast or harder wood. The acetyl modification method has been subject of extensive studies showing that any wood type can achieve optimum durability, avoid swelling and shrinking almost totally, and become UV light resistant. In principle the modification method can be selected to fully match the prospective use. Durability, dimension stability, and surface paint acceptance can be separately enhanced, as required by the specific prospective use. Method Acetyl modification uses the chemical acetic acid anhydride. Wood cell walls contain hydroxyl groups (OH). They play an important role in the water management process inside the wood and its decomposition via micro organisms. As a result of the introduction of acetyl, the hydroxyl groups inside the wood are substituted by acetyl groups. Swelling and shrinking behavior. As an acetyl group is heavier than an hydroxyl group, the wood becomes heavier as a result of the acetyl being introduced. This weight increase, expressed in WPG, indicates the degree by which the wood has absorbed the acetyl. Contrary to hydroxyl groups, acetyl groups are water repellant making treated wood much drier than untreated wood. This greatly reduces distortion. Durability With fungi inside the wood failing to encounter any hydroxyl groups, their enzyme system fails to identify the wood as a nutrient. UV resistant Impact from UV rays changes the structure of lignin and causes discoloration. Impact from rain and wind removes the decomposed lignin and causes surface weathering. Acetyl causes the lignin bound hydroxyl groups to be substituted by an acetyl group almost totally stopping any discoloration. Discoloration due to weathering can be counteracted by applying a clear varnish with UV absorbers. The wood having ceased to distort, durability of this clear varnish may be as long as 10 years. Compared to the present 1-2 year warranties, this is exceptionally long. Board material SHR is doing studies of board material made of acetyl modified wood. MDF, but multiplex, chip board and OSB as well, might be made suitable for outdoor applications. Environmentally safe Accepted wood preservation methods introduce toxic substances into the wood to prevent fungi impact. The wood preservation method using acetyl is quite different. No toxic materials are used in this process making acetyl treated wood a totally clean product which remains environmentally friendly also over prolonged periods and when incinerated.

Sources: Centrum Hout: SHR Wageningen; dr. Jules J.A. Janssen of TU Eindhoven; Hickson Guarantir Netherlands bv; Leegwater houtbereiding; UULA natural dyes; Flamer Guard bv; Literture: 1. International Condference non-conventional constructin materials (nocmat 97); 2. Wood Protection in Tropical Countries, Hubert Willeitner, Walter Liese, 1992 GTZ.