An ancient Japanese wood preservation technique has gone on trial as part of an Arts Council funded project on the Hampshire coast.
Two ‘rotating observatories’ have just opened at the Winchester Science Centre in the South Downs National Park, designed to be workspaces allowing artists and writers to interact directly with the public.
The innovative structures, which appear to float above the ground but actually sit on a hidden slew-ring mechanism, will also serve as test beds for a fashionable wood preservation technique known as Shou Sugi Ban – Japanese for ‘burnt cedar board’.
This involves applying flame to the surface of the timber, to burn off the softer cellulose and leave only the much more durable lignin fibres exposed. Resistance to rot, fire and insect damage is said to be significantly enhanced.
The distinctive black char finish has recently been appearing on a number of UK projects, both externally and for internal finishes, although there is little research data on how charred timber actually performs in British conditions.
The project has been conceived and managed by creative facilitator Space, Placemaking and Urban Design (SPUD).
Working on it with north Devon based artist Edward Crumpton are four graduate architects from Feilden Cregg Bradley Studios. Architectural assistant Charlotte Knight said: “Traditionally the timber treated with this technique was Japanese red cedar.
“But we considered this would be too low density for the UK. So we’re trialling both home-grown and imported Siberian larch for the main cladding, while one elevation is given over to test panels of Western red cedar, chestnut and oak.”
Knight added that some of the charred timber currently on the market had simply been flamed with a blowtorch. “But this seems to do little more than blacken the surface. Plus a blowtorch isn’t sustainable.
“We’re looking for a sustainable technique to use on locally sourced timber that preserves without toxic chemicals.”
After much experimentation the project team came up with its own version of the traditional Japanese method. Larch planks 3m long were clamped together in threes to form a triangular chimney. A sheet of newspaper was ignited at the base and the resultant internal fire allowed to burn for six or seven minutes before being dowsed with water.
Knight said the team received one vital piece of advice from the Timber Research and Development Association. “They told us not to forget about the end grain and the other face of the plank.
“So we charred the end grain as well, and applied a range of finishes to both faces, including the traditional Tung oil.”
Where high durability materials were needed elsewhere on the project, acetylised timber alternatives Accoya and Medite Trycoya Extreme (see box below) were specified. These were donated by timber merchant James Latham, who also part sponsored the £50,000 project.
SPUD principal associate Mark Drury paid particular tribute to Hatfield-based structural engineer Unitspark, which was responsible for the two supporting structures.
“Basically each observatory sits on a steel base frame mounted on a slew ring that enables the user to rotate it through 3600 by a hand-crank,” he said.
“Another steel frame supports the slew ring. The ingenious thing about this lower frame is that it was equipped with shipping container lugs that allowed the observatories to be safely mounted on the vehicle that transported them to site.
“Then wheels could be fitted to enable them to be moved into their final position.”
After six months the observatories will migrate to the New Forest National Park. Other future locations include the River Tamar and the South Dorset Ridgeway.
How reagent used in heroin production can harden wood
Acetylisation as a process, while not as ancient as Shou Sugi Ban, has been around for more than 75 years. It involves the impregnation of low durability softwood species with acetic anhydride, a commercial reagent widely used in the conversion of cellulose to cellulose acetate and in the production of both aspirin and heroin.
All untreated timbers contain a mixture of acetyl groups and free hydroxyls in the walls of the cells that make up their structure. Free hydroxyls readily absorb water and are vulnerable to fungus attack, while acetyl groups are much more resistant.
Hardwoods typically contain 4.5% acetyls, three times as much as softwoods. Acetylisation, however, raises softwood levels to higher than even tropical hardwoods, rendering the treated wood resistant to fungi and insects, and much more dimensionally stable.
Accoya products are derived from sustainably sourced softwood sections and have achieved a Class 1 durability rating, equivalent to most tropical hardwoods. A more recent development is the use of acetylised wood fibres to produce Medite Tricoya Extreme, a much more durable medium density fibreboard.