See the trees through the wood

Researchers in Sweden have succeeded in making a sustainable, renewable transparent wood that is strong enough to use as a structural element. It will be a while before it is widely available, but it has the potential to profoundly affect the construction industry.

Glass changed how we see things – literally

The use of glass is as old as humanity itself. Our distant Stone-Age ancestors made their finest tools from obsidian – a naturally occurring volcanic glass – but it was only much later that humans figured out how to make glass. According to the renowned 16^th-century metallurgist, Georgius Agricola: ‘The tradition is that a merchant ship laden with nitrum [saltpetre or potassium nitrate] being moored at this place, the merchants were preparing their meal on the beach, and not having stones to prop up their pots, they used lumps of nitrum from the ship, which fused and mixed with the sands of the shore, and there flowed streams of a new translucent liquid, and thus was the origin of glass.’

It’s a lovely story and, whether it’s true or not, glass was manufactured in Ancient Egypt, and was even used (very sparingly) for windows in Ancient Rome. But it was only with the construction of Joseph Paxton’s seriously game-changing Crystal Palace that glass became a mainstream structural material.

The limitations of glass affect design

We are all so used to the beautiful multi-paned windows of Cape Dutch houses that we don’t stop to think why they were made that way. It was not a conscious decision by the designers or builders of these lovely houses – it was the only way they could build them. Glass was imported from Europe by ship, and only teensy weensy little panes would survive the voyage, so the settlers had to divide their windows up into little squares with strips of wood. And then – as is so often the case – necessity bred an accepted aesthetic.

These were a huge improvement on the oiled-cloth or even animal-horn windows that their ancestors had got used to in Europe, but they just wouldn’t cut it today. Now we want huge expanses of glass that let in light and views, and can stand up for themselves. This is all possible with laminated plate glass, which has significantly changed the look and feel of our homes and cities.

Glass is eco-friendly

Glass is an almost perpetually recyclable substance, but it is still a non-renewable resource that requires mining of silica, and significant energy input for production. And laminated glass requires the use of plastic, which is not quite so eco-friendly. Also, laminated glass is – technically – recyclable, but only with a vast input of energy and other resources. So, while it is still a great building material, it does have its drawbacks. And that’s where the concept of ‘wood glass’ seems like a great idea. Of course, it goes (almost) without saying that this would necessitate the wood being sourced from certified sustainable plantations.

The manufacture of transparent wood

In 2016, researchers at Sweden’s KTH Royal Institute of Technology created a see-through wood by stripping out the lignin. While this gave them a translucent wood, it was also pathetically weak, so they replaced the lignin with a fossil fuel-based polymer, which restored its structural integrity. But this was clearly not sustainable, so the researchers kept looking for alternatives to the synthetic fossil-based polymers and eventually came up with limonene acrylate – a monomer made from waste citrus peels from the manufacture of orange juice.

More than just windows

The resulting transparent wood is sufficiently strong and flexible to be used structurally, and it can even be used for heat storage, and a built-in lighting function. ‘We have looked at where the light goes, and what happens when it hits the cellulose,’ says Professor Lars Berglund, the head of the KTH’s Department of Fibre and Polymer Technology. ‘Some of the light goes straight through the wood, and makes the material transparent. Some of the light is refracted and scattered at different angles and gives pleasant effects in lighting applications.’ This scattering can even be used in a range of nano-technologies, and may be used to create ‘smart glass’.

The research was supported with funding from the European Research Council and the Knut and Alice Wallenberg Foundation.