A simple, environmentally benign method uses steam to make wood transparent. Others have made see-through wood before, but the new technique removes more wood fiber, giving larger and clearer pieces. Stronger and more insulating than glass, transparent wood could be the key for making homes and buildings more energy-efficient.

Windows let in light, but they also are a way for heat to escape in the winter and seep in during summer months. Roughly 25–30% of residential heating and cooling use in the United States for instance goes toward offsetting energy loss through windows, according to the US Department of Energy. Because of wood’s excellent insulating properties and strength, some researchers have sought to make it see-through for use as a novel building material that could replace glass panes. The strategy is to remove lignin, the structural polymer found in plant cell walls, which then lets light pass through, and then backfilling the pores with a transparent polymer like an epoxy resin or poly (methyl methacrylate) (PMMA).

To wash out the lignin, researchers typically boil natural wood samples in bleaching solutions made of chemicals such as sodium chlorite or sodium sulfite. In addition to the use of harsh chemicals, “the remaining lignin content is no less than 1.5% by weight [with] previous solution-based delignification approaches,” says Rongbo Zheng, a chemical engineer at Southwest Forestry University in China.

Less lignin would leave more voids to fill with epoxy, resulting in samples that are more optically transparent and stronger. To do this, Zheng and his colleagues placed natural pine and basswood pieces on grids above a boiling aqueous solution of hydrogen peroxide. After 2–12 hours, when the yellow color of the samples disappeared, the researchers rinsed the samples with cold water and ethanol.

Because the steam penetrates wood samples better than bleaching solutions do, the new technique gives transparent wood composites with half the lignin content of previously made transparent wood. The researchers are also able to make larger pieces because unlike the solution-based methods, the steam technique retains the structural integrity of the wood by keeping the cellulose in the cell walls intact. While the largest transparent wood pieces reported so far using previous methods have been 10 cm × 10 cm and 1.4 cm thick, the new technique gives samples that are 21 cm × 19 cm in area, and up to 5 cm thick.

“The size of wood samples is only limited by the steam container and the manufacturing method is scalable,” Zheng says. The researchers now plan to make meter-scale delignified wood with which they will make a transparent wood house model to investigate the light performance and heat-insulating performance of the building material.

“The work is truly novel in that the steam-based approach has not been used before, and it potentially offers real advantages,” says Lars Berglund, director of the Wallenberg Wood Science Center at KTH Royal Institute of Technology in Sweden. But the researchers have reported data for samples where the wood fibers are perpendicular to the wood plate surface. “This is the easiest case for modification, and perhaps less interesting from an application point of view since the mechanical properties are not very good,” Berglund says. It would be interesting to investigate the steam-modification technique for samples where fibers are in the plane of the wood plate.

Read the abstract in the Journal of Materials Research.