“We chemically modified wood to make it transparent without losing the mechanical properties,” says Céline Montanari, a researcher at the KTH Royal Institute of Technology in Stockholm who recently presented her work at the national meeting of the American Chemical Society.

Montanari and her team took balsa wood and removed its lignin, the polymer that helps make wood rigid, and filled the resulting microscopic holes with acrylic. The resulting wood looked rather like the frosted glass of a shower door. Then they took the material one step further, mixing it with polyethylene glycol, a so-called “phase-change material” that melts at 80 degrees Fahrenheit. When it melts, it absorbs energy and becomes transparent, then solidifies and releases energy when cooled.

In theory, this means the polyethylene glycol-infused wood material could be used to make windows that absorb energy during the hot part of the day and release it into the home when it got cold at night.