Why does wood become darker when it ages? I have lots of pieces of pine furniture which have different colours depending on how old they are. So, I just wondered why that is. I guess it has something to do with structures and chemical compounds that were already in the wood when it was still alive, so I hope this is not off-topic here. What processes take place in dead wood when it ages? What determines the colour it will have? For example, pine and spruce wood looks very similar when freshly cut, but will develop very distinct colours afterwards. Is there also wood that does not become darker? Is it something to do with the processing of wood, or will that happen to any stick you leave lying around in your room (so it doesn't rot)?

While most wood we see comes from stain or oils, wood left on its own will darken. As far as this article suggests, this is caused by the polymerization of polyphenols from lignin into tannins which are dark red or brown in color when they are highly polymerized.

The specific content of the lignin in different woods will affect the density, strength and color of the wood.

This woodworking reference shows that intense UV exposure to wood will darken it to give it an aged look.