Almost any art material you select is going to somehow decay over time. Even if you think about a writer and words, words in their own way decay over a period of time. They're thought of differently today than they were in the 17th century. That's not the sunniest way to look at things, but art materials are no different. When you think about hard materials like marble, bronze, and other sculptural materials, they all undergo this transformation that you just have to accept as being part of the thing.
Oil paint is another example of something that's continually degrading.
The sun or light of any kind is going to affect it and add age to it. It's like how we maintain the human body. We know the human body is not going to live beyond 80 or 90 years. We could be looking at the human body generations from now and come no closer to preserving it than we knew about or pondered at the dawn of civilization. It's going to decay.