Would an oil painting age better than watercolour paintings?
Not necessarily. Early watercolours that have been kept in ideal conditions can last longer than some oil paintings that tend to acquire a crazed appearance as they age. The cave paintings of 15,000 years ago were water based and they still look just as good today as the day they were painted. Will I be able to prove provenance in the future? There is every chance that your watercolour artwork may become collectable in the future. As a professional artist, I catalogue all of the paintings I sell on eBay. Each one is given a unique number along with a signature on the back. The number and an image is kept on a database so that it can be proved to be an original artwork from me in the future. Unfortunately, in the past, some disreputable people have tried to sell poor copies and prints of my work which is copyright fraud, but by having a catalogue of my output, I can prove whether a painting is mine or not. If you should have concerns about a painting which is purported to be one of mine
