I'm not really sure that there is a formal definition of natural and artificial light. Usually we use the term natural light to describe light that comes from the sun, and artificial light to describe light that we have to make somehow after the sun sets or if we're in a basement. In that sense, light from a flame is artificial since we are making it.

On the other hand you could argue that there is nothing artificial about a fire (and in fact the sun is really a big fireball that puts out a lot of light!)

So I think the answer to this one could go either way.