Anybody know a good way to artificially age/weather wood?

I have some reclaimed lumber (not sure what species) from an old barn
that I want to use for the frame of a painting of the barn that will
be a birthday gift for a friend. Machining the wood will expose
surfaces with different texture and color and I'd like to treat/finish
those surfaces to better match them to the look of old and weathered
wood.

I suppose I could just set the frame out in the elements for a few
years, but a more expeditious approach would be preferred.

Tom Veatch
Wichita, KS
USA