Hang a painting on Brick Fireplace?

A client wants to hang a watercolor over the fireplace. The painting will be foam-core backed and covered in glass. The chimney on which the painting would actually hang is uncoated brick and gets warm when there is a fire. I don't think this change of temperatures will affect a watercolor, but is there anything I should know - maybe I should add a second insulating layer of foam core outside the frame on the back? Advice?

Cynthia Kelly