Thinking organic shellac or some locally produced variant of the period? Hung over a fireplace for x number of decades? Looks really nice. The metal finish and blue on these is pretty special.
It looks like a natural reaction to age, heat, or prolonged exposure to sunlight to me. Don't think anything has been added. End grain (like you'd find in the bolt cutout) is sometimes weird in the way oils and moisture can bubble out of it. Fireplace hanger isn't a bad theory.