First thing to know any light is harmful to an autograph period .
Obviously light intensity , length of exposure , distance from light source , type of light exposed to , and light source will all affect an autograph.
Also the type of ink used as well as the color of ink and the type of baseball the autograph is on will affect the signature , and thusly affect the autograph. Also is your ball holder UV resistant .
But I’ve never heard of a direct opinion of a mirror affecting an autograph but it goes to reason that if it reflects light it will affect the signature .
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