Amateurs...
All of this is pointless if you don't first take the precaution of carefully bathing the receipt in a high alkaline solution of four parts distilled water to one part baking soda in order to neutralize the acidity of the receipt paper. Then, of course, you carefully air dry the receipt in a dark, cool place, carefully pressed between two sheets of mylar and two one inch thick metal plates held together at precisely four foot/pound torque.
I mean, if you're going to do it right...