One of the culprits grinds her paint materials using a mortar and pestle. The mysterious giveaway green flecks in the forged art's paint is from the aquarium algaecide tablets, also ground using the same implements. This would seem to be taken from the Stella Nickell case. She copycatted the Chicago Tylenol poisoning case by replacing capsules of Excedrin in stores with cyanide in order to kill her husband, also causing the death of innocent Sue Snow in the process. She was tripped up by the same green flecks when she tainted the Excedrin in the same bowl she used to crush her aquarium algaecide. See Gregg Olsen's account in his gripping novel "Bitter Almonds".
A great episode nonetheless!
A great episode nonetheless!