So, in the interest of science, I soaked it in the same 91% IA for about 90 min - bottom line (TLDR), don't do that:
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After 90 min (just picked 90 min randomly), the stamp came off with just the gentlest of cotton swab touches:
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The 6 year old still living inside of me then proceeded to remove the chip from solution, rinse it in warm water, and continue the abrasion testing - the chip has softened up significantly:
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The whole chip is about half as dark green as before but the original discoloration/blemish on the other side is still there:
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Twisting the chip in my hands/fingers (I wasn't trying to break it, just feeling it - I think I could feel it flexing) caused a fleabite!
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I think if I left it overnight and then came back tomorrow and stirred, I would end up with a green slurry... (I can put 1mm deep indentations into the edge of the chip with my thumbnail right now)