I’ve been a proponent of using (or at least seriously studying) propylene glycol instead of alcohol for collecting, for shipping, and for long term storage of insect specimens. I’ve always thought about it from the standpoint of the specimen (internal/external morphology, color, DNA), but not the label. Victoria Bayless asked me about how propylene glycol interacts with ink, so I did a quick study. Maybe someone has done this in the literature already.
I used 1) a pencil, 2) a Pigma pen, [three ball point pens] 3) black Bic, 4) blue free pen, 5) red Pilot, 6) black Sharpie. I used our standard label paper: HP Q6608A Glossy Laser Brochure Paper, White.
Treatments were as follow:
1. Control, left dry.
2. immersed in 95% EtOH for 8 days, removed and air dried.
3. immersed in 95% EtOH for 8 days, transferred to 100% propylene glycol for 8 days, removed and air dried.
4. immersed in 100% propylene glycol for 8 days, removed and air dried.
5. immersed in 100% propylene glycol for 8 days, transferred to 95% EtOH for 8 days, removed and air dried.
All the treatments basically reacted the same. Based on what I tested, if the ink is not dissolved in alcohol, it will not dissolve in propylene glycol (pencil and Pigma) and vice versa. The ball point pins aren’t too surprising because they often use propylene glycol as a solvent.
If you do run across an instance of an ink that stood up to ethanol, but was dissolved by propylene glycol, please sing out and be heard!