Long ago (practically at the dawn of time) the list ran a thread on preservatives for gum. In those days I still had some formaldehyde and mentioned that one or two drops in a 2 to 4 ounce dropper bottle of gum preserved it, as far as I could tell, permanently.
I bought some of the other materials mentioned, but didn't test them rigorously -- tho I mention for those whom it may concern that they included salicylic acid and 15% thymol in methyl alcohol. I don't think I got but it was mentioned, some form of lavender, which can give a lovely aroma, if nothing else.
While I was in the cabinet, meanwhile, I picked up a tube of something marked "MEK Peroxide in Dimethyl Phthlate: Danger: causes severe burns."
Does anyone have a clue what that is or does?
Finally, there was another chemical, probably a powder, which smelled like a doctor's (or dentist's) office, and had the word -- as I recall -- something carbon or carbonate. That was in the cabinet too, but the name escapes me now, the smell is gone, and I don't find it... Any clues?
I don't use any of the above myself these days because I found that Daniel Smith's prepared "regular" (not "Premium") gum arabic was excellent & bought an extra gallon from the lot I liked best. (Now, needless to say, it costs 2 or 3 times what I paid for it, but the dry stuff probably does too.) The prepared gum has the preservative already in it, & seems to keep forever.
IME they all darken in time -- IN THE CONTAINER -- but I've tested them with the dichromate, pigment and exposure, and never found any difference in the printed color, except SLIGHTLY darker in a pure pale yellow.
However, the caveat is that each year gum arabic is different, according to several variables, including rainfall in the acacia groves. So try it before you buy a barrel-full.