I have not heard the rumors about String Dope before but upon a cursory reading, it seems like that cook does infact produce some unknown and possibly dangerous drug even if it is not meth.
"string dope" is a pure myth; no psychoactive compounds are being produced. There is no more reason for ammonia and the carbon in activated charcoal to randomly assemble into methamphetamine than there is for it to assemble into the debilitating neurotoxin MPTP.
Also, gun blue contains selenium dioxide, which is an oxidant. Given how the process for making methamphetamine normally requires a reducing agent (i.e. the opposite of an oxidant), that also speaks against it being useful in a meth synthesis. Oh, and it's also kind of toxic, so any "meth" produced this way would have to be thoroughly purified to make it free from selenium impurities.
But again, there is no plausible reaction mechanism for a psychoactive compound to appear under these conditions (unless you consider the symptoms of selenium poisoning to be "psychotropic" in nature). The crystals forming on your string would probably just be toxic ammonium selenite, (NH
4)
2SeO
3.
Basically, the explanations for the origin of this myth are:
a) that someone used a string as a nucleation site when recrystallizing their meth. Other tweakers saw the pretty crystals, but apparently didn't realize that recrystallization actually requires you to have a supersaturated solution of methamphetamine in the first place
b) ...as to why it supposedly involves ammonia and gun blue... I could see someone misinterpreting some basic info they'd heard about the "Birch reduction" method for making meth, which involves mixing ammonia and alkali metals to create a blue-coloured solution that functions as a reducing agent. But you'd still need to get your hands on pseudoephedrine as a precursor to reduce it with the aforementioned solution.
c) And let's face it, tweakers can sometimes have a bit of an overactive imagination. Maybe they came up with the string dope recipe the same way they keep coming up with ways to "improve" household appliances by taking them apart and reassembling them over and over.
d) ...or maybe a cook just made up the myth to fuck with amateur "shake'n'bake"-cooks trying to compete with his business.