Steel etching with Winsteard’s reagent. It is a bit dangerous because if done wrong it forms explosive dust. It was also long and tedious because the liquid must be near boiling and stirring so it evaporated quickly and has to be topped off and brought back to temperature often. The etch itself requires a long temper of a quenched sample and has an iterative process of etching and back-polishing to gradually remove surface roughness but leave the slightly deeper grain boundaries.
It took several hours of preparation and several hours of active work per sample and even then had a 50/50 success rate. I was professionally trained by a third party who learned this process from the person who perfected it, George Vander Voort.
I work in a chemistry lab
Okay, question, what’s the least hazardous reaction you wouldn’t want an amateur to ever attempt? Somewhere in between etching art into something and making your own Teflon lives a cutoff line for future shed projects, but I don’t know where that would be exactly.
Anther chemist stepping in here: Anything that produces an off-gas of any kind that does anything other than smell bad should be considered potentially lethal. People have died from working with liquid nitrogen or dry ice without proper ventilation. In addition, a gas explosion can be far worse than any other explosion you are likely to pull off by accident, and if you have a leak somewhere you may have no clue how much explosive gas is in the room with you. Some gases will react and form acid when it gets into your airways, essentially acting as an invisible acid that can jump from the table into your face.
In short: Stay away from dangerous gases and stuff that makes them, and consider pretty much all gases as dangerous unless you know for a fact that they aren’t. Other than that, the potential dangers of backyard chemistry can largely be mitigated by using common sense and working with small amounts of chemicals, good luck :)
What if I do have strong ventilation, or even a lab-style fume hood? “Don’t produce any gasses” is a lot more restrictive than “don’t plan to deliberately work with gasses”.
Also, more exotically while we’re at it, what about pyrophoric gasses? If you your silane pipe breaks it should “just” start a fire.
If you have a fume hood that’s good of course, but since the question was about advising amateurs on safety, my advice is restrictive, because gases can be very dangerous in subtle ways.
As an amateur: Do you know how to properly work in a fume hood so that it protects you? Do you know its capacity, and what to do if something unexpected leads to gas development over that capacity? Have you had training in using this stuff, so that you can react properly and quickly if something goes wrong, rather than freezing up?
In short: Because the potential dangers when working with a lot of gases are harder to detect, and harder to mitigate, than when working with other stuff, I’m taking a restrictive approach in my advice.
For you question on pyrophoric gases: They can remain in contact with air for a while (several minutes, depending on concentration) before igniting. Worst case, the room around you can fill with gas from a leak before causing a gas explosion. In principle you can also inhale gas from this leak, such the the explosion also takes place inside you :)
