At high concentrations, it will pretty much burn right through your flesh like other strong acids. The truly scary thing is that at lower concentrations, you can spill enough on you that it will threaten your life, and require amputation and not be aware of it for hours…
This reminds me of the time I used a trebuchet to launch a pepperoni pizza into a giant soap bubble filed with super heated argon and Beanie Babies. The results were equally as spectacular.
They were using 48% HF.
From Wikipedia, in case you were wondering why a relatively small amount on skin can be fatal:
“By interfering with body calcium metabolism, the concentrated acid may also cause systemic toxicity and eventual cardiac arrest and fatality, after contact with as little as 160 cm2 (25 square inches) of skin.”
Science, sometimes dangerous. Usually fun. What struck me the amount of protection the chemist was using. Something this dangerous probably demands more protection.
The knife-cut seems to me as fairly logical. The bulk of the acid is sinking the heat from the filament; the glass above the liquid surface is heating more. The just-at-the-surface line is where there is the acid from the outside, and the heat conducted from the top; a thin layer of acid hotter than the bulk. Therefore the reaction goes faster here. My hypothesis, at least.
HF is a pretty nasty beast. It penetrates skin, does not cause immediately visible burns, but kills the cells. The molecule does not dissociate much, and is lipid-soluble, so unlike the ionic-behaving materials it penetrates skin fairly easily.
The heart hazard is from its affinity to calcium ions; it forms calcium fluoride that is fairly insoluble, therefore depleting the blood and cellular calcium; which is important for all sorts of signalling, so nerve activity (and heart activity) gets impaired.
If you etch glass (either making a hydrophilic surface, or matte surface, or, using e.g. beeswax with nail-scrapped pattern, a line-art), I’d suggest immobilizing it in a gel or (which worked for me) a piece of cotton wool. It’s a watery fluid and annoyingly mobile. You can also opt for a HF-NH4F combination.
I would be pretty careful with it, but not outright hysterical; a small drop won’t kill you (though it may make a painful blister); larger spillage on hand is however something to avoid. (Knowing that it is not a magical all-killing poison will also help you to not have shaky hands when manipulating it.)
It is not anything close to dimethyl mercury, toxicity wise.
Interesting stuff here, especially the part with the fire-related releases.
Edit: @Roclaf: you beat me to some of the aspects.
I also wonder why they used sodium carbonate to neutralize it, instead of precipitating calcium fluoride…
I was kind of hoping a lab rat, wearing an Evil Knievel suit and riding a miniature motorbike, would have jumped over the beaker of acid. You know… for science!
Could the reaction be accelerated by something in the air? I was surprised to see so much of the submerged bulb left over.
The deadly-contact-poison problem kinda broke that bathtub-through-the-ceiling scene in Breaking Bad. It was meant to be funny, but they’re standing there with exposed skin while gallons of the stuff is sloshing around and pouring down from above so instead of laughing as the writers intended, you’re all “OMG! FLEEEE!” 
Yo! Science, bitches!
HF is nasty stuff, but “the most feared chemical there is”? Hardly. I’m no chemist, but I absolutely love reading Derek Lowe’s Things I Won’t Work With blog. It’s full of colorful descriptions of compounds that make HF look about as hazardous as crystal pepsi. Things that explode violently if they get too warm (as in slightly above absolute zero) or if somebody sneezes in the next room, or for no reason in particular. Things that burn inextinguishably upon contact with just about anything. Things that can kill you from across the lab if you look at them the wrong way. Gloriously horrifying stuff.
What’s worse than hydrofluoric acid? Something that exothermically produces hydrofluoric (and hydrochloric) acid vapor when you get it wet.
Thanks for reminding me to look at that site for the first time in a while:
Concentrated peroxide has a long history in rocketry, going back to the deeply alarming Me-163 fighter of World War II.
Ah, such a way with words.
I am. It is. Sorry.
I once worked at a lab where the senior chemist (mostly retired) had brought some in to etch glass, for fun. I am sure he knew his stuff, and the lab routinely handled hazardous waste of every variety. I worked with PCBs, pesticides, and volatile organics… Every chemist on staff was less than happy that the stuff was even in the building. The lab manager literally threatened to quit. Was anyone overreacting? I dunno. I was happier when it was gone though.
Damn, I was hoping the answer would be, “And that’s how we created the first ever wormhole to the Alpha Centauri System”
It isn’t actually that strong an acid. HCl is stronger.
But it has some other interesting properties…
I am glad that I don’t have to work with anything that will put concrete or sand on fire.
HF is on the other side of the line where arguing if something is less or more scary is pointless. I don’t want to work with it, FOOF or ClF3. Surely that is enough.
Sincer the_borderer has mentioned FOOF, I get to say “Satan’s Kimchi”.
I think shaddack has it right. Warmer at the surface, less heat dissipation through the liquid, maybe a little surface tension, a local thin surface convection layer to carry the dissolved silicate away and the air-exposed part of the bulb is warmer still but gets less acid …
My parents are master glassmakers. They’ve stopped using HF because they thought it was too dangerous, even though it meant they’d have to reject some orders. When I was a child, I remember they used it in open air in the garden and I had to stay in the house as long as they’d be working outside. For storage, the (special) bottle was wrapped into several layers of thick trash bags, tightly closed. The whole package was stored in a locked box, inside a locked garden shed (in order to keep it in a ventilated place where nobody would stay and breathe the vapors). As a result, rust developed quickly on everything in the shed. Somehow, vapors had managed to escape the special bottle and the many layers of closed trash bags. Really scary. They eventually got rid of it through a chemistry lab.
