rrkss - 1-1-2010 at 09:01
How would one get charring off glassware after soap and scrubbing failed without using something dangerous like chromic acid?
12AX7 - 1-1-2010 at 10:07
"Fire. And lots of it." - Dr. Julius Hibbert
Of course, you really ought to have an annealing lehr to do that to glass. It would do a fine job, though.
The whole point of "char" is it's tenacious, which is why ordinary chemicals don't do jack and why H2CrO4 or H2SO5 is necessary.
Oh, in case it's actually something tarry and not entirely charred, try a variety of solvents. Pet ether, acetone, THF, etc...
Tim
entropy51 - 1-1-2010 at 11:01
There are several threads already on cleaning glassware. Use the search button.
It usually takes something corrosive, like 10% KOH in ethanol. Soak a day or two, pour out and save for later use. Rinse with HOT water, and if need
be, apply a brush and some gentle scouring powder.
DJF90 - 1-1-2010 at 11:33
Shaking the flask with acetone and salt in it is pretty good as far as gentle abrasion goes. You could always sit it on the mag stirrer overnight.
psychokinetic - 1-1-2010 at 12:37
Go the whole hog. Melt it down and re-blow it 
rrkss - 4-1-2010 at 17:07
Sodium Hydroxide in Ethanol seemed to work well in removing most of the charring but not all of it.
Mr. Wizard - 5-1-2010 at 10:09
I've had good luck removing age hardened paint, charred grease, 50 year old oil and grease with a liberal application of oxygen through a cutting
torch. After getting over the shock, think about it.
I have started the cutting torch, then quickly, with no preheating, apply it and the oxygen to the organic material you want to remove. Don't linger
in one place. This has worked well for me in removing organics and even Bondo from thin metal. Often the metal is just warm to the touch after the
removal. Cast iron parts often look brand new after a cleaning like this
A modification of the practice would be to flush the container with O2 while gently heating the area, from the other side, gently oxidizing the
target material. Don't use this where there is a lot of organics or you will just catch it on fire and ruin your glass.
A small ozone generator, with a piece of aluminum foil hooked to a HV source behind the offending stain, and a water bath acting as the other
electrode would also effect some oxidation. I know it works in air, because it bleached some capacitor dielectric I was using. The effect could be
improved using a gentle flush of O2.
NADPH - 5-1-2010 at 15:29
When usual methods fail I use piranha solution by mixing H2SO4 + H2O2 in situ and gently heating. It helps. But be careful with this thing...
Fleaker - 11-1-2010 at 19:14
I would strongly consider using Caro's acid as the last resort, and only after you've physically removed as much as possible. Making and heating such
a solution is a recipe for a violent explosion that probably won't be in anyone's best interests. Piranha solution is no joke, I would only use it on
precleaned glass.