What are the best cleaning agents for carbon residues on glassware? Introduction

Recenly I wanted to isolate some ascorbic acid (Vitamin C or chemically - $\ce{C6H8O6}$). I've put the filtered solution (I had to filter it because of the binder) in a flask and heated it. It didn't go as planned because then I didn't know that ascorbic acid decomposes at high temperatures, but I still decided to keep the product of the reaction. I left it on a hotplate at a high temperature and it all went back to carbon because it was there for too long (It was dehydrated).  





The cleaning agent that I know of - $\ce{NaOH}$

I used a saturated solution of $\ce{NaOH}$ to clean it but there were still some flakes stuck on the wall that I couldn't reach.  

It worked well for me - cleaned ~90% of the residue.  



The question

Are there any other cleaning agent that'll work better than aqeous Sodium Hydroxide?
Are there any other cleaning agent that'll work better than aqeous sodium hydroxide?


Yes, but I would not recommend them to anybody running experiments at home!

Back in the days in the lab we used arm-deep tanks filled with a saturated solution of potassium hydroxide in isopropanol to soak dirty flasks. You need long (!) chemical-resistant gloves and once in a while, the whole mess has to be discarded and replaced. Down the sink is not an option ;-)

And there is piranha solution, a mixture of 30% hydrogen peroxide and concentrated sulfuric acid. While is is effective, it is infamous for going off with a bang, see this report.

To sum it up: Stay with the sodium hydroxide solution. Don't risk your health and don't screw up your own environment to rescue a spoiled flask.