[Count of Monte Cristo]

I have an Erlenmeyer flask that I recently used in a reaction, and even after washing and rinsing, there is still residue in the bottom on the sides of the flask that I cannot get with a brush. I have read that boiling water in glassware is effective for cleaning them, and want to know if it would be unsafe to place the flask (filled with water) inside a small cooking pan and heated on an electric stove. My reason for placing the flask inside a pan is that I have read that direct, or close to direct, contact with glassware on an heating coil can cause damage to the glassware.

EDIT: Not sure if this makes a difference but the flask is small (125mL).

[Mitch]

I assume washing meant soap and water. If it still has a film, place it into a water bath that has lye in it. Make sure to wear gloves during this process and not to touch the solution. Leave it overnight and rinse with water. Should be very clean after that.

[Arkcon]

So you want to scrub the inner lower surface of the sides of a flask, and you can't get good leverage with the brush?  Here's what to do -- mix a slurry of soap and water, and drop in some unpoped popcorn, or other grain.  Or if you have a well outfitted lab, some boiling chips.  You can swirl the flask, and the pieces will scrub the spot for you with the soap.

[Hunter2]

Or use the standard cleaning solution potassiumhydroxide 10 g/l and isopropanole 30% w.w in water. Store the glasware at least 1 to two days in it.

[Zyklonb]

The king glass cleaner is hexavalent chromium in conc. sulfuric acid.
I use it in extreme cases only.
Be careful! All hexavalent chromium compounds are toxic due to their oxidizing power. They may be carcinogenic, especially when air-borne.