[FlaskBreaker]

One day I made a batch of iron(III) Oxide-Hydroxide, and I'm finding hard to clean it out of my 250ml erlenmeyer. I tried distilled water, sodium carbonate in water (to make it basic), acetic acid, and acetone. None of these worked. I also have it stuck in my 150ml erlenmeyer.

Do any substances that it's soluble in come to mind, or will I have to use a strong acid?

Thanks,
FB 

[Arkcon]

That's the quickest answer.  Just a few mL of hardware store quality muriatic acid will leave a yellow solution that's easy to discard and rinse away.  If you want to try acids, hardware store oxalic acid is called wood bleach, and its good for rust stains as well.

[wildfyr]

I've had trouble before with iron oxides in glassware after an HCl soak. Sometimes a few rounds of soak and elbow grease are necessary.

[FlaskBreaker]

Looks like I'll have to get some HCl, unless there's some other way to make it myself.

[billnotgatez]

Looks like I'll have to get some HCl, unless there's some other way to make it myself.

Many citizen scientist know that  Muriatic acid at the hardware store is HCl.
https://en.wikipedia.org/wiki/Hydrochloric_acid
I would buy it.

[FlaskBreaker]

Oldly, I was able to clean my glassware by adding some calcium carbonate, and pouring distilled water in slowly 5ml at a time. This created a sludge that must have caught the iron and washed it off the glass. The yellow tinge that was in the erlenmeyer is now gone, and no particles can be seen.