Mineralogie » Cleaning Pyrite. HELP.

Hello

In one exhibition of minerals, one of the exhibitors suggested that I put my pyrites in phosphoric acid.
So I bought it and put a pyrite nodule, rusted on the surface, in a small container of 75% P acid.
Except for 3 mini-bubbles at the start, everything remains intact: the rust does not leave.
The question is how long I have to wait and whether I should also "brush" the Pyrite.
If not, it is the solution to restore gloss to a rusty pyrite nodule.
Same question for cubic pyrites that have tarnished over time.

I didn't find the right solution from the internet.
References:

http://www.geoforum.fr/topic/35136-nettoyage-pyrite-help/

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