you can etch the iron out using hydrochloric/ muriatic acid..... And
then clean the piece and try re pickling in normal sparex/ sulfurous
acid type pickle, if you have an ultrasonic at your disposal it
wouldn't hurt to run it thru....
Side note: if the iron gets back into the sparex type pickle it will
cause electro deposition of the dissolved copper... Aka copper
electroplate on the surface of the piece.....
If you have access to nitric acid you can try a quick etch in there
it will also eat the iron off and the copper....
just my 2 cents,