How to get back my beautiful crystals made from iron and sulphuric acid I have put some old nails and screws in the 30% sulfuric acid to prepare an electrolyte for plating. After some bubbling and horribly irritating smell (probably $\ce{SO2}$), the reaction slowed down. Overnight, beautiful green/blue crystals formed:



I just love them. After another day, the reaction almost stopped. I suppose the acid was all bound to iron. To continue the process, I poured some concentrated sulfuric acid into the solution. That was a big mistake - the crystals reacted with concentrated acid and formed ugly white dust:



What did I create? What are the crystals and what's that dust? How to reverse the reaction? Does the dust dissolve to be electrolysed?
I realize this question is over two years old, but I have to add the correct answer here: the iron (II) sulfate you had at the beginning was in the heptahydrate form.  SEVEN molecules of water for every iron (II) sulfate ion pair.  Now, what do you think would happen to that crystal structure when you add one of the most powerful dehydrating agents in existence?  That's right, it sucks all the water out of your crystals, and you're left with a powdery, noncrystalline form.  You could have just added less concentrated acid, that was diluted enough to not be such a strong dehydrating agent.

Incidentally, iron is a surprisingly strong reducer of sulfates.  I did this reaction recently and noticed gold flecks floating around in the solution.  I was shocked to see gold, since I obviously never added any, until I realized that the iron had reduced the sulfate all the way down to sulfide, and what I was seeing was iron pyrite -- fool's gold.  Adding more acid eliminated that almost immediately.