Black Sludge Appearing When Making Iron Oxide Through Electrolysis I am trying to make iron-oxide through electroylsis to make thermite. I'm using nails for the iron on the positive electrode in salt water. While there definitely is a good amount of rust collecting, especially at the surface, after dissolve multiple nails, there are also these black sludge-like solids forming: 



I can pick them out of the solution if i'm careful, but they are very brittle 
and break up and dissolve almost immediately if I  use too much force. Additionally, if I stir the up or shake the container, more of this stuff comes up from the bottom, most of it dissolved in a black-green watery, solution and you can't even see the rust anymore. If you give it time to settle, though you can see the rust again.

My question is what is this stuff, and will it make the rust impure? Do I need to get rid of it to make the thermite? Will boiling the solution to get rid of the water like I had planned be enough?

Thanks in advance.
The black stuff is your iron oxides. FeO and Fe3O4 are both black, Fe2O3 is red. I would guess you are making some complex iron oxide/ hydroxide/ hydrate. I have seen XRD analyses of many aqueous iron corrosion products , they are typically a complex mix.