Rust Removal by Electrolysis - A Detailed Illustrated Tutorial, page 28
5. After-Electrolysis Cleaning: How to Clean Black Coating Off De-rusted Artifact
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After you covered the artifact with your prepared cleaning mix, brush the surface with a stiff nylon brush.
Brushing Black Coating Off
You will see the results right away: the black coating is coming off.
Black Carbonate Comes Off
Rinse off the black substance and inspect the artifact. If more black coating still remains, use the scouring pads again to get off the final traces of black carbonate if necessary, or repeat the procedure of brushing with cleaning mixture until you see the bare-iron surface. Just DO NOT let the black coating dehydrate!
Sometimes you may encounter a tough spot of the black rust covered with the black carbonate on the artifact's surface. This spot can be chipped out with an awl. Some enthusiasts use an engraver which easily knocks out the black carbonate and any remaining rust out of cracks.
Applying Awl to Remove Black Rust Encrustation
Black Carbonate/Black Rust Encrustation Gives In
5) And finally, rinse the artifact thoroughly while still brushing it. As soon as the dirt is washed off, you can try the following:
You can rinse the artifact in boiling water which will oxidize the iron surface to a flat, pleasing black color. But do NOT rinse it for too long because iron artifacts may rust even in de-ionized water unless gluconates (they act as rust inhibitors during the rinsing), such as gluconic acid, sodium gluconate, or sodium glucoheptanate, are added to the rinse water.
After being electrolyzed and cleaned, the surface of the de-rusted iron object is in very reactive state (unstable), and, therefore, susceptible to rusting. As soon as you are done with rinsing, proceed with next treatment WITHOUT A DELAY!