Black streaks in deposits of nickel are caused by bubbles of hydrogen gas, which form in clusters on the surfaces of articles and then burst. They may be prevented by gently agitating the articles whilst being plated, or by stroking the clusters with a stout feather and thus bursting them.

They appear frequently when nickel solutions have not been agitated for some time, and have consequently settled in a stratified condition. It is therefore advisable to stir the solutions occasionally in the evenings, and thus thoroughly mix the contents.