A method of electro-depositing an envelope of metal on a cathode. The metal envelope (120) comprises metal sheets (122, 124) on either side of the cathode plate (100). A groove (150) is provided along one edge of the cathode plate whereby metal deposited on and adjacent to said groove forms a frangible portion (140). The groove is shaped such that a line of weakness (A) is formed in the metal deposited within the groove such that separation of the two sheets of deposited metal is initiated along the line of weakness.