Mint-made errors are called "type-2 blanks (or planchets)" after an upending mill adds uniform, rounded rims. Type-2 blanks may also be considered striking errors as they are prepared correctly, but are released without having been struck. A misfeed can occur when the metal strip is fed through the blanking machine. The punches sometimes overlap the leading edge of the metal producing a straight clip. Sometimes, the punches strike an area of the strip which overlaps the hole left by the previous strike producing a curved clip. Sometimes punches strike the irregular trailing edge of the metal strip producing irregular clips. Coins are
Glass-to-metal seal dark. Oxygen-free copper has to be used if the metal comes in contact with hydrogen (e.g. in a hydrogen-filled tube or during handling in the flame). Normally, copper contains small inclusions of copper(I) oxide. Hydrogen diffuses through the metal and reacts with the oxide, reducing it to copper and yielding water. The water molecules however can not diffuse through the metal, are trapped in the location of the inclusion, and cause embrittlement. As copper bonds well to the glass, it is often used for combined glass-metal devices. The ductility of copper can be used for compensation of the thermal expansion