During the early period they usually consisted of a blue glass base with a white overlying layer,[6] but those made during the later period usually have a colourless background covered with a translucent coloured layer.[5] Blanks could be produced by fusing two separately cast sheets of glass, or by dipping the base glass into a crucible of molten overlay glass during blowing.[6] The most famous example of a cameo from the early period is the Portland Vase.