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The Celts used the same metal techniques as other civilizations, but the Celts do not seem to like rules. You know what a Celtic knot looks like: swirling, flowing, never ending. When introduced to Roman art, the Celts decided that the Roman ways were more popular, so they copied the Roman style: strait lines, strict measurements, and so on. Gradually, the Celts went back to their engravings of enormous bunches of hair and unending designs.
As Christianity spread, however, the Celts combined the Cross with their curves and swirls, and came up with the Celtic Cross.
Examples are found in the Ardagh and Derrynaflan Chalices and in the Tara Brooch. http://www.visual-arts-cork.com/irish-crafts/celtic-metalwork-art.htm