Original: Eugène Viollet-le-Duc was responsible for the choice of copper in the statue's construction, and for the adoption of the repoussé technique, where a malleable metal is hammered on the reverse side.
Simplified: Eugène Viollet-le-Duc choose copper in the construction of the statue, and for the adoption of the repoussé construction technique, where a metal is hammered on the reverse side.