The bowls never wanted washing

The bowls never wanted washing because the boys licked them clean.... they were so hungry that even the smallest fleck of gruel wasn't left behind.

The bowls never wanted washing. The boys polished them with their spoons till they shone again; and when they had performed this operation (which never took very long, the spoons being nearly as large as the bowls), they would sit staring at the copper, with such eager eyes, as if they could have devoured the very bricks of which it was composed; employing themselves, meanwhile, in sucking their fingers most assiduously, with the view of catching up any stray splashes of gruel that might have been cast thereon.

Why did the boys stare at the copper ?