High-quality antique bathtubs are typically constructed from Cast Iron with Porcelain Enamel or Acrylic/Fiberglass. Cast iron is the gold standard for durability and heat retention, though it is extremely heavy and requires floor reinforcement. Acrylic options are lightweight and cost-effective, making them easier to ship and install, but they lack the authentic 'heft' and thermal mass of metal. For a premium look, Copper or Brass tubs offer natural antimicrobial properties and a unique patina over time.