What should be added to amalgam to enhance its strength?

Adding copper to amalgam is a well-established practice that significantly enhances the material's strength and durability. Copper plays a crucial role in the microstructure of dental amalgams. It improves resistance to corrosion and increases the amalgam's overall strength, which is vital for withstanding the forces of chewing and grinding.

Copper enhances resistance to wear, decreases marginal fracture, and improves the amalgam's ability to form a solid bond with the tooth structure. The incorporation of copper reduces the amount of tin in the amalgam, leading to a more favorable balance of mechanical properties, allowing for better performance in clinical situations.

In contrast, other materials listed do not serve the same purpose. Resin is typically added to composite materials for aesthetic purposes rather than affecting the strength of amalgams. Water is not used as an additive in amalgam, as it can compromise the setting reaction and overall integrity of the material. Iron can affect amalgam properties but isn't added to enhance strength and could lead to undesirable characteristics.