Metal weapons receive bonuses based upon the metal used for construction. Essences will raise the level requirement by 1-8 levels based on the quality of the item and essence. As a general rule, lower level items (bronze) will be more affected than higher level items (Damask). See Weapon guide for details.
Metal armours receive bonuses based upon the metal used for construction. Each piece of an armour set grants a specific damage immunity: Slashing for body armor, Bludgeoning for helmets, and Piercing for shields. Due to the base cost of items, applying a resistance essence may increase the level requirement by 1-2 levels. Again, lower level items will be more affected by essences than higher level items.