Amalgam fillings are the most widely used and time-tested filling material. The advantages of this material is the cost, strength and durability. Traditional amalgams were plugged into teeth and not bonded. Previously, healthier tooth structure had to be removed to make the filling more retentive in the cavity. Nowadays with modern techniques of amalgam bonding the tooth preparations are more conservative, preserving healthy tooth structure. They are often used to build up a base to allow a crown or bridge to be placed over the tooth, and usually placed on back teeth for their strength.