The Hammer is unlocked at level 13. It can be used for free in that level. To use it in subsequent levels, it must be fully charged. This can be done by spending coins to charge it instantly, or charge it slowly by killing pigs. A full charge will be retained in the next level if not used. The amount of coins needed to charge it varies, and depends on how much more it needs to be charged to full. Farming previously completed levels for pig kills will also charge the hammer.