Although you're probably using fully-cooked ham, it's essential to thaw your ham completely to maintain good food safety practices. There's something called the Temperature Danger Zone, which is when foods are between 41 to 135 degrees F. In this temperature range, bacteria grow faster than in cooler or hotter temperatures. The goal is to get your chilled, thawed ham through this temperature range as quickly as possible, ideally within 2 hours. If you cook a frozen ham, the centre of your ham will stay in this danger zone for much longer than is considered safe. You'll know the ham is fully cooked when it reaches 145 degrees F in the thickest portion.