Spiral hams come to the grocery store already cooked. The need for cooking it again is a matter of preference. The trick is reheating the ham without drying it out. The secret to this is to heat it very slowly. While raising the temperature of the ham, it is best to avoid putting it in the oven as much as possible. You will want to raise its temperature before and after you place it in the oven.
Place your ham into a large pot. Do not take the ham out of the packaging first.
Pour enough hot tap water into the large pan to submerge the ham. Let it sit for 45 minutes. This will raise the temperature of the ham slightly.
Pour out the water, and fill the pot back up with fresh hot water, and let it sit for another 45 minutes.
Preheat the oven to 121 degrees Celsius.
Remove the ham from its packaging and place it into an oven bag.
Place the ham into the oven.
Use a meat thermometer to check the temperature every 15 to 20 minutes. The ideal temperature is 40.5 to 43.3 degrees C.
Remove the ham from the oven and place it somewhere to rest. While the ham rests, it will continue to cook. The ham can rise in temperature by as much as -9.44 degrees C over a 15 minute span.
Use the meat thermometer to check the ham every five to seven minutes. Ideal serving temperature is 37.7 to 48.8 degrees C.
Remove the ham from the oven bag once the desired temperature is reached. Serve.