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275 °F (135 °C) is an average temperature for long, slow roasting. You may adjust the temperature, but don't go below 200 °F (93 °C). Use higher temperatures to cut down on cooking time.  City ham is what's commonly sold in supermarkets. They're often pre-cooked, so they won't need to spend as much time in the oven as uncooked hams. Spiral-sliced ham is basically a pre-cut type of city ham. It's also pre-cooked. The labeling on the packaging will tell you what kind of ham you have so you know how to heat it. Uncooked city hams exist, but are harder to find. They'll need to cook a bit longer than pre-cooked ham. Try to keep the side thicker with white-colored fat facing upwards. You may need to cut off a little bit of fat off the bottom to get the ham to lay flat. Leave most of it until later. When roasting half a ham, place the cut side on the pan. Baking sheets, broiler pans, and casserole dishes may also be used. Keep in mind that pans with elevated sides are better than flatter pans. Hold the ham flat and, using your chef's knife, begin scoring lines into the fat. Cut lines across the ham, then cut more down the ham's length. The pattern looks similar to a checkerboard. If there's a lot of fat, you can slice off some of it. Leave a thin layer about 1⁄4 in (6.4 mm) thick. Avoid cutting into the meat. Fill the bottom of the pan with 1 cup (240 mL) of water. If your pan doesn't have high sides, you'll need to check the pan as it cooks and replace the water as needed. The steam provides extra heat and protects the pan from burns. If you choose to cook the ham uncovered in the next step, you'll only need 1⁄4 in (6.4 mm) of water. Place a sheet of foil all the way over the pan. Wrap it tight around the pan's edges so it forms a tent over the ham. Using foil is optional, but it'll lead to a juicier, tastier ham.  Alternatively, wrap the ham directly in foil or place it in a cooking bag. Include the water inside the foil or bag. You may also leave the ham uncovered and cook with less water. 2 hours is an average amount of time to wait before checking the ham's temperature. The specific amount of time will depend on the size of your ham. Estimate it'll take about 20 minutes per 1 lb (0.45 kg) of meat for an uncooked or partially-cooked ham and 15 minutes or less per pound for a fully-cooked ham.  Bone-in hams have a little less meat than boneless hams, so adjust the timing as needed. If you're planning on glazing the ham, it's a good idea to make the glaze while the ham's cooking. Pull out the ham after 2 hours and unwrap the foil. Push the end of the thermometer into the thickest part of the ham. The temperature should read about 120 °F (49 °C). If it isn't, rewrap the foil and continue cooking. Remove the ham when it reaches the proper temperature. When using a liquid glaze, use a pastry brush to coat the outside of the ham. For dry glazes, carefully pat the glaze on by hand. The score marks you made will hold the glaze in place. Generally, the glaze should be added within the last 30 minutes of cooking time. You can also pull the ham out halfway through the cooking time to reapply the glaze. Put the ham back into the oven. After 20 minutes, the glaze will have finished cooking. Pull out the ham and test it again with the thermometer. Look for an internal temperature of 135 °F (57 °C) and continue roasting as needed to reach this temperature. If you used a cooking bag, you should skip this and let the ham rest for 15 minutes. If you need to glaze it and continue cooking, you can use a roasting pan. During these 15 minutes, the inside of the ham continues to cook. It also causes the ham to hold onto juices, preventing it from drying out. After the 15 minutes are up, slice the ham and serve it.

Summary:
Preheat the oven to 275 °F (135 °C). Place the ham in a roasting pan. Cut a diamond pattern into the fat with a chef's knife. Pour 1 cup of water into the roasting pan. Cover the ham with foil to prevent the ham from drying out. Cook the ham for about 2 hours. Check the ham's temperature with a meat thermometer. Spread the glaze over the ham if you're using one. Cook the ham uncovered for another 20 minutes. Let the ham rest for 15 minutes before cutting.