Hot ham sandwiches provide a simple meal choice for a large event or afternoon picnic, but keeping the ham warm as everyone serves themselves can prove challenging. It's vital that the sandwich slices are kept at a safe temperature until they are eaten or stored; otherwise, they can harbor illness-causing bacteria.

Illness-causing bacteria can blossom in ham slices if they fall into the danger zone. For heated, precooked ham sandwich slices, the danger zone is between 40 and 140 degrees Fahrenheit. Generally the ham remains safe to eat for two hours after you remove it from refrigeration or heat. If you're trying to keep ham warm for sandwiches, a food thermometer will let you monitor the temperature at all times so the meat doesn't fall into the danger zone.

Cook or heat the ham as close to serving time as possible. If heating ham from the freezer, move it to the refrigerator to thaw 24 hours prior. Thawing at room temperature can cause bacteria growth. Large whole hams intended for slicing may take up to 48 hours to thaw completely in the refrigerator. If you must heat the ham in advance, cover the pan with foil and place a thermometer into the meat so you can view it easily. Keep it in the oven at the lowest setting until you are ready to set it out for serving. The foil prevents the ham from drying out in the oven.

You must plan ahead to keep the ham warm once you set it out for serving. A slow cooker set to the warm setting works well if the ham is precut into slices for easy serving. The ham may dry out if kept in the slow cooker for more than an hour. Prevent this by placing 1/2 inch of water in the slow cooker. Set the ham on a rack so the meat is kept above the water. Chafing dishes use a gel fuel canister to provide steady bottom heat to the dish. These heating dishes are commonly used at catered events and buffets to keep food at a safe temperature. Disposable models are available for home use, and work well for ham slices.

It's more difficult to keep the ham warm for a packed lunch or picnic. Heat the ham well into the safe zone immediately before packing it. Leave the ham in the hot heating dish to minimize the amount of heat lost, since a cool dish immediately begins leaching warmth from the meat. Insulated packs for transporting lunches or casserole dishes help keep the ham warm. You can create your own insulated pack by wrapping the dish with foil and packing it into a bag filled with crumpled newspaper, which adds further insulation. Follow the two-hour rule: Use the warm slices within two hours to prevent contamination.