Using the coarse side of a new, dry sponge, scrub garlic bulbs and remove any dirt. Do not peel the bulb or separate the cloves. Also, avoid washing the bulbs with water.

Set a slow cooker on the warm setting (low isn't low enough). 

Place the bulbs inside the cooker. Make sure you leave some space between the bulbs. The bulbs should not touch each other as they are heated.

Cover the slow cooker and allow the bulbs to rest in the heat until they turn black and tender. This should take around three weeks. Maintain temperature between 140F to 190F to age the bulbs without cooking them.

Check the slow cooker regularly to make sure it is set on warm, not low-temperature settings. The bulbs of garlic will turn hard at first, but will soften as your curing process approaches three weeks.

Once ready, remove from the cooker to cool. Store fully cooled bulbs in an airtight container and move to the refrigerator. Use within three months.