Buy black fire cement from your local hardware store or online.
Allow your potbelly stove to cool if it's been in use. Then, remove any fuel from its body.
Wash the area around the crack you're going to fill. Make sure the crack is free of dirt and debris. If there are any loose fragments of iron around the cracked area, remove these with a metal file. Allow the stove to dry thoroughly before you proceed to the next step.
Pack the crack with fire cement using a trowel or a knife. Make sure the crack is completely filled with cement and that the exterior finish is smooth. Most fire cement products do not require mixing.
Allow the fire cement to dry per the manufacturer's instructions.