From Salem Wiki
A Fireplace is an Ignitable Structure used to cook or boil Foods. It has room for 4x items and either a Clay Pot or Copper Pot. A Fireplace is also a Secondary Source of Fire, so items such as a Torch can be lit from it. Standing next to a lit Fireplace will raise your Body Temperature and help prevent Hypothermia.
As a Fireplace burns down it builds up Ashes (right display meter), as long as there are Ashes certain Foods can be cooked inside a Fireplace. Ashes continue to smolder for some time even after the open Fire has burnt out. An open Fire is required for cooking most other Foods or boiling. Food cooked over a source of Fire lowers your Insanity a little and has a chance to give you a Gelatinous Lard.
How to Boil Over a Fireplace