The Fire Hammer is the first hammer that increases the temperature of your sword craft. an upgraded version Inferno Hammer is available from special quest.
Since hammer strikes add heat by 1 degree each strike and 2 degrees for a critical strike, Fire Hammers are more commonly used on tables that will not see many hammers. Since swords will lose heat at 24 degrees and less, at the rate of 2 degrees each hour. A sword starts at 10 degrees and will take 10 hours of no hammer strikes before any penalty from cold is applied, -5 quality @ -10 degrees. Since hammer strikes alone will be enough to keep a sword from reaching the cold stage, Sword Smiths with only a few active friends will require FireHammer strikes to prevent a a sword from reaching the -10 penalty. Since swords being overheated and requiring ice hammers is a more common problem it is always good to use your "See All" Magnifier before using a Temperature Hammer. You never want to use a fire hammer on a craft that many smiths are working hard on trying to keep from overheating, or are Ice Stones.
Another popular use of Fire Hammers is to bring a Fire Stone quickly to 30 degrees so the overheat and burn temperature bonuses for 40, and 45 degrees occurs quickly.
After being used, the Fire Hammer applies 5 degrees to a sword and has a recharge timer of 30 minutes, so if you know you will need it, plan accordingly. A Fire Hammer can not heat a craft above 30 degrees, so the only way to heat a sword over that temp limit is to have friends hammer it.