Once the fire is hot, it's time to hammer. Which hammer to use depends on the project. In general, hammers should be light enough that you can swing them easily and with sufficient control for a few hours at a stretch. Some smiths grip with their thumbs pointed along the handle, others prefer to keep it out of the way. When hot metal is hammered, metal flows out from the point of impact, so the shape of the hammer head affects the shape of the deformation created in the workpiece.