All I know is a door into the dark.
Outside, old axles and iron hoops rusting;
Inside, the hammered anvil's short-pitched ring,
The unpredictable fantail of sparks
Or hiss when a new shoe toughens in water.
The anvil must be somewhere in the centre,
The older outdated and rusted tools are contrasted with the continuing work in the blacksmith’s shop.  Though times change and new equipment is developed the work continues. 
 Note the strong, hard consonants throughout the first  quatrain . Notably effective is the compound adjective “short-pitched ring” which, while not quite  onomatopoeic  is resonant and memorable.
The Forge