A Clapper or Tailor’s Clapper is basically a piece of hard wood which you place on steam pressed seams.
As the wood absorbs the steam when left for about 10 seconds, you will have a crisp fresh seam.
I thought I would give this a try so googled some facts on which wood to use and asked hubby what wood he had in his wood stash. It had to be a hardwood like Maple or Tulipwood (who knew?).Oak (although this can split over time). He dug deep and came up with a piece of wood called Sapele, which is like a mahogany. I then gave him some dimensions and showed him some pics on the internet (as, like many, he’d never heard of a clapper before).
He soon got to work in his work shed and was happy using his new Router tool…no varnish is needed as this could transfer to the fabric.
This turned out just great so decided to make it ‘pretty’ by first painting it black then using some adhesive backing to cut out some fabric hexis and ironed on…
He kindly obliged to make another one for a good friend who is also a quilter, so I personalised it…with a candy stripe of course…
Now.. I tested mine out a few times. I never use steam for pressing so used a spray gun on the seam and placed the iron on then rested the clapper on to absorb the steam for a while. This was a bit slow but results were good…a lovely flat seam. If you’ve got lots of seams to press it would take ages! So you couldn’t go like the proverbial clappers 
Brilliant!