the felt between 1830 and 1850 used by Pleyel on their grands, and Erard on everything they built, probably..
It was the first felt ever used on pianos and it was invented by Henri Pape.
Before then, leather was used and it had to be carefully chosen and tensioned by hand.
This felt was a revelation for the piano industry because 'anyone could make a hammer-set'
The felt was applied like a stamp, with no tension and a mild gelatin for glue.. but it wore-out after a few years of playing...
Pape also made a hammer shaped like a wheel, so that when the felt wore-out on the striking-point you could turn the hammer and expose a new surface..
this is possible because the last outer layer, which is the only felt layer, the others being leather, was applied without tension..
so I did a similar thing... I took what was left on the side of the hammer, which was unused and new (you could tell it had never been shaved-down because it has the external burn marks made by a hot sheet of metal applied at the final stages of the felt-making process) and I applied it on the tip with gum arabic.. the felt is very light and not dense at all.. then I glued some similar rabbit-felt on the sides..http://2.bp.blogspot.com/-ir5pRFmLWCw/VJCAHJBAIzI/AAAAAAAAAaY/-OgPhMeOZDw/s1600/IMG_4578.jpg
the outer grey layer is roughly equivalent to the 'pianissimo' layer techs make today by needling the striking point, but it is a discrete layer.
you can't usually do that with hammer felt or leather because most felt is tensioned before it's glued on the old style of hammers.. they left the tip unglued and the felt was more resistant to tension.. this particular felt is quite loose and flimsy by comparison and it was glued-on everywhere including the tip..
P.S. that's not me playing