Felt Quality

[David C. Stanwood]

The art and science of felt and hammer making a complex subject for 
sure.  Whether your using cold pressed, hard pressed, lacquered or 
not, the best description of what we all seek is best described by 
the father of the modern production made hammer, Alfred Dolge who 
wrote in 1911 in his book Pianos and Their Makers:

"The art in hammer making has ever been to obtain a solid, firm 
foundation, graduating in softness and elasticity toward the top 
surface, which latter has to be silky and elastic in order to produce 
a mild, soft tone  for  pianissimo  playing,  but  with sufficient 
resistance  back of it to permit the hard blow of fortissimo playing."

I'd like to make a comment on Ric's mentioning that Yamaha uses 
Wurzen and  Ray's comment that the Wurzen tends to keep more 
tension...  The Yamaha is a hot pressed hammer and Ronson cold (or 
warm) pressed.  Hot pressed hammers loose tension as a result of the 
heat.  One might imagine heating up a spring that is stretched to a 
nice red glow.  The annealing effect of the heat causes the tension 
to "melt" and when the spring cools off it doesn't take up its 
tension again.  The same is true for hammer felt...  so there is a 
distinction between tension and resiliency.  Wool fibres are more 
resilient than any other natural or manmade fibre unless degraded by 
harsh heat or chemical processing.  So in a hard pressed hammer it is 
a question of restoring resiliency and allowing room for the heat 
densified fibres to move.

The test of tension is to take a razor blade and cut down through the 
crown of the hammer and watch the immediate effect of the cut opening 
or making a cut through the middle of the hammer molding and seeing 
how the hammer opens.  With a hot pressed hammer the opening force 
from felt tension is always diminished compared to a cold pressed 
hammer.   With a cold pressed hammer the tension then becomes a 
function of the complex structure of the felt and this is very 
difficult to quantify.

I did have an interesting experience years ago.   I had used a couple 
of sets of cold pressed hammers which had extraordinary tension and a 
most beautiful quality.  I order more sets and found the tension 
greatly diminished.  Nothing the hammer maker did could create the 
tension of the first sets I had ordered.  I sent samples of the fibre 
to Texas A & M for analysis and the only difference found was that 
the average micron diameter of the sets with good tension was 21 
microns.  The sets with the poor tension had an average micron 
diameter of 23 microns.  So in this case fibre diameter seemed to 
have some bearing on the ability of the felt to hold tension at the 
crown.  I have seen fibres diameters of as low as 18 microns in turn 
of the century hammer felts... but it's a complex issue...

gotta run...

David Stanwood