The maker of the felt is only one part of the equation and, arguably, a smaller part than the press technology.
In other words, relatively soft felt can be pressed to form a quite hard hammer given enough moisture, heat and pressure. There was a time in the 1970s and 1980s during which technicians were convinced some Asian hammermakers were chemically hardening their hammers before shipping them to America. They could be used to drive nails! Over time I've been able to figure out what they were doing and I now understand how they came up with such dense hammers. The brand of the felt was only a small part of the problem.
Once I started my own experiments in hammermaking at Baldwin I was able to figure out what they were probably doing. I found I could pretty much duplicate the density of the hardest hammers of the day using Bacon felt. This was no small trick since, at the time, Bacon felt was among the least dense hammer felt available and the Asian hammers we were getting were harder than anything we'd ever seen. All it took was modifying the pressing procedure -- I increased the moisture content of the felt, increased the pressure (both in line with the molding and on the shoulders) and upped the heat. The result was a hammer that no technician would want to use on a real piano, of course, but they could be pressed efficiently and consistently.
And I think this was the motivation that led to all those super-dense hammers of the day. Too many hammers department didn't have much experience with, or knowledge about hammermaking. They had purchased or copied hammermaking processes they were unfamiliar with. Their emphasis was on production and consistency -- they were making product, not piano hammers. The processes that evolved were filled with the wrong solutions to the problems that arose. The upside was that there was a lot of tolerance in the system. All of the hammers were all going to come out hard and dense and looking great. No problem, just have the voicers drive the needles in as far as they can and work right over the crown. Do this a bunch of times -- I've seen electro-pneumatic machines set up to do this "pre-voicing" automatically. -- and then, once the hammers are on the piano, go back and shallow-needle some more right over the crown. The piano could easily and quickly be made to sound, if not great, at least decent and consistent with a minimal amount of skill and experience needed on the part of the final voicers in the factory. The piano wasn't going to sound all that great six months after it was sold and in the customer's home but that wasn't the factory's problem. If the piano couldn't be made to "sound just like a Steinway" it was the tuner/technician's fault. He/she was simply incompetent.
On the other hand, many years later and with another company, I was able to press what we technicians would call very "soft" hammers using fairly dense Japanese-made Royal George felt. Ideally, the characteristics of the hammers should match the piano to give the desired tone with minimal voicing either at the factory or during dealer prep. To get them to come out of the press this way does require more skill on the part of the entire hammer department, however. Minor variations in felt thickness make audible differences in voice. As do variations in felt moisture content, press pressures and press temperatures.
So, to be sure, the manufacture and physical characteristics of the felt are important, but there are many other equally important aspects to piano hammer making. Ideally we would like to have it all: good felt -- What is good felt? That is a whole subject all by itself. -- along with a carefully worked out process with good QC controls at every step. Not an easy result to achieve.
ddf
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Delwin D Fandrich
Piano Design & Manufacturing Consultant
6939 Foothill Court SW, Olympia, Washington 98512 USA
Email
ddfandrich@gmail.comTel 360 515 0119 -- Cell 360 388 6525