With respect to the use of the term "fulling" in the process of making hammer felt, I defer to David Stanwood or others for current usage. My understanding of the term has to do with its longer historic use (back to Roman times, and the English term seems to derive directly from the Latin), which was directed at a range of processes by which woven wool cloth was altered so as to be, essentially, partially felted, and which included many distinct activities, one of which was pounding. That's a subject that has long interested me, and I have read quite a bit about it over the years (first got interested from Don Quixote, a scene where he hears the sound of a hammer mill - a water mill pounding wooden hammers on wool cloth to "full" it - and imagines other things).
I guess the term "fulling" has been transferred to mean pounding and a couple related processes in the making of non-woven felt, something that came later in history. These matters of word usage are not particularly germane to the topic, but of interest as it does apply to other wools used in pianos, particularly such things as bushing cloth, which is a "fulled cloth" (as opposed to a felt). What does matter to our discussion is the final, unwoven hammer felt product (or range of products), a dense unwoven felt in which fibers are "interlocked."
In any case, I think the nub of where we have competing viewpoints lies in our use of the word "tension." I have been trying to explain my mental conception, perhaps not clearly enough. It might be helpful to distinguish between what might be called "active" tension and "potential" tension. Active tension could be defined as what acts on a large scale, affecting the mass of the hammer and how it behaves in a direct way. Potential tension is something that is in the felt itself, and can be adjusted so as to become active tension, under certain conditions and using certain techniques.
Active tension is what we can see and feel when we cut open hammers as in the photos in one of my previous posts, and I surmise (and I think this is a fairly commonly held opinion) that it is a large part of what gives a hammer its ability to a have a wider range of tonal gradient, because it allows for movement of fibers within the hammer in a dynamic way. I would expect Bacon felt hammers to spring open more than the Abel hammer I experimented with, partly because of what I think of as "heat set" in the Abel hammers, partly because the additional density of the Abel inhibits movement of the fibers. I would be curious to see the results of heat pressing and cold pressing identical felt, without glue, everything else the same as a normal hammer pressing. What would each felt strip look like when removed from the press? That might tell us something, and would probably be a good promotional pair of photos for a hammer manufacturer. I would expect the cold pressed felt to come closer to its original shape, but how much closer would be interesting to know.
What is "heat set," and how permanent is it? I don't know, in analytic terms, what happens to the individual fibers, but I don't believe it is permanent, and in pragmatic terms it seems to be reversible by the process of needling (or by other processes, like "voice-gripping" and steam and chemical means). The individual fibers have not lost their memory, and continue to try to go back to their original shapes when no longer constrained - unless so much heat has been used that some internal transformation has taken place, which I don't believe to be the case based on experience with various felts. I don't know for sure, but it certainly seems reasonable to assume that the pressure and heat of the original felting processes would have as much effect on the integrity of the fibers as the same heat and pressure during pressing of the hammers. Granted, there is a more "global" pulling and squeezing together of fibers, but from the point of view of the individual fibers, the forces applied would be analogous to what they have undergone in the initial felting processes.
"Potential" tension is part and parcel of the individual fibers of the felt. In my conception, hammer felt itself contains within it tension and compression. That is, the individual fibers all have "memory" of the crimp shape they had originally, and are "trying" to get back to that shape. Their individual situations are fairly random, some being bent more than in the natural state, others being pulled straighter than they were. If they are "pulled" straight, they are "tense" and trying to become crimped again. If they are "pressed" into a greater degree of bending, they are trying to straighten again, pushing outward. The former is tension, the latter compression, and the two are operating against one another within the matrix of the larger body of felt. This gives felt its "springiness." The forces of the individual fibers are minute in isolation, but taken all together become very significant.
The inherent tension and compression in a sheet of felt is somewhat altered when it is pressed into a hammer, as the large scale (millions of fibers being involved) action of forcing the felt around the core and fixing it in place causes considerable tension and compression, focusing these forces in different directions and creating a gradient in the large scale. Still, the initial tension and compression of the individual fibers continues to be there, altered somewhat, but it is a large part of the whole picture, and contributes a great deal to the behavior of the whole. The forces related to the individual fibers are magnified, as the fibers close to the core are more densely packed, but they continue to have the same memory and exert the same sort of force, even if constrained very tightly or stretched very taut.
How can it be that inserting needles could increase "active tension," when the needles go through the outer layers to get to the inner ones? In my conception, that has to do with the fact that the inner fibers are closer together, and needle work that penetrates deeply will "loosen the ties" of many more fibers than are either torn or "loosened" on the surface. (By "loosening" I mean that the felting ties, the connections between the scales of the individual fibers, are pulled apart, caused to be no longer effective in holding the fibers together - there is also some degree of tearing of fibers). So the inner fibers are able to "spring outward" and thereby cause additional tension on the surface, but also more tension in the interior of the hammer, as other "felting ties" are activated. Once the outer layers have been opened up from an initial needling, additional penetrations will have less of an effect, will tear less fibers, because the interlocking is looser (as we feel when doing additional needling later). Overall tension is no doubt reduced, but continues to be adequate to maintain a balance with a high degree of compression. The active tension compression gradient of the hammer is altered by these means.
I would add that needling technique can be a big factor here. That is why I insert needles deeply, insert being a key word, as opposed to stab. I believe that much stab voicing tends to penetrate more on the order of 5 - 6 mm rather than the 9 - 11 mm required, and that the result is a weakened outer tension and little effect toward the core of the hammer. The resulting sound tends to be pretty dead and colorless. Sometimes such a hammer can be revived with deep needling, but often not. I guess I should say that I have seen voicers who have a fairly precise "stab" technique that does penetrate to 10 mm or so consistently, though this "stab" is generally a bit slower than what I was describing as 5 - 6 mm penetration. I prefer a somewhat slower penetration, starting at the surface of the felt rather than moving when it hits the felt, as I am convinced it retains the integrity of the felting far better - and that is based on the experience of voicing the same sets of hammers over a period of a couple decades, and observing the results.
Well, I can see that this gets more and more convoluted to explain. As I try to make one part clearer, it requires expansion of another part, and the whole becomes less well organized. I think I will just leave it as it is, and hope that at least some of the concepts I have been trying to convey will come through with at least a little clarity. It is a mental model that has seemed to serve me well over the years, and to correspond pretty closely to what I experience when I work with hammers. It is only a model, not really verifiable. The techniques work, so I am not all that worried about whether my mental model is accurate.
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Fred Sturm
University of New Mexico
fssturm@unm.edu http://fredsturm.net "The true sign of intelligence is not knowledge, but imagination." - Einstein
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Original Message:
Sent: 04-02-2013 01:17
From: David Love
Subject: lacquer voicing
Fred:
No point in disputing felting versus fulling. There are several basic steps to wash, process, mix and interlock the felt create these thin little sheets which are stacked up on top of each other and then take this 12 inch or whatever thick pile of wool sheets and reduce it to a thinner and more dense piece of felt. The use of oscillating rollers, steam and heat in that later part also serves to further interlock the fibers. The result of good hammer felt making is a uniformly dense sheet of felt that is well interlocked and where the fibers are not damaged. The Weickert process is considered to be superior in terms of controlled and uniform density (over Bacon, VFG, "Japanese" methods): a good blend of both long and short fibers, health of the fiber when the process is over (not too many harsh chemicals that strip the felt of its natural substances), and the degree of interlocking. That's why the best hammer makers want it and use it. That's probably sufficient for our purposes however we wish to call the processes.
You need to be careful with straw man arguments here. You are misrepresenting what I have said before making an argument against it. I never said that tension was "absent" in a "hard pressed hammer". You can press a hammer very hard and it can have tension in it. The starting density of the sheet can impact how much stretching can be done but that is another issue. I said that the use of heat in the process of hammer making has an effect on tension, it lessens it, it does not increase it. The application of heat while the hammer is under tension alters the fibers structurally. But even with the use of heat there will likely be some tension, just less. Too much heat, however, and you can effectively destroy any or most tension. Since the voicing process itself (needling) reduces tension further, then starting with less leaves you with less to work with and it is more quickly dissipated.
If the debate is whether you want or need tension at all in a hammer, that's fine. You can argue that. I don't agree, but it's point that can be debated. A hammer without tension will still hit the string and impart energy. Neither am I suggesting you can't get acceptable results with hammers with less tension, or no tension for that matter. With some skill you can make just about anything work, even work fairly well, we do it all the time. I'm talking about what is desirable. I can't see a reason to reduce or eliminate tension in a hammer on purpose before you even start under pretty much any circumstances. Heat is the main culprit in that.
The interlocking fibers are, in fact, quite strong. Otherwise the felt will tear (or the molding will fracture if you are trying to shove it down into the caul without some prepressing). Poorer felt, in fact, can and does tear and the amount of stretching that can be done can be limited by that. Lack of space between the fibers does not insure that there is more tension (or less for that matter). You can have dense felt with no tension or not so dense felt with lots of tension. In terms of your experiment of cutting hammers apart, the less dense Bacon felt often opens the most in that experiment. Tension is not to do with the cauls, though they have a role in stretching the felt in the pressing process. The tension is in the felt and is only created through stretching. If the felt is not stretched there will be no tension. That's what tension is, it's the pulling of the felt in the opposite direction from which it has been or is being stretched. Once that stretching force is removed, there is no more tension. I donât see how you can introduce stretching by needling. Needling releases (removes) tension.
I suppose that if you could expand the felt from underneath the hammer surface so that it stretched the outer layers then you could introduce tension into the outer layers of the hammer. But when you "needle" hammers, you needle from the outside in (generally). So when you say loosen the fibers (I assume you mean the outer fibers) to increase the tension that doesn't make sense to me. By "loosening" the fibers you are necessarily releasing or removing tension in the outer layers (they are no longer as stretched since you've now separated the fibers some. Just as when you loosen your elastic belt you remove tension from it. Your stomach may expand and reintroduce some tension back into your belt with, perhaps, some room to move now, but then you are saying that the release of tension of the outer layers that is constricting the hammer allows the hammer to expand and therefore introduces more tension into the outer fibers than was there in the first place. Not sure I buy that. You may well "open up" the hammer, an apt description, but you are not, as I see it, increasing tension. I suppose you could argue that deep needling expands the layers underneath more than the piercing of the outer layers removes tension that is either there or forming (or both) on the outside of the hammer. I can't comment on that. I'll have to rely on the scientists there. What I hear and the manner in which I see the hammers respond doesn't seem to suggest that however.
Rather, I think in that case you are simply making the hammer less dense, allowing it some room to compress thus introducing a degree of flexibility into an otherwise rigid body. Cleary that does work and is an important structural consideration when voicing hammers that have no tension but are simply too dense. We do this all the time with hammers that are too hard and have devised all kinds of enhanced interrogation techniques to accomplish this including: pliers, steam, water, dremel tools, needles, fabric softeners and vodka (such a waste). But creating a "density gradient" should not be mistaken for introducing tension that is somehow hidden in the hammer or confused with what happens in a tensioned hammer when you release tension by needling. A portion of the density in a hammer that is under tension is a function of the tension itself. As you needle and release the tension from the outer layers, the hammer becomes less dense. It's more complicated, of course, because in a tensioned hammer it seems that you can release tension in certain parts of the hammer to impact the density in other parts of the hammer. When you needle tensioned hammers low in the shoulders they can get brighter, or louder. There may be other reasons for that too which are beyond this discussion, already too long. But when the tension is gone, your only hope to replace the density lost by the loss of tension is to basically pack the hammer down--the ultimate fate of even the most tensioned hammer. That won't return the tension to the outer layers, it simply provides the next best substitute. That's why we should treat the tension in hammers as a limited commodity, something to preserve and be cautious with. Every needle stick lets air out of the balloon.
In many ways hammers with no tension that are simply density driven are easier and less sensitive to voicing mistakes. Hammers filled with lacquer, for example, can take all kinds of abuse: make then harder, make them softer, needles in the crown, you can get away with a lot. Good, well matched to the belly (doesn't always happen), tensioned hammers sound better, I think, more lively, more resilient, more open, more bounce, but they require more of a surgeon's touch. You can make a mistake and let all the air out of the balloon at once if you are too aggressive and not careful. Who knows, maybe that's the reason they are not valued as much.
I could be wrong, of course. Wouldn't be the first time (or the last). But I don't think so.
David Love
www.davidlovepianos.com