A few comments about hammer leather ....this is a research area I have spent
some time on - not as much yet as on the reproduction iron wire, but enuf to
have found a few answers.
The problems with hammer leather material are much the same as those
associated with iron wire - there is really nothing modern which is the same
as the old stuff. Enuf samples from old pianos have been analysed to be able
to state that it is almost always tanned sheepskin which was used on
hammers. The problem now is that the domestic sheep from which skins are
obtained in modern times have been selectively bred to the point that the
skin layers are now different from those of historical sheepskin. So, even if
you follow the old tanning recipe, you will not get leather of the same physical
properties as the old hammer leather, using a sheepskin.
The tanning recipes are not secret, and are well-enuf known. Other species, less
domesticated, will probably yield similar results to the old leathers, if
they are used, tanned according to the old recipes. Here another couple of
practical problems come up, but these too have been largely solved, although
I've never pursued this to the endpoint of actually getting the stuff made:
(i) economics dictates that you will probably have to have a few hundred skins
tanned - who's going to pay for that?;
(ii) the old recipes are not very environmentally friendly - many countries
probably would not permit them, and, for sure, the vast majority of tanners will
not want to be involved with potential lawsuits for effluent infringements.
Solving (i), i.e. finding someone who will tan just a hide or two, will also
solve (ii), since the scale is low enuf that pollution is not a serious concern,
except in environmentally ultra-regulated countries like Germany. As for which
species to use for this, reindeer was proposed to me by a leather expert who is
familiar with the old recipes and old leather types. (Now this is highly
appropriate, given the seasonal content, but don't tell Santa we plan to skin
Rudoph and turn him into piano hammers.) Reindeer skin is closer to
that of the wild European sheep, and likely to give good results. It is also
relatively easy to obtain, but quantities are an issue, since you might be
expected to purchase 1000 hides. (That's a lot of hammers.) Also,
possibly European deer (doe) skin might work, but apparently not deer skin from
North American animals. This was told me by the director of a leather trade
school in Germany. So, to solve the whole shooting-match, you need to find a
small, local tannery, and a few reindeer skins (not hard). I know the recipes.
Without an adequate equivalent to old leather, the solution for hammer
coverings is to look for something, anything, which has similar properties
to the old hammer leather. Leather samples can be found on old pianos, of
course, and studied. You need something which is supple, resilient, has a dry
feel to it, and has absolutely no stretch, in at least one direction (the one
that is destined to be parallel to the hammer head). Forget about trying to find
particular types of leather and expecting these to be automatically "best". It
doesn't matter what species or tanning process you end up with, and this is not
a good predictor of the leather qualities with modern leathers.
All that matters is that is has the right physical properties, which very
enormously between different samples, even within a particular piece.
You might find a big piece with a small section that is just right. If you're
lucky you'll come across a uniformly good piece of reasonable size. Stretchiness
is the factor which disqualifies much modern leather. Putting leather with any
stretch, even a bit of it, on a hammer is a big mistake - the stretch just grows
and grows (like the pink bunny in the battery commerical) with hammer use, and
the stuff starts slapping about pretty quickly, killing the tone. With no
stretch - like the old stuff - the leather covering stays resilient despite
constant use. Another factor that disqualifies much modern leather can be
characterized by a wet feel, slippery, sleezy sort of touch. You want what can
best be described as a "dry feel" - this is the thing which is most instantly
striking about the old leathers. You can use the wet stuff for
undercoverings, but keep it well away from the outer layer(s).
Armed with knowledge of what you want in a leather, go to a leather merchant,
coat or leather goods maker, shoe repair shop, etc. and check out the remnant
bins. Stroke, push, stretch as many samples as you need, to find the stuff you
are looking for. It will be generally quite obvious when you've found it. Buy
it, even if it's red or blue. Of course this is also the cheapest way to get
what you want. I know a piano maker who buys used suede jackets in the local
flea market and cuts them up, but I have not had trouble finding the right stuff
by sifting remnant bins in the leather goods repair shop.
The physical demands on hammer leather are serious. This has been a problem for
piano designers from the first pianos, through the felt era, even to modern
builders - it is not easy to get a consistently good product that is just right
for the job. There is still a big subjective element in hammer covering
manufacture. For leather hammers, the material needs to be:
(i) resilient, so it is tonally responsive;
(ii) non-stretchy, so it doesn't get loose and start to slap, killing the
tone; and
(iii) durable, so it doesn't lose its desirable properties too rapidly with
use.
These are mutually incompatible, of course, especially (i) and (iii).
Tonally-nice leathers will be less durable. Rock hard leathers that would
work in the space program will not give nice tonal gradation. You have to
choose a happy medium in that respect. Generally it should be expected that
hammers will likely need to be re-leathered, at least the top layer, every few
years on a piano that receives regular use. Keep in mind, though, that the
1814 Streicher piano in Stuttgart was certainly not kept in storage for its
long life, and the mostly original hammer leather still sounds very
charming. Now that piano has up to 7 layers on the bass hammers, which
helps.
When you leather a hammer you are building up graded layers of resiliency,
i.e. compressibility varies from more on the outside to much less on the
inside, to very much less on the core, just like the compressibility
gradation in felt hammer coverings. In the case of leather hammers you
build up this gradation layer-by-layer, so you have some control over how it
comes out, first by the type of leather you use - hardest on the inner layers,
softest on the outside - second by the way it is glued, and third by any final
voicing you do to adjust the resilience of the outer layer. Of course, with felt
hammers, you control this gradation mainly by voicing after the felt is
installed on the core.
Hot animal glue is a must for leathering hammers. It should be made very
thick and sticky. Thick regular hide glue is ok, but you can use something like
rabbit glue too, if you like to experiment. It's a bit fussy to use, but it
grabs like nobody's business. Contrary to popular belief, glue under the
strikepoint was not totally avoided. I've seen enuf leather removed form old
hammers to conclude that the lower layers were very often glued all the way
around the head, and this is certainly how hammers like Cristofori, with a
single leather layer, were glued. For a Viennese hammer, say with 5 layers, you
might find a couple of them glued all the way around under the strikepoint
(enhancing the hardness gradation), then the glue stops away from the stikepoint
in the outer layers, with more unglued leather under the strikepoint as you move
to the outermost layer. That's also why stretchy leather is so bad,
since it tends to work loose and get really sloppy on the outside where it
isn't glued to the layer below.
You take your piece of leather, cut strips overwidth as compared to the hammer
head width. The strips should be sorted and laid out in a row so that similar
thicknesses are next to each other. Wrap a strip around a hammerhead, and cut it
to the proper length. Skive the strip on the underneath
surface (shiny side) at the two ends, with a very sharp knife, tapering the
thickness to nothing, over the area which will be glued. Glue one end of the
strip to a hammer, and continue working on about 15 hammers at a time. By the
time you get to #15, the first is ready to do the second side. Put on the glue,
pull it tight on the head, and press it down, clamping with your fingers until
it sticks (not long with the right kind of glue). Each subsequent layer goes a
little further around the head, but, by skiving the thickness to nothing, you
don't build up a wad of leather on the outsides of the hammer head. Once you've
finished you take your really sharp knife - use the strop every couple of
cuts - and slice down the side of the hammer, toward the wood, taking a
sliver of wood as you go, giving a really clean cut and nice edge. Straight
razors work well for this leather cutting operation. If you do too many hammers
at the same time the glue gets too hard and more difficult to cut (15 is about
right if you work at a reasonable pace with thick hide glue - probably less with
rabbit glue, which sets with extreme tenacity).
For clamping, some builders use small, purpose-made clamps, to hold the layers.
These are easy to make using pieces of wood the same thickness as, and large
than, the hammer head with its leather. Cut out a profile of the hammer head
inside, and cut one end open so you can insert the head into the opening. Put
the clamp on the head and tighten the open ends to hold the leather
compressed, using a small spring clamp, or purpose-made screw mechanism. If you
clean up the side of the leathered hammer while it's in the final clamp it makes
it pretty easy to get a nice clean edge.
Some builder have experimented with continous layering, with mixed results. This
may have been the way semi-production 19th Century shops made leather hammers en
masse, with up to 10 layers of leather. You build up strips of leather of
reducing widths, piling them up lengthwise, maybe five in
a pile, with a cross-section like a pyramid. Each layer is glued to the one
underneath before you install the leather layers all together onto the
hammerhead. The lower (wider) layers are glued well toward the centre, or they
will bunch up when you pull them around the core. But the old hammers are just
like that, as I mentioned. Your pile cross-section looks something like this,
with the length about 8 inches or so:
XXOOOOOOOOOOXX
XXXXXXOOOOOOOOXXXXXX
XXXXXXXXXX00OOXXXXXXXXXX
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
where X = glued, O = unglued. Cut the pyramid piles into hammer-width (overwidth
again) strips cross-wise. Put on your glue, force the strip around the hammer
core (biggest side toward the core, of course), and clamp the head
(one of those purpose made clamps works great for this). This is a real time
saver, doing it this way.
Hope this is useful - Diane, I'll reply to your specific questions in a separate
letter.
Stephen
Stephen Birkett Fortepianos
Authentic Reproductions of 18th and 19th Century Pianos
464 Winchester Drive
Waterloo, Ontario
Canada N2T 1K5
tel: 519-885-2228
mailto: birketts@wright.aps.uoguelph.ca
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