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Introduction
Discussion rages between
proponents of thread wrapping and cork lapping on tenons in flutes.
Thread wrapping has been around since flutes (and other woodwinds)
started to be made in multiple parts in the baroque period. Cork
lapping seems to date from France in the mid-19th century. Thread
wrapping enthusiasts quote the simplicity of the method, its stability
with changing weather or flute usage patterns, the ease with which
non-technical people can make any adjustments as what attracts them.
They dislike cork because of a perceived risk to the socket if the cork
is put on too thick, although this applies to thread equally if not more. Cork enthusiasts (like the writer) feel they
have overcome any corking problems, enjoy the method and the results
they get, and are concerned that the tightness of thread can pose
problems to the flute bore, especially over time.
Over recent years the writer
has received for repair two 19th century flutes, one in boxwood, the
second in cocuswood, where too-tight thread wrapping does appear to
have damaged the tenons, causing significant reduction in bore diameter.
Indeed, so significant, that the direction of the normal bore taper
under the tenons was reversed! And, when the thread was removed
from the tenon, the tenon was distinctly "hour-glass" shaped, not
cylindrical. The word "strangled" came
immediately to mind. I'm defining strangulation as sufficient bore
compression to cause a significant bottleneck in a bore that should have
had a straightforward reducing taper.
Most other 19th century flutes also
show signs of less extreme bore compression. Reporting all this on the Chiff & Fipple flute forum led to a
very lively debate as to whether or not
thread tension could be enough to cause such damage, although no-one
seemed able to suggest any other probable cause for the distortion.
Clearly, there was a need to investigate.
A short survey
I went looking among flutes I
had easy access to for evidence of bore compression and strangulation.
I hoped to find flutes showing varying levels of damage, and, guess
what? Richness beyond the dreams of avarice! The results are
shown in this graph:
I've sorted them into
provisional categories, and advanced definitions for each.
Seriously strangled flutes
These are flutes where bore
compression is so advanced that a significant bottleneck forms under the
tenon area, constricting operation of the flute. The bore minimum
under the tenon is more than 1mm less than the rest of the bore might
suggest at that point. The acoustic and aerodynamic impact on the
performance of the flute is dramatic.
-
The anonymous boxwood
strangled flute, in yellow. Note the massive compression - a
reduction in diameter of over 1.5mm - in the middle of the tenon
area, stretching all the way in to 50mm along the flute. The
degree of compression reduces as you go further - this makes sense -
it's much easier to compress the thin wood of the tenon than the
relatively thick wood of the body. But in the boxwood case,
the process has continued well into the thicker body.
-
The cocuswood strangled
flute, in pink. Similar pattern to the boxwood, but milder in
terms of both diameter and length, perhaps due to the greater
strength and stability of cocuswood.
-
In aqua, a 1-key flute in
stained boxwood by Schuchart (presumably John Just Schuchart,
flourished London 1731-c1753), from the Bate Collection in Oxford,
drawn by Australian Ken Williams in 1984. Average of vertical
and horizontal measurements (both show the same level of
strangulation). Nice to have an example on the public record
predating this study. I'm not making this stuff up! Ken
records the observation: "Tenon hollowed, matching socket is
straight". We know why it's hollowed, don't we....
Moderately strangled flutes
Bore compression under the
tenon forms a mild bottleneck (less than 1mm) followed by a region with
reversed taper.
-
A boxwood flute by
Richard Potter, circa 1790, in orange, showing substantial
strangulation, distortion going back to around 50mm.
-
An extremely famous
original 18th century boxwood baroque flute, shown here in cobalt
blue. It's the GA Rottenburgh owned by Bart Kuijken.
Note the even larger bore of this early flute (from 40mm onwards),
and that the compression also extends well past the tenon.
This flute has a number of corps de rechange (alternative left hand
pieces used for different pitches). Although wear patterns
indicate that some of the corps were rarely used, they all showed
bore compression, suggesting that the combination of thread and
weather was enough to cause it. I.E. a thread-wrapped flute
doesn't have to be played for the damage to set in.
-
A boxwood flute by Bilton,
in very light green, with the area under the thread wrap dipping
below the region to follow.
-
My Nicholson's Improved
flute, cocuswood, in sky blue, "just past the cusp of
strangulation". Note the larger bore on this flute, compared
to most of the other 19th century flutes.
Flutes "on or about the cusp
of strangulation"
Bore compression has been
enough to flatten out the taper, but not yet cause a bottleneck.
Nonetheless, the mixture of acoustic and aerodynamic disruption is
likely to weaken flute response.
-
A cocuswood flute by
Camp, showing bore compression a smidge beyond "the cusp of
strangulation", in brown. You'll notice that, in the Camp, the
compression is limited to the tenon area, the first 25mm from the
left. It also starts earlier than the cocuswood flute -
this might be because there is less of a tip shoulder before the
threaded area starts.
-
A perfect case of taper
negated, the very cusp of strangulation! (The Camp had come
close, but this is perfect.) Check out the 20 to 25mm region
of the Clinton 8-key in cocuswood, in light yellow.
Flutes showing bore
compression, but not strangled
Such flutes will show
compression to the bore, but not to the extent that the original taper
is negated or reversed. At this level, impact on performance
should be slight.
-
My late Rudall Carte,
cocuswood, shown in navy, with only mild compression. By my
definition, this flute is definitely not strangled - at no point is
the bore taper negated or reversed. But you can see there is a
definite dip in the area of the tenon. Now, very
interestingly, this flute has an original wrap of a thick black
thread, overlaid with some additional thinner white thread.
Keep that in mind when we get to the topic of serial
strangulation...
Flutes with no sign of
compression
Such flutes will show a
straight or otherwise plausibly intentional taper. They serve to
remind us what general form the other flutes should take.
-
An anonymous Pratten-like
flute with cork lapping and no sign of compression. In grey.
Note the bridging taper over the first 6 mm, presumably intended to
make a smoother transition from the bore of the head (19mm) and the
main taper, which would otherwise have come out at around 18.3mm.
This may not have been an original feature, as the taper is quite
crudely cut, unlike the rest of the bore which is very nicely cut.
But note no compression to the taper in the lapping zone X = 5 to
22mm. Unfortunately for our study, English conical flutes with
original cork lappings are rare. It's unlikely that we'll come
across enough to draw much in the way of conclusions.
-
And finally, the Rosetta
Stone - a threaded flute that appears to have suffered no
compression at all and shows no sign of any tampering. Shown
in pea green, it's my Geo Rudall, Willis Fecit - one of the flutes
made by Willis for George Rudall in 1820, before Rudall teamed up
with Rose.
Time to count heads
I could go on, adding flutes
to the survey above, until I run out of flutes. But the graph is
already rather cluttered, so I think time to pause and do a tally.
Ignoring the cork-lapped flute, then, we have looked at 11 thread-lapped
flutes. Of these, and rounding the numbers:
-
10% are undamaged
-
30% are seriously
strangled
-
60% are strangled
-
80% are at or
beyond the cusp of strangulation, and
-
90% show some
compression
Ideally of course, we would
have a much bigger sample size - that could be achieved by a trip around
the museums such as I had done in 2002. But even if a large sample
halved the figures above, they would still tell a dismal story, and one
we can hardly ignore.
Willis' Secret
Now, what was Willis' secret?
How come his 190 year old flute has survived unscathed, while earlier
and later ones have suffered compression or even strangulation?
The answer is actually pretty obvious on inspection. Willis' tenon
is thicker than those on more recent flutes. Indeed, it's more
reminiscent of a baroque flute, although that didn't seem to have helped
our boxwood baroque instrument!
Unfortunately, the solution
available to Willis is no longer attractive to us. In Willis'
time, flutes were generally much thicker all along, and bores were
thinner. The outside of the barrel at the base of his socket is
29.7mm, the inside diameter 23.5mm. The tip of the tenon is 23mm,
the inside at that point 18mm. Since then, it's been realised that
a thinner head plays more adroitly, and bigger bores give more power.
As the flutes became thinner (all over, for more elegant appearance and
comfort in holding), and the bores bigger, the wall strength was
whittled down from both sides. And as the strength of a piece of
wood varies at approximately the square of its thickness, a small
reduction in thickness brings a bigger reduction in strength.
Willis had a second trick,
and I suspect this has even more to do with his freedom from damage.
He also made the thread trough very shallow - so shallow that the
current fairly thick thread has room for one layer only.
This thread is 0.4mm (estimated under the microscope) and, from its
condition, is probably not original. Less thread to cause the
damage, more tenon wall to resist it. Bravo, Mr Willis!
Indeed, here's an interesting
observation. Supposing you deepen Willis' thread trough just
enough to accept a second layer of thread. And you might have good
reason to - as it stands there isn't much room there to accommodate any
wood movement due to seasonal change. The second layer of thread
will double the constraining force on the tenon, and
accommodating it will slightly weaken the tenon's strength to resist
that force. A third layer would treble the force, and
weaken the tenon further. The ratio of thread depth to timber left
might be the critical element in Mr Willis' legacy.
This seems to be confirmed
when we contrast the unscathed Willis with two of the worst affected
flutes in the survey - the strangled boxwood and cocuswood flutes.
They both had very slim barrels, around 28mm, leaving little room for
all that has to fit inside. But the Schuchart from the Bate seems
to have pretty juicy dimensions (34mm OD and 24.8 ID in the middle of
the socket, around 22.5 and 17.2 for the tenon) and yet its tenon is
dramatically crushed. Confirming it may not be enough to consider
wood strength alone, but necessary to look to the ratio of that and
thread thickness. The Schuchart's tenon is so distorted we might
never be sure what thread thread depth was supposed to be. But the
current difference between the socket's 25mm ID and the tenon's 22.5mm
OD suggests that a lot of thread would be needed to fill that gap.
I'd suggest serial strangulation might be the killer here, see the
discussion on this topic later.
Measuring compression
I should mention that
measuring compression is not altogether straightforward. Most
people measuring flutes, eg when preparing to make copies, use the
regular T-gauge, in conjunction with the micrometer. Set the
T-gauge to say 18mm, and see how far down the bore it will go.
Then move on to 17.8mm, repeat etc. But, as you can see from the
graphs, once you pass the constriction, you need to start opening up the
gauge again, not just go on to the next place it stops.
Follow the yellow Strangled
Boxwood curve in the graph above. Once you get down to 16.3mm, it
skims along from 10 to 15mm, and then won't stop until somewhere near
the middle of the section! That's a dead giveaway that something
is wrong. Obviously, it won't be so noticeable on flutes with mild
compression, but you'll still notice a bigger step than normal.
Follow the brown Camp trace, and note that there's only about a 10mm
step across the cavity. But you can still detect that.
So, the golden rule has to
be, when you find a step that's bigger than any other step, it needs
full investigation, not glossing over. Open your T-gauge a step,
introduce it on an angle to duck under the low overhang, straighten it
up and note how far in it will go and how far back it will come.
Keep increasing the setting until you've mapped the whole chamber.
What effects can we expect?
We can expect two kinds of
effects. There will be an increase in aerodynamic losses, as the
oscillating air column encounters an increased constriction in the
middle of the flute. So performance will be down on that basis.
But there will also be a
marked acoustic effect which will evidence itself in two ways.
Tuning is going to be different to what the original maker expected,
and, if the spacing between the octaves changes, that is likely to bring
a weakening of performance, as the harmonic alignment will suffer back
at the jet.
I ran a few computer
simulations to predict what we might expect to see. The software
is currently being developed by a group involving the Physics Department
at the University of New South Wales, the Powerhouse Museum and myself.
It's not quite ready for use in computer modelling of flutes yet, but
more than adequate for comparative analyses like this, where only small
changes are involved.
You'll see three traces in
the graph below:
-
The blue trace is the
boxwood severely strangled flute we met at the top. You'll see
that the model predicts sharpening of the low octave, to a startling
24 cents around low A, and a flattening of the second octave, again
by up to 24 cents around 2nd octave G#. With those midrange
octaves now being narrowed by up to 48 cents, we can certainly
expect significant loss of performance, as well as distinctly odd
tuning!
-
The yellow trace shows what a
moderately strangled flute such as the Richard Potter will suffer.
Same general shape, but with a bit less than half the effect. I'd
still expect some performance loss from both acoustic and aerodynamic
sources.
-
Finally, in pink, a flute
that was on the very cusp of strangulation, i.e. the bore is
sufficiently compressed to flatten the taper under the wrap, but not
enough to actually cause a measurable bottleneck. But enough to
narrow the G# octave by about 14 cents. That may or may not be
enough to introduce acoustical misalignment losses - this would depend
largely on luck.
I could model more of the flutes,
but it would mostly serve to confuse the graph. They can be expected
to come out between the pink and blue traces. A flute like my Rudall
Carte that only shows mild compression would come out between the pink trace
and the horizontal axis. That's about as much uninvited change as I'd
like to see.
We can draw much from these simulations:
-
Note the general shape of the tuning change is one we
are already familiar with. Most old flutes have LH notes that tend
sharp in the low octave. Some of that is attributable to other
reasons, but it's certainly not something to be encouraged further!
-
There isn't that much between the Cusp and Moderately
strangled flutes. Once the bore has constricted enough to negate the
taper, significant errors are already being introduced. Which is
logical enough. The maker made the flute tapered to bring the octaves
into line. If part of the taper gets cancelled, so does that part of
its beneficial effect.
-
Given that we found above that most old threaded flutes were
strangled or beyond, we can now say that most old flutes no longer perform as
the maker intended.
-
If we have an old flute that is at the cusp or moderately
strangled, we have to ask ourselves what are we doing to prevent the problem
advancing.
Just how strong is thread?
I took a length of the thread
taken from the strangled cocuswood flute, supported it from a retort
stand, tied the free end to a hook, and carefully added calibrated
weights until the thread broke. I got to 700 gms weight, or 1.5lb.
That's pretty strong for a thread with a diameter of about 0.1mm!
I measured the thread from
the Richard Potter "somewhat strangled" flute. This was very old
thread, and no doubt weakened by time and compression. It also
broke at 700 grams weight, but unless we can find a modern equivalent,
we don't know what it was capable of when young and fresh. There
were 5.4 metres of that thread on the top tenon. It was squashed
too flat to measure by normal mechanical means, but using a stage
micrometer on the zoom microscope, I'd estimate it as varying between
0.3mm and 0.9mm, depending if I was looking on the flat or the side.
It had become a ribbon, rather than a thread. If we took the
average of 0.6, it's 6 times thicker than the modern sewing thread.
I also measured some
"soft-looking" yellow hempy sort of thread I'd bought 30-odd years ago
from a Scottish bagpipe shop, sold for the express purpose of wrapping
tenons. It measured about 0.3mm on the stage micrometer. A
single length of that broke at 2.7Kg (6lb). Thread is strong!
Another useful observation I
can make about these tests is that none of the threads seemed "elastic",
in the bungee cord sense. When the first calibrated weight went
on, the thread straightened out. As I added more weights, there
was no sense of the thread getting longer, up to the point where it
broke. I can obviously measure this if needed, but I suspect the
observation is enough. A coil of a hundred or more turns of any of
these threads will present a formidable barrier to expansion.
Calculating the forces
An informal attempt to calculate the possible maximum forces involved lead to
numbers so great that some readers rejected them outright as
preposterous! I asked my friend
Professor Neville
Fletcher for a confirmation (reproduced below in italics). Characteristically thorough, he
responded with two ways of looking at it - the total force
applied by the thread on the tenon, and the pressure the thread
applies to the tenon. You'll remember that pressure is force
divided by area, and so takes into account the area covered by the wrap.
Total force applied to tenon
From what you write,
you want to evaluate the total effective inward force acting on the
tube. Suppose the string is tightened to 700 grams weight and that
there are 13 layers each of 150 wraps. Now imagine that you slice
along the tube on both sides and remove one half of all the threads,
but that magically the thread stays in place and taut. To make this
happen you have to put a weight of 700 grams on each end of each of
semicircular wrap. This makes a total weight of 13 x 150 x 700 grams
or 1365 kg on each end of the semicircle, making 2,730 kg
altogether.
For US readers, this
translates to 6020 pounds.
Pressure applied to the
tenon
If the question you
are answering is "What is the effective compressive pressure
provided by the winding?" Then each turn has an inward force of T/R
per unit length, where R is the radius of that turn, acting over a
length 2.pi.R, giving a total inward force of 2.pi.T. (Note that
this is independent of R.) If we have N turns in a single layer and
M layers. then the total inward force is 2.pi.N.M.T. But this force
is spread over a total area 4pi.N.R'.r where R' is the average
radius of the winding and r is the radius of the thread, so the
inwards force per unit area, or equivalently the inwards pressure,
is (2.pi.N.M.T) / (2.pi.N.R'.r) = (M.T) / (R'.r). To evaluate this
you need to decide whether to use SI units and get the answer in
pascals. Doing it on the back of an envelope, I get about 2x10^8 Pa
or about 2000 atmospheres.
For US readers, this
translates to about 29,000 psi.
These are indeed frightening
figures. But keep in mind the following:
-
we based the thread
tension on the breaking strain of the thread. It's
unlikely that anyone would wind that hard. But even if they
wound at one tenth that tension, one tenth of those forces is still
a lot to apply permanently to a thin-walled tube of wood.
-
I'm told that this is no
news to serious kite flyers. It seems that, if you wind up the
kite string onto a hollow plastic reel at the tension the kite is
exerting, the reel can crack and collapse under the accumulated
force!
-
The moment you apply
anything like that
pressure to the tenon, it will start to compress. And will
continue to compress until it takes off enough pressure that it can
support the remainder of the force. In other words, it will
crush to reach equilibrium. We will see that happen at the start of the
experiment below.
-
A more realistic way to
look at these figures is that they represent the maximum
resistance to expansion that the threadband (even if relatively
loosely wound on) could present before the thread would break.
As we'll see later, that is totally relevant.
-
The actual value of force
applied to a real tenon is not really calculable, because of all the unknowns.
But it suffices us to know that it is more than enough to cause
problems!
A Test Tenon
In order to test the
possibility that a thread wrap could damage a tenon within a reasonably short time frame, I decided to make up an
under-strength test tenon, wind it firmly and subject it to some
rigorous climatic change.
It has to be noted that this is an experiment
"in extremis". The process I used is a form of
artificial
aging. Such an experiment is designed to bring results in as short a
timescale as possible. It is therefore not designed to mirror
reality accurately. It will be enough for now to prove possibility, and to
learn anything else we can from it.
As can be seen from the first
column, Just Made, the bore of the tenon was 18mm, and the outside
diameter 20mm. There is an 18mm wide trough for
thread lapping with a floor diameter 19.6mm, but of course we'll lose
track of that once the tenon is thread-wrapped. The full
width of the tenon being 30mm, this left shoulders of around 6mm wide on
both sides of the thread trough.
In the Just Wrapped column,
we can see that a wrapping of 0.5mm depth was added, producing a wrap of
20.6mm diameter. The thread I used was the first to come to hand,
a left-over of a domestic sewing project. The manufacturer
advises: "Rasant - the functional polyester/cotton core spun thread.
Rasant is a functional sewing thread with a wide variety of uses. The
perfect synthesis of polyester core and cotton covering makes Rasant
outstandingly efficient, not only in the sewing process, but in the seam
as well."
Because the test tenon was small and a bit fiddly
to hold, the thread was wrapped on firmly, but not overly tight.
It was also not wound on as neatly as had been intended, which would
have achieved a stronger wrap.
Immediately, a small reduction of
the bore under the wrap was noted. This is to be expected, as
mentioned above, as the tenon will be compressed until the force exerted
by the thread is balanced by the restorative force produced by the
distorted wood. The fact that we can measure compression
immediately after the thread has gone on does however confirm that we
are dealing with a significant force. I left it in this condition overnight to make sure that balance has
been reached before proceeding.
Effect of Weather
The next day, there was
little change, so it was felt appropriate to start the "Effects of
Weather" phase. The tenon was subjected to an environment at 25%
RH overnight. Naturally, the timber shrunk, as can be seen in all
curves.
The tenon was then subjected
to a very humid atmosphere by suspending it in a closed plastic ice-cream
container over, but clear of, a wet sponge. My laboratory grade
hygrometer reads an impressive 99.9% RH in there. Note that, in After Humid1, all the
wood swelled, excepting that bound by the thread, which continued to
shrink slightly.
Subsequent drying and
humidification cycles have presented an increasingly interesting
pattern. The bore under the wrap (pink) has continued to collapse
in diameter. But the ends of the tenon have started to increase
in diameter, both outside and in. This dramatically enhances the
"hourglass" shape which was a very visible feature of the damaged flutes.
And it is consistent with a previously unexplained issue with the
damaged cocus flute - all three tenons, even when their thread wrapping has
been removed, jam noticeably as they enter their sockets.
I was concerned that I
mightn't be giving the drying phase enough time for it to equilibrate,
so, after the third drying, I gave it some extra time. As you can
see, not much more happened in the More Drying phase. It was felt
at this time, it was appropriate to let the tenon then equilibrate to
local atmospheric conditions, the results of which you can see in the
"After Airing" column. Not shown there is the length, which has
returned to the original 30mm. It had increased to as much as
30.3mm in humidifying cycles, and reduced to 29.8 in drying. The
fact that it is back to normal suggests we have equilibrated. This
pattern was to be repeated throughout the tests.
Summarising the Effects of
Weather phase, we can see that:
-
the diameter at the
middle of the wrap on the outside (aqua) has dropped by 0.3mm
-
the outside diameter of
the ends has increased by nearly 0.4mm,
-
the bore diameter under
the wrap has dropped by 0.8mm,
-
the bore diameter at the
ends has increased by just over 0.5mm
Explains a few bumps
Interesting now to look at
the RC7174 (Rudall Carte, aqua) and R&R 5501 (Rudall & Rose, black)
curves in the chart below. Note the flares at the ends of the LH
sections (at around X=210mm) and the RH sections (at around X=320mm).
Are they, as sometimes claimed, deliberate flares introduced by the maker, or
are they the result of
severe bore compression? The location and amounts are consistent
with bore compression. Most of the flutes also seem to have signs
of compression at the top ends (around X=15mm).
Effects of Playing phase
With clear trends in the weather established, I think
it's now time to try emulating the effects of playing.
I simulated an hour's
practice, by lightly stuffing the tenon with a damp rag for an hour. I
felt it may not
have any different effect from the humidifying we did in Effects of
Weather, but it seemed possible that only humidifying from the inside
could
produce a different result.
Certainly, the first hour
"playing" illustrates that a wet bore is a much faster way of getting
water into the tenon than a moist environment! Not really
surprising, I guess.
In order to keep things
moving, I popped it back in the dry environment for an hour or so, then
let it air on the bench. As you can see in the final After Airing,
there hasn't been much change since the airing before playing.
Like most things, change is faster at first and slows as time
progresses. There probably would be more change if we kept this
up, but I think we've reached the point where we can draw some useful
conclusions.
How collapse manifests
The image below is of a
slightly strangled flute, the (Richard) Potter mentioned at the top.
I've shown you one edge of the top tenon, with the rest of the LH
section out to the right of image. You'll see that the worst of
the reduction in diameter occurred in the first 14mm (although the
minimum in the bore was at 15mm). But more surprising is that the
width of the combing (the thread retention grooves) varies, being a
regular 1.5mm, then a narrow 1mm one, followed by several at 2mm.
Note also the absence of
shoulders at each end of the thread band. They were a feature of
later flutes. Their absence might be of significance in the shape
of the distortion. Indeed, the more strangled cocuswood flute
presented a little differently. The bore trough was a more
predictable hourglass shape, but there was still evidence of a change
near the middle, with the land areas between the thread retention
grooves normally flat-topped on the tip side, but sawtooth on the body
side.
We're tempted to imagine that
the tenon is made of a homogeneous stuff like plastic, that will behave
in a linear, predictable sort of way. But in fact it's made of fibres,
running lengthwise, made up of empty cells. And the outer ones
have been slashed across every mm or so by the thread retention grooves,
not necessarily to the same depth. And with a tapered bore inside,
leaving the tip end wall the thinnest. Put a death grip on that,
and why should it behave in a simple way? But short of slicing along the
tenon and examining it on the scanning electron microscope, how can we
find out what it is doing?
Hmmmm, CAT scan?
Comparison of bore
distortions
I thought it might be helpful
now to compare the distortion we had introduced into the test tenon with
the kinds of distortions I had found on the two "strangled" flutes.
Not directly comparable of course, as the flutes had tapered bores,
while the test tenon had a cylindrical bore. So I corrected the
test tenon figures to introduce a taper, based on my guess as to what
the cocuswood flute taper might have been originally. So, in the
graph below, we see:
-
The boxwood strangled
flute, in yellow,
-
The cocuswood strangled
flute, in pink, with my guess as to what it might have been
originally in pink dashes,
-
The "corrected" data from
the test tenon, in the purple trace.
You can see that the test
tenon distortion lies between the two other well-strangled flute traces.
Even uncorrected, it pretty much followed the pink solid trace, whereas
it should have been (if it hadn't been compressed) horizontal along the
18mm diameter line!
Note that, had our test tenon
been part of a real flute body, the distortion from the middle of the
tenon onwards (X between 15mm and 30mm) would have been less, as the
more rigid body on that side would have helped resist the distortion.
Although we would have seen some compression in the body. In order
not to confuse the image, I've chosen to map only the first half of the
tenon, which is analogous to the other flutes. I would expect to see the purple trace following a
path approximately halfway between the pink and yellow traces.
Not just the top tenon...
Now, I've been concentrating
in this article on the top tenons of all these flutes, probably because
it's that tenon that has the most profound effects. A constriction
at this point of the flute will effect all the notes, but in different
directions and to differing degrees. Constrictions in the lower
tenons will mostly impact on notes there and lower.
But it needs to be remembered
that most of these flutes are also constricted where the LH meets the
right, and where the body meets the foot. Compression there is
actually easier to detect, as it forms a single point of inflection,
rather than two. There is also no chamber formed, until you plug
in the mating section.
Serial Strangulation?
Now, let's just imagine that
the test tenon on our make-believe flute had suffered the kind of
distortions we've seen above. It had started out with a thread
wrap of 20.6mm, which would have been exactly what it needed for a nice
snug fit in the socket. But that wrap has now reduced to 20.25mm
in diameter, 0.35mm too small for a snug wrap. Indeed, well before
this stage, the joint would be sliding about uncontrollably, and even
leaking. So what would we do about that? Put on 0.35mm more thread
of course, or perhaps even remove the old thread and start again.
Either way, we now have a tight grip on the tenon again, and an even
stronger band of thread. The game starts over!
Remember my Rudall Carte we
met right at the top? Black thread covered by some additional
white thread. Only mild compression so far, but clearly we're on
the way...
Isn't it one of those cruel
ironies that the more you look after your flute, lovingly changing or
augmenting the lappings when it gets a bit loose, the more likely you
are to kill it! I'm reminded of Oscar Wilde's terrifying poem
the Ballad of Reading Goal. "Yet each man kills the thing he
loves..." Indeed, the very benefit touted for strung flutes, that
seasonal variation can be taken up by the owner, might in fact be part
of the strangulation cycle.
The Good Old Days?
Was it better in the Good Old
Days? Some have suggested that perhaps threads were softer then.
Glancing at the graph at the top though seems to dispel that hope.
The most damaged bores were flutes from the 18th and 19th centuries, and
the baroque instruments in particular would not have seen much if any
use until the Early Music Revival in the 1970's. By then the
Schuchart was in museum hands, yet it it one of the most damaged.
The Richard Potter is missing several keys, and has probably not been
played for 100 years or more. There was nothing soft about the
thread I took off it.
Duplicated Distortion
Doubled
Imagine this ghastly scenario. A modern flute
maker faithfully copies a period flute without making allowance for the
thread-induced bore compression. So the copy is also distorted.
But then the maker also uses thread to wrap the copy's tenons.
After some time, it starts to work its ugly magic, and the copy is now
more distorted than the original! I'm told it happens regularly in
the early flute field. Perhaps it happens in our field too?
Clearly, we must understand bore compression and take
steps to guard against it.
What about re-reaming?
Some have suggested that a flute displaying signs of
constriction should simply be re-reamed to remove the offending
in-growing tumour.
A moment's thought though should ring warning bells. When you ream
off the protrusion into the bore, you're further weakening the tenon
wall. Carry on like that - putting more thread on the outside and
reaming off the wood on the inside - and the two will finally meet.
Probably with a bang!
But wait, there's more....
What happens when you play a country song backwards?
According to the popular song, it reverses all the sad things that
usually happen to you in country music. Your truck engine springs
back into life, your dog doesn't die, and even your wife comes back to
you. Powerful magic indeed! If we reverse what we did to our
strangled tenon, could we reverse some of the damage? Can we use
the tenon to work out possible cures for strangulation in real flutes?
We need to know, so...
I measured it once more, to ensure no further changes
over the last few days. Nothing worth reporting. I then
pulled the thread off. For the record, it was 20 metres long.
(Easier to measure coming off than going on!). That is
substantially less than I had taken off the cocuswood strangled flute.
I remeasured the tenon, no
immediate change other than the bore under the thread increased in
diameter to 17.45 as the remainder of the thread tension came off. Originally 18mm, it's still pretty squashed.
We can also now pick up the diameter of the bottom of
the thread trough that we lost track of when it was wrapped (purple
trace).
As we can now see, it had started at 19.6mm, been crushed down to 19mm.
The first test is to simply humidify it. It seems
too hopeful that it will obediently spring back into place, but let's
see. Back into the ice-cream container....
After rehumidifying overnight (humid 3 column), we can
see that all the diameters have increased (as expected). Indeed
the crushed bore under the thread wrap (pink) and the crushed bottom of
thread pack (purple) are now well over their original uncrushed
sizes. So, what will happen when they dry out?
Oooh, that's promising! After the final airing, on
28 January, we can see that the bore under where the thread wrap went is
back to pretty well normal, as is the outside diameter at the bottom of
the thread trough. So this does suggest that removing the thread
wrap and then humidity cycling might be helpful in attempting to deal
with a collapsed tenon.
Surprisingly, it's the free end(s) of the tenon that
have ended up still wrong, and oversize at that. Now who would
have thought? But I think I can see the basis of a way of dealing
with that too. If one were to dry the tenon first to make it all
undersize, wrap some thread around the free end(s) to constrain them and
then humidify and dry, one could probably get that back to size too.
If wrapping thread isn't the answer (might be too hard to predict the
end size), then maybe installing a delrin ring
which can be popped off or turned off later.
The proof of the pudding...
... is traditionally located in the eating. So,
can we really cure a real flute from the distortions of the past, by the
same processes that got us there? I resolved to find out. I
opted for the approach used by violin, guitar and harpsichord makers, suitably
adapted for flutes, but there would be many approaches. This
method offers minimum water uptake and so speedy recovery.
I
turned up a brass plug following the bore shape I guessed at for the cocuswood
strangled flute. Holding the tenon in boiling water, and then
pushing the heated plug into it produced steam, which infuses into and
softens the wood. Each time I did it, I could push the plug a
little further, until it finally reached the point I wanted. I let
the wood cool and dry with the plug in place, to prevent it sneaking
back when my back was turned. (If you want to try this yourself on
an affected flute, be careful to constrain the outside of the tip of the
tenon from expanding too far, as the thin wood of the tenon is at
considerable risk of splitting from the wedging action of the plug.
Paradoxically, wrapping some thread around the tip as mentioned above
might be just the answer!)
You can see the result in this next graph, with the
steamed cocuswood flute in green. Comparing my previous guess
(pink dashed) with
what we have now, it looks like I might have overdone it at the tenon
tip, but that's easily fixed if necessary. But looking at the
steeper bore taper down around the 60-70mm mark, I'm also wondering if
this approach might not have reached far enough into the thicker wood
just beyond the tenon, circa 30-40mm. A longer, slower soak might
do that. But the flute has to be far better off now than with the
constriction seen on the pink curve in its throat! I decided to let its performance tell me if
we're done yet.
The strangled cocuswood flute is now back together, and
working very nicely. It is impossible to say how important fixing
the choke in the top tenon was, as I also found lots of other little
things that warranted attention along the way. This is normal with
old flutes. They might have one or two dead obvious things wrong
with them, which is usually why they were sent in for attention.
But it's often the accumulated effect of a dozen or more little
unnoticed problems that pull an old flute's performance down. If
you want to give it back its will to live, every one of them has to be
detected and ruthlessly eradicated. Bore strangulation is now a
new one on the agenda.
Flute, heal thyself [Luke 4:23]
(See, it's true. Even the devil can misquote
scripture!)
Our experience with the test tenon might bring us some
hope for a self cure for strangled flutes. Imagine you have a
flute with bore compression. You remove the thread-band tourniquet
and replace it with something that won't restrict expansion, say cork.
You put the flute back in service, meaning it gets subjected to wide
humidity cycling. If our test tenon's recuperative experience is
any guide, slowly the damage might be undone. At least you know it
won't be getting any worse!
Wrapping it up (get it?)
Puns aside, I don't think there's much point in
attempting much more on the test tenon, as we're now getting into finer
points, which is likely to depend on matters like how compressed a real
tenon is, what timber it's made from, how thick it is, how long the
damage was sustained over, how long since the damage was sustained, and
so on. But we can certainly thank our test tenon for confirming
how this compression comes about, and offering us some hope that it can
be reversed or at least ameliorated.
I'm very happy with the in-extremis test procedure adopted. In just over a week, we were able
to simulate what might have taken years under normal circumstances, and
yet end up with entirely plausible results. And discover an
unexpected side effect. And explore a possible
road to recovery. Rewarding!
And to apply our new-found
knowledge successfully to a real-life patient and effect a cure.
Priceless!
Conclusions so far....
To summarise, let me advance
this analysis. For tenon-wrapping (cork or thread) to work it must
just firmly fill the void between the socket and the tenon. Any
less and it will leak or not hold the flute together satisfactorily.
Any more and it will put unacceptable outward pressure on the thin wood
of the socket. But the tenon wood is going to get wet during
playing, and will want to expand. If the tenon wrap is
thick-enough cork, it will have that room, provided by the resiliency of
the cork. But if it is thread it will not, and the wood of the
tenon will be compressed. Even if the thread wrapping is applied
loosely, it will be compacted by the same expansion-contraction
wetting-drying cycle, until more thread is needed to secure and seal.
Once enough thread is applied, the pressure goes onto the wood.
This article started out
titled "effects of extreme thread wrapping...", but I have taken out the
word extreme. Most of the old thread-wrapped flutes surveyed
(indeed, all but one) showed considerable bore compression or
strangulation. The test tenon was wrapped in a common
polyester-cotton sewing thread, with considerably fewer turns than I had
taken off the strangled cocuswood flute. The only extremes involved were the humidities of 25% and close to 100%, applied in quick alternation to
accelerate the passing of time. For most of us, these are not
uncommon conditions.
Perhaps it's appropriate to
ask ourselves why, so long ago, makers of Boehm flutes, clarinets, and
oboes shifted to cork, leaving only conical flutes string-wrapped.
The simple, if painful, answer is that conical flutes were the cheap end
of the market, and string is cheap.
It is now safe to
conclude that it is clearly possible, indeed almost inevitable, to damage a tenon by wrapping it
in thread, and subsequently exposing it to the rigours
of weather and playing. Indeed, the question now becomes what can
we do to ensure such damage doesn't happen? Unfortunately, the rigorous and numerous experiments needed to prove
what is safe using multiple layers of thread might take dozens or
even hundreds of tenons, and years,
tens of years or even more! It may be misdirected energy to
conduct such tests. Indeed, maybe we should be putting our
energies in identifying newer, more appropriate materials and methods
than either cork or thread.
What, me worry?
So, having read all this, and
having a flute that is thread wrapped, should you worry that
strangulation should happen to you? Unfortunately, although our
experience shows that flutes get strangled, and our experiment shows a
mechanism by which it can happen, we can't yet predict whether and when it's
going to happen to your flute.
But here's one circumstance that
would definitely ring alarm bells. If you find, from time to time,
you have to add thread, but you don't find you ever have to remove any,
I'd be worried. Unfortunately, it's probably a bit late to be
worried, as it probably means serial strangulation has already set in. But the sooner you stop making
things worse, the better.
I'd be more worried about the
softer and less stable boxwood than the stronger and more stable
blackwood. But having said that, it's only because I have no
experience of blackwood compressing, and that's because I have no
experience of threaded blackwood flutes.
Cocus, from our survey above, is obviously at risk. The older the flute,
the more likely the risk. The thinner the tenon, the more likely
the risk. The deeper the thread pack, the more likely the risk.
I'd keep up the oiling, as
this should slow water intake. I'd be very thorough about mopping
out, as that at least terminates the water uptake period. I'd
definitely unwrap any flute with a tight binding, and look
for something softer I could apply more loosely. And while I had
the wrapping off, I'd look to see if the thread trough is uniform in
diameter or hourglass shape. Holding a rule against the tenon
shoulders and looking at the size of the gap along the trough is a
reasonably sensitive test. if in doubt, I'd measure and graph the bore for
the first 50mm (2") or so at each tenon, and look for signs of
compression.
If you don't feel motivated
to go that far, at least investigate the top few layers. If you
found more than two or three separate windings were involved on any one
tenon, I'd be worried that serial strangulation might be happening.
If that were the case, I'd definitely check out the tenon below using
the rule method.
A single point check
Although to really understand
what's going on, you need to measure and graph the bore for at least the
first 50mm, our survey at the top suggests one pretty easy "single point
check". The typical 150mm (6") vernier callipers can reach about
16mm (5/8") down the bore if you use the internal anvils (the pointy
ones). Larger callipers have longer anvils, but often only the
first 16mm or so are chamfered to prevent interference, so they can only
read accurately to the same depth. Fortunately it's enough to take
us to the middle of the tenon, where the shrinkage is likely to be
mostly felt. If you haven't any callipers, you probably know
someone who has, even if it means a trip to your motor repair shop.
Be gentle in taking the measurement, you don't want to rough up the nice
bore on your flute with the sharp pointy ends!
So the question becomes what
would we expect to see at 16mm down the bore. That's obviously
going to depend on the flute type, but we could hazard a few guesses.
A small-bore flute, if we take the Geo Rudall, Willis fecit as an
example, should be around 17.75mm. A small bored Rudall style
flute more like 17.9mm. A larger bored Rudall or Nicholson's
Improved say 18.1mm, and a Prattens around 18.3mm. If you compare
your measurement with the flutes on the graph at the top, you should be
able to form a good idea of what condition it's in.
Checking the lower tenons on
either the LH or RH section is much easier. Measure just inside
the end of the tenon, and compare it with the measurement you get when
you insert the callipers to full depth. The end measurement should
be smaller. If the full depth measurement is similar or smaller,
then you have clear evidence of bore compression.
There's one case where that
might be misleading though. If the foot on your flute flares
dramatically (eg on a flute with a Short D foot), the back reaming might
have been taken past the foot and into the lower end of the RH section.
In which case you might legitimately see a very small reduction in
diameter when you insert the callipers fully. But if it's a large
reduction, it's bore compression.
Getting it corked
You might decide, on the
evidence above, that it's time to get your pride and joy corked.
In theory, that's a straightforward operation. Any woodwind
repairer who handles clarinets (and that's probably all of them) is
accustomed to replacing cork. But there are some significant
differences between old wooden flutes and clarinets. Clarinets
were made with cork in mind, so the trough depth is ready-to-go.
Clarinets have smaller bores than the top end of flutes, but similar
outside diameters, so the wall thickness is much greater, lending much
greater strength. Old wooden flutes were usually designed with
thread in mind, so the trough depth could be anywhere. And if bore
compression has set in, it may well be deeper in the middle than at the
ends. Putting cork on tenons made for thread is likely to put too
much force on the thin socket walls, unless the tenon and/or socket
dimensions are adjusted to accept the cork safely. That's a job
for a wooden flute specialist, although once done, any woodwind repairer
should be able to replace worn or damaged cork in future. An
alternative process is to put on the cork, then sand it to fit the
socket nicely.
It would be a shame to
compound your tenon compression issue with a split socket issue!
So, ask around and find someone specialising in wooden flutes and cork.
Or at least alert your woodwind repairer to the issue and ask them how
they intend to deal with it.
Future work
Perhaps the most surprising
discovery was that the free ends of the test tenon actually swelled in size,
probably explaining why the three tenons on the strangled cocuswood
flute jam on entry to their sockets.
This means that we may be misinterpreting what
the bores of old flutes originally looked like, and a more realistic test might be
warranted to help guide our interpretations. This
test would still take some time, because the thicker tenon attached to a
real flute body would have much longer reaction time, but at least such
a test would be achievable. Even so, it would still have to employ
artificial aging techniques, or the results may not be available within a
useful timeframe!
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