Effects of Thread Wrapping series:

A Survey

Introduction

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 had dramatically damaged the tenons, causing significant reduction in bore diameter. Indeed, so significant, that 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 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.

Lapping options - Cork vs Thread

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 came much later - it seems to have made it to England in the early 19th century, but only deployed on distinctly up-market flutes.

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 surely 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.

Rockstro on the topic

Now, it would be useful to get an historical view on lapping materials. And if you want an opinion on something, why not turn to someone known to be opinionated? Rockstro, of course! He tells us...

The members of the flute are united by what is termed a pinand-socket-joint. The pin, A, fig. 17, was originally bound with waxed thread, technically called "lapping," to cause it to fit tightly into the socket, B. The old-fashioned ivory mount is shown at C.

The extra thickness at the socket is for the sake of strength. This bulbous excrescence has an inelegant appearance, and is not really necessary. It was discarded in England for many years, but it has lately been re-introduced, and it is now once more the fashion.

The head-joint generally carries the socket, the pin of the joint below therefore enters in an upward direction, the other joint, or joints, being reversed : sometimes all are made to point downwards, with the idea that the condensed breath may thus be prevented from running into the socket, but if the joint be good it will be water-tight as well as air-tight, and if bad it may as well leak in one direction as another. Thread lapping is still in use on low-priced flutes, but it fulfils its purpose very imperfectly, and the thread requires frequent renewal.

Ouch! Low priced flutes, eh? But Rockstro's right. As the new cylinder bored flutes gained supremacy, our much loved conical 8-key flutes became the student flute of the day. They cost about one fifth of a Boehm flute by the same maker. The Boehm (or similar) flute got the cork, the 8-key flute got the thread.

But Rockstro might have missed the boat on why the thread needed frequent renewal. He probably assumed wear, but, as we'll see as our story unfolds, it requires frequent renewal (or at least augmentation) because the wooden tenon below is shrinking away from below it. That's useful to be aware of, as it implies the problem was making itself felt even by Rockstro's time (he was playing and teaching 8-key flutes around the middle of the 19th century).

A short survey

I went looking among flutes and flute data 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 milder 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.
A flute in ebony, by William Henry Potter, in lime green. Like father, like son - well strangled, with distortion running back to 45mm.
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 (only noticeable from 40mm onwards because of the strangled tenon!), 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 finished. 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 virtually no compression 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.

I say virtually no compression, but try this trick. Hold a sheet of paper to your screen so that the bottom edge of it meets the Geo Rudall Willis Fecit pea-green trace at both sides of the graph. You'll detect a tiny amount of compression, about 0.1mm, at around X=15mm. But we can comfortably live with that - the strangled boxwood flute is 15 times worse!

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 12 thread-lapped flutes. Of these, and rounding the numbers:

8% are undamaged
25% are seriously strangled
66% are strangled
83% are at or beyond the cusp of strangulation, and
92% 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.

How collapse manifests

The image to the right is of a well strangled top tenon, from the strangled boxwood flute mentioned at the top. You'll see even from the outside that the worst of the reduction in diameter occurs in the first 15mm, the shoulder at the tip is visibly smaller than the other shoulder, and the shoulders are noticeably tapered toward the thread trough.

The strangled cocuswood flute presented a little differently. The bore trough was a more pronounced hourglass shape, but there was 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.

If we look again at the survey graph (replicated here for your convenience), we can see a wide variety in the shape of the damage. The main compression always takes place within the thread-wrapped area (generally from about X= 5mm to X= 25mm), clearly implicating the thread. But the degree of compression at the opening (X=0) varies a lot, as does the distance down the tube that the damage continues. Some are U-shaped, which is what I'd expect, but some are V-shaped. I imagine there are a lot of factors involved here, such as wood type, wall thickness, thread band depth, existence or not of shoulders forming a thread trough, and so on, depth and perhaps even regularity of thread combing. This is going to make it hard for makers and restorers to work out what the dimensions should have been!

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. And open at one end, but more well supported at the other by the thicker body wall. So, complex and asymmetrical. 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? CAT scan, maybe?

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).

Conclusions so far

I must say I was surprised to find so much evidence of thread wrap-induced distortion, and to find some of it so pronounced. Working with old flutes, one is accustomed to finding bores that seem a bit odd, but it's only when we get a bunch of them together in one place does the pattern emerge so forcibly. It's probably worth pointing out that, while the topic of bore compression has been mentioned from time-to-time, nobody to my knowledge has previously delved into it and presented their findings.