Resin infusion

World/Folk Flutes & Winds
1

It occurs to me that most of the gain from resin infusion would come from just increasing the density of one or two millimetres depth of wood outwards from the bore, so I wondered if the expense of infusing entire billets is the best approach. In carbonfibre manufacturing, there’s a technique called vacuum bagging where epoxy is infused through the material by a vacuum which is held by a plastic sheet rather than a strong vacuum chamber, and it occurred to me that the same idea might be adapted to wood. Suppose you have a piece of wood bored out almost all the way to the far end and also reduced to the required external dimensions for a flute, it would be easy to fill it with the resin that you want to infuse through it - the only hole into it would be at one end, so it’s just like a very long wooden cup. You could then attach the plastic to cover the exterior part, sealing it only round the very top. Then you apply the vacuum and suck air out of almost the entire external surface of the flute, while the resin is at atmospheric pressure inside and can be forced into the wood. So long as this is able to push the resin one or two millimetres into the wood, that would be enough to do the most essential job of ensuring that when the flute is played the air molecules bounce elastically off the walls of the bore without the big energy losses that kill the tone if the wood is less dense. Has anyone experimented with this approach?

(Edit 1: The air molecules will always bounce elastically, of course, but the energy losses must result from the material of the bore wall moving when an excess of molecules hit it at the same time in a pulse from a sound wave, and if the wall’s moving away from them, they’ll bounce off it with less energy than they brought in. Think about what happens in a swimming pool when you push off the end wall: practically all the energy is converted into your movement away from the wall while the wall doesn’t move. If you push off a floating log instead with a similar mass to your own, only half the movement energy will end up with you while the other half is given to the log. This is what is happening in a flute with a soft bore wall, so to fix it, you need to make it immobile. Density shouldn’t be the critical thing, but rigidity. A lot of rigidity comes from having higher density, of course, but I have a bamboo quena with good tone which isn’t greatly dense, so it must be achieving that trick by other means.)

It’s possible that the low pressure would crush the wood a bit, so it may be necessary to keep the diameter wider than the required end result. The amount of resin used could also be kept lower by putting a metal rod into the bore to take up most of the volume, while you would watch the height of the resin reservoir in the bore and top it up from time to time as it falls, while keeping a count of how much you add, as that’s a measure of how much has infused into the wood, so this would tell you when to stop. Because the resin would flow best lengthways through the wood, you might need to seal off the bottom end to prevent all the resin being sucked out through there instead of going deeper into the wood sideways. I’m using birch which is said to be particularly good for resin infusion, and its dust is safer than most woods, plus I can get sustainable supplies easily, so I’d like to keep working with it, but I can’t afford to spend a lot of money on experiments that might fail, so I’m hoping someone’s already tried this and can recommend it or rule it out. Even if no one has tried this specific approach, I’d appreciate any advice on which resin or other kind of juice might be expected to perform best and produce the best end result.

Edit 2: Failing to increase the density at the outer surface of the flute will lead to it continuing to dent easily, so there would be an advantage in infusing the wood all the way through, though that would also make the whole flute heavier, so perhaps it would be best just to focus on hardening the surface layers both inside and out. A vacuum may be overkill for this too: just filling the tube with the resin you want to infuse through it and leaving it like that for a long time might provide plenty of penetration without needing to force it.

Incidentally, I’ve been reading through all the old threads I can find that mention epoxy, and while looking into marine epoxy I found a thing called craft epoxy which may do the same job of sealing a flute while being food safe. https://www.amazon.co.uk/dp/B08SHFYB82/?coliid=I2G2LT6W892DLL&colid=2UFU44BZJRP5&psc=1&ref_=lv_ov_lig_dp_it - a final layer of this could remove all the toxicity worries over the safety of the final product, but it might also be possible to make an entire flute out of the stuff and end up playing an instrument that looks as if it’s made of glass. You can add colour into the resin too, and there are even glow-in-the-dark colours that you can mix through the epoxy.

2

Are you familiar with the WEST SYSTEM?

3

I haven’t used it, but I know a lot of people with boats who mention that one before any other. (I’ve been heavily involved with boats too, but have never needed anything beyond Araldite for repairs.)

I’ve actually just used yacht varnish on the bores of two of my prototype birch flutes and it transformed their tone, making them highly responsive and better than my best bought quena, and it hasn’t dried yet from yesterday’s final coat. I think I’ll go on using yacht varnish for this in future too, letting that soak in before putting a layer of craft resin over the top to have a food safe layer sealing everything in while likely also adding greater rigidity to the surface, hopefully with a further small gain in the tone. I don’t want anyone to be poisoned by the instruments I make, which is why the craft resin looks like the right road to go down. (Incidentally, I used varnish from a half used tin of International Yacht Varnish that was left over from a job coating boomerangs over forty years ago, and it still works as if new: it’s brilliant stuff.)

4

I would say that the primary reason for resin infusion isn’t about the inner bore surface. I’ve been doing resin-infused wooden flutes of various kinds for the last seven years, so I’ve been able to track the various pros and cons. Certainly, anything that helps to create a hard, smooth inner bore surface is beneficial, but this can be done simply by applying multiple coats of marine epoxy thinned with alcohol. It will soak in a good millimeter or more, then you polish, re-apply, repeat. You can get a very satisfactory bore surface this way.

The argument behind a complete resin-infusion is to stabilize the entire billet of wood to prevent shrinkage and (secondarily) to create density. If I’m going to make a flute that has joints and/or a tuning slide, my concern is related to having the wood retain some degree of mobility that might later result in cracking, a loose joint, etc.. But vacuum resin stabilizing is ideally done to the stock before you make it into anything, because if you use a resin that requires a heat cure you can’t apply it to anything that is pre-shaped. It will distort during the heat cure.

I’ve never tried any sort of vacuum application of resins or finished that don’t require the heat cure, however. Not sure what is possible there.

5

Thanks for sharing all your knowledge so extensively through this forum, and on your own site - it’s helped set me in the right direction without making a lot of costly mistakes. I was convinced that birch wasn’t good enough for the job, but your work persuaded me that anything can support a bore coating which essentially becomes the real material of the instrument while the wood beyond it is primarily there to hold it and provide an attractive interface to the player.

The argument behind a complete resin-infusion is to stabilize the entire billet of wood to prevent shrinkage and (secondarily) to create density. If I’m going to make a flute that has joints and/or a tuning slide, my concern is related to having the wood retain some degree of mobility that might later result in cracking, a loose joint, etc..

That’s a part of it I failed to pick up on before: I’m so focused on making one specific type of single piece flute that I wasn’t paying attention to what’s needed for joints. If I was to make a two piece quena to make it tunable, that would then bring in all that extra complication. As you’ve pointed out before, increasing the density of the wood all the way through may also transfer the vibrations better to the player, making it feel as if it’s more responsive, so there are still good reasons for going to all that trouble even for single piece flutes.

I’ve never tried any sort of vacuum application of resins or finished that don’t require the heat cure, however. Not sure what is possible there.

I no longer think it’s necessary for me to use a vacuum, but it was worth floating the idea. I now suspect it will be sufficient just to fill the tube with yacht varnish, seal it, leave it for a week or two, then open it and tip the content into the next flute tube, topping it up with some new varnish from the tin each time. The right kinds of wood should soak it up well enough. I don’t know if the same would work with epoxy as it would solidify rapidly even without the presence of air, giving it less time to soak in deep. A final layer of epoxy is likely crucial though to lock in the smell of the varnish which could last for months otherwise, and it probably also has a slightly more rigid surface. (The yacht varnish already gives it a surface that looks like glass and which acts like a mirror when you shine a torch through the instrument, so it’s a similar end result.)

6

With one-piece flutes things are much easier, for sure. You can use all sorts of things to harden the bore. I’ve mentioned it elsewhere on the forum, but one of my methods for some of the woods is to re-bore the stock between applications of finish. Because I use an epoxy, it cures pretty quickly at a warm temperature (I have a curing cabinet in my shop that stays about 80 degrees or so when in operation). This will cure the epoxy hard enough to machine within 24-36 hours. So I’ll bore out my stock, coat the bore, cure it, re-bore it and then reapply the epoxy. If the wood had bored particularly roughly (lots of micro-tears) I might even do it again, but that’s usually overkill. After re-boring, I finish the flute then coat the bore a final time (sometimes twice).

However, the down side to using a warm environment to cure the epoxy is that the warm temp can encourage wood to settle more quickly. So if there is any tension in the wood, it comes out and the billet (especially if it is longer) will bend a bit. This is a big problem when you go to turn the flute, because now you have asymmetry in the walls of the flute. Because of this, I only do the re-boring on woods that I know will remain stable during the epoxy cure. With woods that are a bit more squirrel-y I don’t take this approach. I actually treat the bore after I’ve shaped the flute, and instead of re-boring I sand and polish the bore and reapply the finish. Once I know that the walls are uniformly thick, I’m not too worried about a moderate amount of warping after the fact. I make a lot of long, thin flutes of various kinds and they all warp in degree, even though it is rarely visible at a glance.

But the point is that there are lots of effective ways to reach the same end. Just putting thing coats of finish in the bore, sanding and polishing in between is what I do 95% of the time. You’ll end up with a wonderfully smooth and evenly hard surface.

7

Can you actually hear the difference between lots of thin coats and one thick one? I find that a thick coat automatically forms a smooth surface without having to do a lot of work: I rotate the flute from time to time while it’s drying so as to prevent it being thicker at one side and thinner at the top. I’ve been applying it using a knitting needle and lots of circular movements round the bore rather than a brush (I don’t have any brush that would work in there) and rely on it not being thin so that all the marks disappear.

I’ve just varnished the bore of my original prototype which was made with a much lighter wood and which struggled to produce notes at all. It’ll be interesting to see if I can transform its tone in the same way as the much more dense birch ones and how close it can get to matching them. I’ll find out tomorrow when it’s dry enough to blow. I read that you did experiments with wood with a density not far off balsa wood for people who struggled with holding the weight of normal flutes. How close do you reckon those were in tone to equivalent flutes made of dense wood? I ask this because I’m now wondering if it might make sense to work with less dense woods in order to cut the amount of wear and tear on the drill and bit and speed up the boring. Of course, the ability to carve holes neatly will also be a limiting factor which may make such a downgrade in materials impractical other than as special exceptions for people who need that minimal weight.

[Note: I’ve just edited an earlier post to name the yacht varnish correctly: it’s “International Yacht Varnish”, which today appears to be called boat varnish if it mentions any marine aspect at all: while the bigger tins just call it “International Original”. I’m hoping that’s the same stuff as I’m running out of it.]

8

My experience so far is that thickness isn’t really an advantage beyond a certain point. So long as any micro-fibers have been removed, and anything that is porous has been sealed, then it does not take much of a thickness to create a responsive flute. I don’t know how thick the coating of yacht varnish ends up being, and in fairness I’ve never attempted and actual measurement on any of my flutes. It wouldn’t be a very meaningful exercise in most cases because the bore of a freshly made flute is going to shrink to some degree almost immediately, depending upon the wood and how dry and stable it is, etc.. On softer woods, I tend to use at least 3 coats, sometimes more.

If you are making any type of flute that has a specific bore profile (conical bore flutes for example) you don’t want to be too heavy-handed with any coating because a really thick one can really change the interior profile of the bore. So I usually do the re-reaming trick mentioned earlier if I’m making a one-piece conical bore flute from something other than stabilized wood. My shakuhachi flutes are often made this way.

I’ve made some flutes from paulownia wood, which is unbelievably light. About 30% of the density of something like maple, and less than 20% of the density of something like rosewood. However, that is a wood that does not machine well at all–frankly it’s a bit of a nightmare. Tears like crazy. Not too bad with a straight gun drill, but if you use a reamer on it the result is very rough indeed. When I use it for my shakuhachi flutes, it gets re-reamed at least twice, with epoxy treatments in between, just to get the bore to a state where it is like the hardwoods. However, once I’ve smoothed the bore, the resulting flute is highly resonant and projects quite well. Certainly as well as the denser woods. It’s just one more example of how the inner bore surface seems to be far more critical than the overall density of the material. You couldn’t find a more extreme example of this than paulownia wood.

Softwoods are certainly easier on tools, but they don’t machine cleanly and they create a bit more labor in terms of cleaning them up and sealing the grain, etc..

9

Thanks for resolving more questions for me: I now see that you’re working against extra complications which I don’t have to deal with as my sole objective is to make cylindrical bore quenas where I can get away with simple methods. I’m doing this primarily to put the finger holes closer together in two clusters which remove all the stretch issues, thereby allowing me to play faster and more accurately without repetitive strain injury kicking in. I can’t buy what I need, so I have to make it myself, and it’s a relief that I’ve been able to get the tone I out of these that I hoped might be possible, and with good notes in the lower part of the third octave too. Without your ideas, I doubt I’d have got there. By luck I’ve also started with what may be the most suitable local wood, although with the cosmetic disadvantage that it looks as if I’m making them out of broom handles, though I’ve just bought a new batch and two of them have magical patterns on them. The lightest rod of the five is 4/5 the weight of the heaviest, so it’ll be interesting to compare how they perform: I suspect the lighter one will absorb more varnish and may end up being denser where it most matters.

10

Update:-

I’ve now made a quena out craft resin (epoxy) and the sound quality is only marginally better than using birch with two coats of yacht varnish soaked into the bore. The density of craft resin is a bit more than that of water, putting it on a par with tropical hardwood. (Wood may handle condensation better by absorbing it for a while, but if that was an advantage it would outperform gold flutes, so there may not be any gain there to be made.) The craft resin quena feels like a cheap plastic pipe, though it’s fairly rigid, having just enough give in it to prevent it from breaking bones in a fight, though it could still be useful for defence. I made it inside a paper tube, taking four goes to make a square bore (which produced bad sound, as expected) and then turning it into an octagonal bore which sounds fine. There may still be room for improvement by making it closer to circular, but it’s getting into diminishing returns. As for the visual impact of it, parts of it look like glass (after sanding off the original paper tube and coating with an extra layer of resin), but most of it is cloudy as I kept working on it without giving it time to cure fully. It fills up with condensation within a few seconds of playing anyway and doesn’t look attractive in that state, so full transparency probably isn’t the right route to go down. The big gain over using delrin though is that there’s no waste from drilling out a bore, oh, and it’s also a third the price.

I’m now starting the next experiment of building a paper maché quena using craft resin as the glue, starting with a tube the width of the bore and then working outwards. I don’t expect a radical improvement in tone from this, but it should be a bit more dense, a little more rigid, and the bore will certainly be better shaped, so there ought to be a small gain.

11

Just out of curiosity, what prompted you to take this unusual route to making a quena? As opposed to simply drilling some wood or using a piece of PVC?

12

With craft resin there are possibilities with transparent colours, and also using layers of resin with a mixture of transparent, translucent and opaque colours at different depths, so there are opportunities to take things in new artistic directions while also trying to make professional quality instruments. The low-tech drilling was fun as a challenge, but it’s noisy and dusty and there’s a limit to how many instruments I’d want to make that way, so I’m just exploring other options and testing materials. I have some ideas which I think can be realised. Instead of putting a few diamonds on a flute, the whole flute can become the jewel.