Brass & ebonite versus acid (and hot water)

In this article I take a close look at what acids we can use for cleaning, and why mouthpieces go green

I had an email a little while back from a chap who'd read my articles on cleaning mouthpieces and crooks, and wanted to give it a go on his own gear. Trouble was, he couldn't abide the smell of vinegar - "Makes me sick to my stomach, just can't bear to be around the stuff". Could I suggest any alternatives? Well, yes...there's always formic acid, which is commonly used for descaling kettles - but it smells terrible, so I didn't think it'd be any less offensive to him than vinegar. There are also plenty of much stronger acids that don't niff quite as much, but these are risky to use...both to the gear and the user.
A much more sensible suggestion would be citric acid. If it has any smell at all it's so faint as to be of little offence to anyone - and having spent quite some time splashing it all over the workshop I think I can safely say it's odourless. It's also (reasonably) readily available, it's cheap and it's quite safe to use...as long as you don't rub it into your eyes, of course.

There's a problem though. Vinegar is typically sold at a strength of 5 or 6%, so it's a known quantity. I can base my advice on how it acts over a set period of time at a given temperature and be confident that you'll achieve the same results as I do. But citric acid is usually sold in crystalline form. To make an acid solution you have to mix a quantity of it with water. But how much? And how does it compare to vinegar?
There's only one way to find out - and that's to run some tests.

The first thing I needed was some citric acid, which is where I ran into a slight problem. I trotted down to my local chemist (pharmacy) and asked the assistant behind the counter for a packet of citric acid. I got a rather stern look, and then she asked what I wanted it for. I said it was for cleaning, and was told that they could only get it in on special order. However, on nipping round to the local hardware store to pick up some other bits and bobs, I spied a box of the stuff right alongside all the other far more dangerous things you might use for household cleaning. So I bought some...and wasn't given the old third degree at the checkout either. I later discovered that it can have some nefarious uses (drugs/bombs), hence you may be quizzed about your wanting to buy it.

Before we get started I guess I should do a bit of health and safety and advise anyone who's going to use this stuff that proper goggles and gloves should be worn. Personally I didn't bother with the gloves - and had no issues at all...though you might be wise to consider them if you have any cuts on your hands. It can sting a bit. Goggles are a wise move, because if you splash any in your eye "it can sting a bit" doesn't even begin to cover it. 'Nuff said.

I did two rounds of testing: one with tarnished brass strips and one with a selection of old ebonite mouthpieces - and we'll kick off with the brass.

I made up four pots of acid solution of 110ml each (about half a cup in kitchen measures).
Pot 1 - malt vinegar (pickling) at 6% strength
Pot 2 - water mixed with half a teaspoon of citric acid powder
Pot 3 - water and 1 heaped teaspoon of citric acid
Pot 4 - water with 2 heaped teaspoons of citric acid
All solutions were heated to 50 degrees, which is the top end of hand hot. Going hotter speeds up the process and going colder, as you might have guessed, slows it down. 50 degrees is a sensible bet, and avoids loud screams in the event of any spills.

I cut four pieces out of a strip of tarnished brass and degreased them to remove any substances that might have acted as a masking agent. With the pots full of acid the brass strips were popped into place, leaving a short length of brass above the solution to act as a colour comparator. The effects were almost immediate; in each sample the brass had begun to change colour within the first minute.
I noticed no fizzing or bubbling - in fact absolutely nothing at all.

After five minutes the strips were removed, rinsed with clean water, dried and photographed - and here are the results.
We can see a progression between sample 2 and 4, there's a distinct colour shift from yellow to red which appears to be due to the strength of the acid.
The vinegar sample (1) looks to me to sit between 3 and 4 at present. It's definitely a darker hue than 2 and has some spots that match those on sample 4.
If you're wondering why the top of sample 2 looks so yellow it's because I fumbled the extraction and it submerged into the acid for as long as it took me to get my fingers in there, lift it out and rinse it. I may also have spent a couple of seconds cursing - and a further couple more when the acid got into a cut on my forefinger (shoulda worn gloves). I'm not sure too much can be read into the coloration at this stage because some of the original tarnish still remains, and that's likely to skew the results.

After thirty minutes things have settled down a bit.
The submerged surface are more even in colour and the hues are likely to be a more accurate representation of what's going on. 4 in particular has evened up considerably, and 1 is looking much more uniform.
It still looks like 1 sits between 3 and 4 though, but 2 seems to have got a little lighter, if anything.

If you were cleaning out a crook, half an hour with warm/hot vinegar is about all you'd need - and going by these results I'd say that mix 3 is the most likely contender.

I popped the strips back into the jars for an extended soak and removed them after a total of 150 minutes' immersion.

This is considerably longer than you'd need to clean out even the mankiest crook, but it's at least worth seeing what prolonged immersion in these acids will do to brass. In fact it's not a lot really.
Sample 1 has consolidated - the surface is much more uniform than before. You might note I've flipped the sample over, but it makes no difference...both sides are evenly matched.
Sample 3 is the outlier in this group - if anything it's got slightly more yellow, which is a bit of a surprise - but it seems to be the closest match to the performance of the vinegar.
4 is showing a more grainy and reddish appearance. Could it be eating into the brass?
2 seems to have stopped dead in its tracks - it looks hardly any different to the 30 minute sample, and that small blemish on the lower right is still intact. Notice how the sample isn't much different from the top section, which stood above the acid the whole time...save for the few seconds when I knocked the strip into the jar.

From this test I deduce that sample 3 is the best match for vinegar, and that sample 2 is rather too weak. Sample 4 is very effective but I was a bit concerned about the coloration. It could be a sign that the acid is eating in to the zinc part of the alloy. This is going to happen whatever strength of acid you use, but it's wise to keep it to a minimum and it could be that this strength of solution is rather pushing it. Or it could be a false flag - so I decided to do another test.

Here's another strip of brass, and I've selected one that's been subjected to some heat treatment in the past. This has left both sides looking very tarnished indeed, and it's already got some red markings on it. If the acid is aggressively attacking the zinc we should see an evening-out of the coloration at the end of the test.
I took the same test jar (110ml) and loaded it up with four heaped teaspoons of citric acid, then heated it as before - and then left it for two hours, leaving the top of the strip out of the solution.

The strip is clearly lighter in colour than the untreated section but there's no sign of the reddish hue having spread uniformly. In fact all it looks like is that the tarnished areas are simply cleaner than they were before. What this suggests to me is that the coloration post acid dip is entirely dependent on the colour of the brass going in, and that to get past the point of merely cleaning things up a bit, a far stronger acid would be needed. As indeed it would.
But just to make sure I ran one more test.

This time I took strip 4 and buffed up the lower portion of it before dropping it into a fresh solution containing four heaped teaspoons of citric acid. As before, I heated it up and left it to stand for a couple of hours.
The pre-soak example is on the left, post soak on the right - and there's really not a great deal to call between them. The post soak strip is a little lighter in the unpolished section, but the buffed section looks exactly the same (bar some difference in the lighting when the shot was taken). Even under a magnifying glass there's no sign of pitting on the polished section, and were it not for that slight reddening of the middle section you'd be hard put to know that it had been sat in acid for two hours.

But let's not get too carried away. It clear that bog standard vinegar is more than adequate for the job in hand, it's quite safe to use and it'll allow for a margin of error (such as forgetting you've filled your crook up with acid) - so all we really need is a solution of citric acid that's of roughly the same strength.
Let's come back to the proportions then. Scaling up the mixtures to a half-pint quantity I'd suggest (give or take a pinch) one and a half teaspoons per half pint for sample one, two and a half for sample three and five for sample four. But the amount of fluid you'll need to clean out a tenor crook is around a third of the sample size - which very conveniently equates to a single slightly heaped teaspoon of citric acid to the volume of water required to fill the crook. A little more or less either way isn't going to make a significant difference - but you could use a little less for an alto and less again for a baritone. For a soprano, knock it back to a quarter of a teaspoon. If in doubt, measure and mix it up as per jar number three...and pour what's left over down the sink to give it a bit of a clean.
Because it's a weak acid it's unlikely you'll go far wrong with your mixture as long as you're sensible and you limit the length of time inside the crook to half an hour - and as we've seen, even if you significantly overdo it you're unlikely to run into any problems. If the acid does the job in half an hour - great; if not you can fill the crook up again and leave it for a bit longer...or knock up a fresh, slightly stronger batch. But do remember to wash the crook out with clean water afterwards though (the article on cleaning crooks goes into more detail).

By far and away the biggest advantage of using citric acid is that it's doesn't pong like vinegar does...especially when you heat it up. It also means that if any finds its way onto your mouthpiece cork (which it inevitably does), you won't feel like you're gigging in a fish and chip shop for the next couple of months. Chances are, too, that it works out slightly cheaper. My box of citric acid (Dri-Pak brand) cost £1.50 for 250g - and a litre of 6% pickling vinegar comes in at around the same price. I could work out the number of crooks you could clean using each method, but at £1.50 a pop it hardly seems worth the effort.

So much for the brass then, how about ebonite?
Any acid is going to attack the calcium carbonate build-up that appears in mouthpieces over a period of time - so I'm not really all that interested in testing the performance of citric acid in this respect. I am, however, curious as to its effect on ebonite. In my article on cleaning mouthpieces I proposed that filling the bore with cotton wool and then dripping vinegar onto it was a good way of minimising its contact with the ebonite while focussing on the gunge inside the bore. This advice holds true for any acid.
But if I'm going to recommend it as a de-gunking agent, it's just as well to test how it might react in the event of a spillage.

The most likely action of an acid on ebonite is to blemish the surface. This is known as 'greening' - though in fact it appears to look like more of a bleaching/lightening action. Not that it's hard to encourage ebonite to go green - which is something it's likely to do of its own accord over a period of time. By far the most common cause of it is exposure to heat, light and moisture...which pretty much sums up any gig, right?
With this in mind I broke the tests down into two sections. The first was to test the action of cold and lukewarm acids on a selection of samples, then to test a sample with plain water at various temperatures. I dug out a handful of crusty old mouthpieces that had long since gone green.

Why not use shiny new mouthpieces? Because the process of greening takes some time - the older the piece gets, the more susceptible it becomes - and because of the time it takes for calcium carbonate deposits to build up inside a mouthpiece it's more likely that older pieces are going to be subjected to such a vigorous cleaning process. However, for the hot water test I used a much more modern piece. As you can see, these pieces are pretty green already - but just to make it interesting I cleaned and polished all the beaks. This will allow us to see what effect the tests will have on a piece that's been restored. I sanded the beaks with 600 grit paper, then gave them a very light buff (so as not to heat the ebonite) and then degreased them to remove any remaining buffing soap.

Here are the pieces in their test solutions. I placed each piece sideways on and added the fluid until it came halfway up the mouthpiece, so that any changes to the ebonite could be compared to an untreated portion of each mouthpiece.

The tests are as follows:
Sample 1 - plain water at 37 degrees (lukewarm), marked Berg Larsen
Sample 2 - cold vinegar at 6% strength, unmarked
Sample 3 - lukewarm vinegar at 6% strength, marked Louis & Co
Sample 4 - cold water and 1/2 a teaspoon of citric acid, unmarked
Sample 5 - lukewarm water and 1/2 a teaspoon of citric acid, marked 6
The strength of the citric acid solutions equates to that of pot 3 in the brass test (1 teaspoon per 110ml).

The pieces were left to stand for thirty minutes. This is plenty long enough to remove or at least loosen any calcium carbonate deposits - assuming you were prepared to take the risk of soaking most of the piece in acid rather than just treating the bore. They were then removed and rinsed with clean cold water and air dried before examination.

And here are the results. Some of them have very visible marks from the soaking, some are more subtle - so I'll go through them in detail.

1 showed some very faint greening to the beak, but it was patchy and barely discernible in place. There's slightly more visible greening to the unpolished area and a distinct boundary mark between the two halves.
2 had a slightly more visible band on the beak (but only just), with very visible greening to the unpolished area.
3 had no visible band on the beak, which was rather unexpected - but had very considerable greening on the unpolished section.
4 had virtually no visible effects at all (it's the other way up 'cos I put it in the mix the wrong way round).
5 showed some very slight and patchy greening on the beak and a more distinct band on the body.

Sample 3 is by far the most visibly affected; 2 and 5 are evenly matched but show less effect than sample 3 - and 1 and 4 are the least affected and are evenly matched with each other.
I was very surprised by the effect on sample 1 - even just a soak in plain lukewarm water brought about some greening, which I really didn't expect to see. I wondered if I'd contaminated the water in some way, but a taste test revealed nothing untoward.
What was even more surprising is that this piece is the newest of the group, and the one I'd have expected to have been the most resistant to greening.

I also noticed a couple of small round blobs of greening on the edge of 4's beak, which seemed a little odd and completely askance to all the other samples. I suspect that a couple of grains of undissolved citric acid might have made their way through the mixing process, and came to rest beside the beak. This would have caused a temporary and localised increase in acidity...hence the blobs.
I tested the theory by placing a grain of citric acid on another piece and popping a drop of water on it. It only took a couple of minutes for a blob of greening to appear, so it seems very likely that this was the reason for the anomaly.

I noticed that the discoloration was a lot more visible when the mouthpieces were wet, but it proved to be rather hard to get the lighting right and photograph them before the water ran off the surface - so a tried a coat of almond oil. It worked a treat, as you can see - it really made the banding pop right out on these two samples (5 and 1).
Incidentally, coating an ebonite piece with oil (typically olive oil) is said to be a way of restoring a greened piece back to black. In fact all it does is change the contrast temporarily...and make the piece all oily. It's a complete waste of time (and we'll come back to this later).

Because of the unexpected variations in the results I decided to carry out some further tests. It seemed to me that there wasn't really much consistency in the effects, and that this might be due to variations in the composition of the ebonite.
Piece 3 seemed to be the most affected by the warm vinegar, so I tried the other half of it in a solution of warm citric acid. I gave it half an hour and then rinsed and dried it as before.

The arrow marks the darker band that was left untreated by either the vinegar or the citric acid.
To the right is the side treated with vinegar, the left has been treated with citric acid. I concluded that this piece was made of an ebonite that was especially prone to greening, and that the type of acid (of similar strengths) made little or no difference. However, there's now some slight greening on the edge of the beak, which didn't happen when the piece was soaked in vinegar. Because the greening on the beak isn't completely over the area that was submerged in the citric acid I think it's fair to assume that this is just an area that was a little less well polished than the rest of the beak.
You'll have no doubt noticed the very clear mark of where the ligature sat, and this perhaps demonstrates how keeping the light off the ebonite helps to preserve its colour. It could also be the case that the ligature protected the ebonite from moisture and sweat from the hands - and these too are likely to be a contributory factor to greening.

I also retested sample 4.
It had shown few effects the first time around, so I repeated the test with a shallow bath of twice the strength of citric acid solution. I gave it the usual half hour followed by a rinse and an air dry.
You can clearly see a narrow band of greening along right right hand side of the beak. It's clear that this piece has a greater resistance to greening because it took a much stronger acid solution to discolour it. What's interesting about this result is that this piece is identical to 2 apart from the bore size (2 is slightly smaller). There's no makers' name on either piece, but they're clearly from the same source...and yet exhibit different reactions to acid.

My next test was a rather unusual one.
It's clear to me that once the greening process has started, the ebonite becomes ever-more susceptible to it. This is due to a chemical change at the surface of the ebonite, which means that if you remove this top layer you can restore a piece to its former glory. Or so we're told.
The tests above seem to bear this out, because the polished beaks seemed to suffer from significantly less greening than the unpolished areas - but how would these pieces stand up to normal use now? I didn't really want to go to all the hassle of rigging up each piece and playtesting it - so I simply gave them all a long, slow lick.
This produced some very surprising results.
All bar piece 3 tasted slightly acidic and bitter. This is pretty common, and is a result of the breakdown of the surface of the ebonite. However, all the beaks had been sanded down and polished...and still I got the acrid taste. This shows that the chemical change runs rather deeper than the immediate surface.
Pieces 4 and 2 showed immediate and impressive signs of greening. In fact it happened right before my eyes. Piece 5 was less affected, piece 1 even less so - and piece 3 showed no effects at all. However, I noticed that where greening had occurred it was patchy...which got me wondering.
So I took some more sandpaper to the beak, this time running over it with some 240 grit to really get beneath the surface layer before finishing it up with 600 grit and a polish.
I then repeated the lick test...and absolutely nothing happened. The beak stayed black.
So I popped the beak into a solution of warm vinegar for half an hour to see what would happen, and nothing happened. And still nothing happened when I gave it the lick test.

I can draw some conclusions from this experiment, but before I do there's just time for another test.
Here's an old Selmer D alto piece. Apart from the part of the beak which had been covered by a mouthpeice patch it started off all the same colour as the centre section; which is not quite black but is beginning to 'bloom' a shade of dark brown, which indicates that the greening process has started. For this test I began by attacking the shank of the piece. I first immersed it halfway along its shank in water heated to 85 degrees for 15 seconds. I then immersed it up to the body section in 60 degree water for 15 seconds - and then up to the middle of the Selmer logo at 50 degrees for 30 seconds. Finally, I poured boiling water over the beak.

You can clearly see two distinct rings on the shank. The lighter one nearest the end, a slightly darker one up to the body...and then no real change until you get to the beak. From this test I can say that water up to 50 degrees is not likely to have much of an effect - at least over a short period of time (remember what happened to the Berg Larsen in the soak test?). Get much above that and things are going to happen.
It's also clear that pouring boiling water over a mouthpiece is likely to be a very bad idea - and in this case it resulted in instant greening...apart from the area that had previously been protected by a mouthpiece patch.

And just to double-check my results I gave poor old sample 4 another round of abuse - starting with a very hot soak halfway up the beak, followed by a good slooshing with boiling water. You can see that the sides have taken a beating, but the surface of the beak is holding up really well. I can just see some signs of slight blooming here and there, but have to look very hard to find them.

So what have we discovered from all this malarkey?
It's clear that ebonite greening is a surface phenomenon, but that it extends rather deeper than just the surface. Rather than being 'skin deep', it might help to think of it more as a crust. It has a little depth to it.
There seems to be a sort of 'break point'. If the ebonite's fresh and black it'll have some resistance to blooming for a while...but once it gets started (even slightly), getting acid or hot water on the piece is going to accelerate the greening - sometimes rather rapidly - and even extended soaking in something as innocuous as lukewarm water may have an effect. When seeking to dissolve calcium carbonate build-up inside the mouthpiece it's important to confine the treatment to the bore in order to avoid discoloration of the body.
However, it is possible to restore a greened piece back to black - but it requires some reasonably aggressive removal of material down to the subsurface level, after which point the ebonite displays enhanced resistance to acid and heat attack. At least for a while. Greening is inevitable over time, but there are a few things you can do to slow it down - and certainly a lot you can do to avoid making it worse. It also perhaps explains why some players are adamant that soaking their ebonite pieces overnight in coke/battery acid/radioactive waste has done no harm. If the ebonite's in good condition with little or no signs of blooming they may well get away it, but I rather suspect that in so doing they've taken one step further towards greening. If it's ebonite, it's going to go green at some point.

So perhaps the most helpful takeaway from all this is that a weak solution of citric acid works every bit as well as vinegar when it comes to cleaning crooks and mouthpieces - and it does so without all the associated smell. It also carries the same risk with regard to its effects on ebonite. A couple of teaspoons per cup of water is going to be just fine - and a little under or over each way isn't going to make a significant difference.

And at the end of this batch of tests I've got some slightly cleaner brass, a very clean sink...and a bunch of green mouthpieces, which are going to require an awful lot of sanding and polishing if I wanted to bring them back to black.
I could smother them in olive oil, but that's next to useless...or maybe there's another way I can jazz them up a little...?

I picked out a couple of very green pieces that looked more or less the same in terms of colour and condition.
I selected the top one for treatment with olive oil and duly applied it to the front section of the piece. I gave it some time to 'soak in' then wiped the remainder off with a tissue before giving the piece a bit of a buff with a clean soft cloth. Here's how it came up.

It hasn't gone anywhere near black, but it's at least considerably darker than it was - and the surface finish seems more even. You can see the difference between the treated section at the front and the bit that remained untouched at the rear.
It's a quick and easy way of tidying up a tired-looking piece, but the results will be very dependent on how green the piece is to start with. There's been no real change to the ebonite - what little oil that's left on it is sitting in all the tiny pores in the material. Once it dries out or gets rubbed away, the green tinge will return. You don't even need to use olive oil - pretty much any oil will do, though as you'll be putting the piece in your mouth it makes sense to stick to edible oils.

For the second piece I'm going to try something rather off the wall.
I said earlier that the only way to bring a piece back to black is to remove the surface layers - but it's said to be possible to blacken a greened piece with a chemical process.
However, it requires the use of some fairly nasty chemicals and can't in any way be considered a DIY solution. It's also not particularly cheap to have done, nor is it a service that's widely available...which may mean that there's some other difficulty associated with it. But as all we really want to do is to make the mouthpiece look black, and for it to stay that way for as long as possible...why not just colour it?
Well, why not? The olive oil method works (after a fashion) because some of it remains in the pores. What if we could replace that oil with a pigment...one that dries out and stay put?

And indeed we can
We can't really dye the mouthpiece - it may well take, but the process would probably involve a not insignificant amount of heat and/or moisture over a sustained period of time. This would do nothing for the underlying finish, and would quite likely soften the ebonite temporarily...at least long enough to bugger up the alignment of the rails and the integrity of the table.
And we can't really paint the piece - 'cos that would look daft. What's needed is something in the middle, and a spirit stain fits the bill perfectly. It's just a pigment dissolved in a solvent; when the solvent evaporates it leaves the pigment behind...and in such small particles that they cling on to the smallest crevice. You could buy some spirit stain ready mixed, or you could buy some pigment and mix it up yourself with an appropriate solvent. Or you could just buy a large black Sharpie permanent marker pen.
Yep, that's all it really is - a spirit stain that's soluble in methylated spirits (also known as denatured spirits in some parts of the world). You can pick one up from most newsagents and stationers for a couple of quid - and most hardware/decorators' stores sell methylated spirits. It's possible that other solvents will work (cigarette lighter fluid doesn't, unfortunately), but meths is safe to use on ebonite pieces.

Start by washing the piece in lukewarm water to get it nice and clean. When it's dry, give it a wipe over with a cloth dampened in meths (it'll dry almost immediately) and then get busy with the Sharpie. If you're a bit uncertain about the prospect of putting something in your gob that's been drawn on with a Sharpie you could simply avoid the parts of the piece that go into your mouth (though it'd look a bit odd) - and you might also want to reflect on all the stuff that's undoubtedly leaching out of that green ebonite anyway...

There's no need to be very precise with the Sharpie - don't worry about getting an even coat on - just make sure you cover all the bits you want to stain. Once that's done, let the piece dry off. It won't take long.
And having covered the piece with stain, we're now going to remove it...or most of it, at least. You'll have put far more than you need on, which means it'll probably stain your lips and come off on your hands...so we need to 'cut it back' a bit. Simply wet a cloth or a tissue with meths and rub it all over the mouthpiece. You'll see that a great deal of the stain is removed. Don't be surprised if it looks blue or purple - the black pigment is really just a very dark shade of another colour.
Finish off by giving the piece a good buff with a clean soft cloth.

Here's a comparison between the two treated mouthpieces, with the Sharpie example on the bottom. You'll probably be rather disappointed that the meths wipe has removed most of the black - but you have to be realistic about what's possible and what's not.
It's clear from the shot that the final result is a great deal better than pouring a load of old chip fat over your mouthpiece, and the piece looks blacker...rather than merely darker. I don't know about you but I consider it to be a very acceptable result.
There's also nothing to stop you from using any of the other colours these pens are available in. I mean, if your mouthpiece wants to go green why not make it bright green? Or perhaps blue, or red. It's a whole new world of exciting possibilities. Probably. In fact I tested it (you knew I would) and blue seems to be the only other colour that works.
Having just said that I'm now wondering how long it'll be before someone pops up on a forum to claim that tinting their old green Link 6* red has improved their tone...

There's just one final test to carry out on these poor old mouthpieces, and that's to see how the coloration stands up to a bit of wear and tear.
Now, I could pop reeds on them and spend the next six weeks knocking about with a trad jazz band - but, frankly, I can't be arsed...so I'm going to speed up the process with the aid of the dreaded kettle. Yep, I'm gonna pour boiling water over them.
We know from the tests above that this is a sure-fire way to turn ebonite green in an instant, so let's see how these two finishes stand up to a bit of abuse.

And there we have it.
The olive oil treated piece looks much the same as it did before we started. All the oil's been stripped away and the ebonite's gone back to a rotten old green hue.
The Sharpie piece has suffered a bit too, to be fair - it's definitely a little lighter, but still looks rather more black than green. But here's the thing...
The oiled finish gave up the ghost after a mere ten seconds of pouring boiling water over it - but this did virtually nothing to the stained piece. So I kept on pouring. And I kept on pouring until I'd drained the kettle...which was rather a sacrifice as I'd hoped to leave enough water for a very well-deserved cup of tea. If it can take that kind of stick I'm pretty sure it's going to shrug off some handling and a bunch of gigs - and I see no reason why you can't repeat the process as and when you feel like your mouthpiece needs a bit of sprucing up.

Am I going to recommend this as a fix for green mouthpieces? No, I'm not.
It's not been a long-term test by any means - and who knows, maybe a Sharpie-coloured mouthpiece will disintegrate in a couple of months, or your jaw will fall off halfway through Beale Street Blues. It is what it is - a bodge - but one that appears to work rather well. It's entirely up to you whether you try it, and likewise it's completely down to you if all goes wrong.
As for myself...I've got a nice old Vandoren tenor piece that's looking a bit worse for wear and a brand new Sharpie pen. You'll know where to find me...