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I was thinking, we always stress the importance of fit for bridges, or any glue joint, and it's absolutely very important consideration but at the same time when you remove the bridges on some of these old guitars that have been done up with HHG you see these ridiculously deep grooves in the bridge and on the top wood. The idea being that roughing up the surface makes a better joint. And often times you do not see the glue film across the whole bridge as in Steves case. And yet somehow these bridges managed to stay on for decades.

Roughing up the surfaces used to be common practice based on a flawed understanding of glue adhesion. A few scientific studies showed this to be mistaken. So now we know better, right?

The OP's bridge gluing work looks real good to me, and I think that particular wood and that glue just weren't happy together. I've gotten my most consistent (not best, one time; for that it'd be HHG) results with the fish glue by Norland, though others have mentioned issues with it. I hope to try some real Isinglass someday when I've got the bucks, and also the Amanosan 3B hide mentioned in the Maestronet thread, because I have this nagging feeling about my present hide glue.

Interesting Maestronet link … It’s mentioned in that thread and I’ve seen it mentioned elsewhere by violin makers - this (apparently not uncommon) method of prepping a batch of HHG by cooking it, refrigerating it and then heating it again before first use. I’ve never come across a guitar maker who does this.

Guilty But I'm not a huge user.
I prepare in batches (-8oz/batch), cool in an ice cube tray, refrigerate (and then freeze) before use.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Sorry Colin, I should clarify, I do know of guitar makers like yourself who refrigerate / freeze / thaw .. but most cite convenience as the reason for the method - having an ice cube tray full of glue ready to go without delay. I refrigerate unused glue each afternoon for use the next day myself.

What I was getting at was that these violin folks suggest the glue actually performs better once refrigerated and therefore they will not use it until it’s been chilled and reheated at least once. I’ve not encountered a guitarmaker who has cited this as their rationale.

I also refrigerate HHG after use but only for convenience. I make it up in 1 oz bottles and use it several times before I discard it.

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Steve Smith
"Music is what feelings sound like"

I only use HHG that has been reheated once for strength. I read many years ago that the glue molecules get longer after reheating which strengthens the glue. Nice to meet you, Josh.

This is new to me but not a difficult thing to employ.

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Steve Smith
"Music is what feelings sound like"

I sometimes wonder if "toothing" the wood was done, not because they felt a rougher surface held the glue better, but rather that the grooves created a space for the excess glue and allowed for a thinner glue line. In traditional woodworking adding a step to a process isn't usually done unless there is some benefit.

Sorry, I didn't say why I did it, I do that because frets.com mentioned it, for a similar reason as you do it I believe - " if it sits around a day or so, it develops a slightly more complex molecular structure, and becomes stronger." http://www.frets.com/FretsPages/Luthier/Technique/Glue/UseHideGlue/usehideglue2.html

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

I add a bit of salt to my HHG and keep it in small squeeze bottles refrigerated till I need it. When needed I drop the bottle, with a couple SS nuts for weight in it, in the 145deg bath and it's ready in about 15 minutes. The bottles are sealed so they don't loose too much water from evaporating. The salt gives me a tad bit more time for gluing up. So far so good but I have reheated these bottles many times over a 2-3 month period and have no problem.

From what I gather, there are two out of the four general adhesion theories that come into play for wood glue adhesive function - the mechanical model where adhesive locks into key-ways and other surface imperfections on the pieces to be glued together, and the chemical model (aka, adsorption) where lots of weak electrostatic forces - really, van der Waals forces - between the glue and the stuff to be glued keep things together. In the period where Martin scored their bridges, fretboards, etc., the dominant model in craft was mechanical, so more/larger key-ways on ebony seemed desirable...ebony must have appeared to the naked eye or hand loop to need some pores and scoring to have some place for the glue to grip.

As time passed, the craft community has been informed by both experience (gluing well-toothed but weathered wood generally fails despite abundant key-ways) and generally available scientific and engineering knowledge (courtesy of those pesky glue nerds at Franklin) that something else was at work. Thus, having a clean, unburnished wood surface that has high surface energy supports both mechanical and chemical binding, so scoring ebony or other dense, hard, closed-pore wood prior to gluing has largely disappeared outside of reproduction work and those traditionalists that persist with everything the old guys did because...well...the old guys did it.

The proportion of overall joint strength due to mechanical versus chemical bonding is still an active debate, but a plan to maximize both results in that clean, newly-scraped surface free of burnishing or excessive sanding debris seems ideal for adherents of either or both theories.

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For the times they are a changin'

- Bob Dylan

Nice to meet you, Barry! This is very interesting to me. When I was starting out to use HHG I was told that fresh is best for any stressed joint, so I’ve always mixed a fresh batch for bridge gluing and saved the reheated stuff for other jobs. I should reevaluate!

Or both? Chemical as well as mechanical. I do believe the chemical bond is what gives it the most strength but I can also see water based glue under clamping pressure forcing it's way into the wood fibers and making a lock too. And that's what I was getting at with my first post here about torrified wood. There is no soaking into the wood b/c it is so hydrophobic.

Apparently, the mechanical model is more about pores and other keyways, while the chemical model is more the number of chemical binding sites. Ensuring an open, freshly scraped or planed, uncontaminated surface works well because it provides many key-ways on even something like ebony sanded to P80-P180 or so, while also increasing the number of chemical binding sites by increasing the surface energy of adherends.

Certainly, the reluctance of highly torrified wood to absorb excess water (and carry AR/PVA or animal glue with it to lock into surface cellulose fibers) reduces the chemical binding sites available versus recently harvested, untorrified wood, but that may be the most reasonable explanation as to why 315g HHG is so useful in keeping the bridge on. The greater cohesive/internal glue line strength seen as gram strength increases in HHG (the glue's ability to resist internal failure) means those tiny key-ways filled with glue are less likely to fracture under load. That those fractures are avoided suggests that they stay better attached to the parent adhesive film between the adherends, so better resist cohesive failure.

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For the times they are a changin'

- Bob Dylan

Disregarding both the mechanical model and chemical model, empirical data may have shown that in a production setting of less than perfectly made joints, that a toothed surface which allowed the surfaces (which did make contact) to have a thinner glue line to be of benefit.
For hammer veneering a toothed surface appears to be desirable, and I believe it is not so much for increasing the mechanical bond as for giving the excess glue a place to go. Paul Sellers mentions this as one reason for toothing substrates:
https://paulsellers.com/2014/05/the-poo ... ost-video/
As mentioned, "the Old Guys" didn't often add to their workflow unnecessary steps if it didn't make things better.
Building guitars with modern tooling or in a non-commercial setting, toothing bridge bottoms may not be desirable, but "back in the day" it may not have been as foolish as we have come to believe.

Hi Steve

if you place a drop of water on the top at the glue area does it soak in quickly or bead up? Does this piece of wood seem better or worse for water soaking in than you other pieces?

Kevin

Hi Kevin, the torrified spruce does not absorb water well. This is a well known characteristic and probably a significant reason why it is difficult to glue.

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Steve Smith
"Music is what feelings sound like"

Yes I agree. I was wandering among your wood stash was this piece particularly bad.

Not particularly - just seems that gluing a bridge to torrefied spruce requires more care than usual.

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Steve Smith
"Music is what feelings sound like"

I don't know why we would see a thinner glue line for a toothed surface, given that glue line thickness with HHG is a function of the glue staying liquid for long enough to see the joint closed, excess adhesive squeezed out to the joint's edges, and adequate clamping pressure applied. Having done a little hammer veneering, I can see where a combination of a very flexible veneer, rigid substrate, and scoring would give the hide being pushed ahead by the hammer's edge a way to flow out of the joint so as to achieve a thinner glue line. I'm just not sure I see a comparatively inflexible bridge or fretboard behaving - or needing to behave - in the same fashion on a joint with 6 - 40 square inches of area. We can usually close a joint in a fraction of the time that the hammer veneering process takes (at least for those of limited upper body strength ), and can apply far more consistent pressure to the glue line than someone using a veneer hammer. Another example of applications differences between luthiery and allied trades... similar to our crazy fixation with cross-grain construction....works for us, but acts as a self-destruct switch in some types of solid wood cabinetry.

Further, I'm not certain I see how the scoring seen on some vintage bridge and fretboard joints would act as a reservoir for glue being squeezed from the joint, as a) both sides of the joint were commonly wetted with glue before closing, filling the toothed channels before pressure was applied to close the joint, b) the volume of glue applied would be multiple orders of magnitude greater than the volume of the channels and other key-way features (e.g., scraper marks, pores), and c) glue which was not absorbed or trapped in the thinning glue line would have been expelled from the joint so long as the HHG remained above the gel temperature.

Finally, my apologies for any lack of clarity... I meant to suggest that Martin's craftsman were working with what was commonly understood craft lore during the Golden Era and a decade or two after. I did not mean to suggest that they were resistant to new knowledge, techniques, or tools, but rather that the knowledge environment in which craft functioned at that time when many of those vintage instruments were built was a very different place than today.

While there were certainly staff at the factory that shared knowledge on materials and techniques with colleagues and kept up on allied trade and manufacturing developments, the barriers to a line worker or foreman researching emerging adhesive physics knowledge would have been daunting, given local resources, a longer workday, and a 5-1/2 to 6 day work week through much of the country into the early 1940's. Ford and other industrial concerns were at the forefront of reducing the work day to 8 hours from the 1920's onward, but the six day workweek persisted into the 1960's for many lower level salaried workers. Just finding the time and opportunity to visit a library likely to have the appropriate texts and journals available would have been a time-consuming task for those living outside of larger cities or college/university towns.

Today, we can sit on a shop stool at our bench and get a pretty decent understanding of the adhesives science applicable to woodworking glues and emerging knowledge in the field in a few hours of time on a Sunday evening. This does not make us smarter or wiser than previous generations of craftsman... it simply means we have a lot more resources available and at an astoundingly low cost if we choose to use them.

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For the times they are a changin'

- Bob Dylan

I think the idea of toothing was craft folklore but inadvertently what came of it was that the teeth acted as squeeze out channels for the glue to move out sort of like a fluted dowel.

I agree that the deeper tooled scratches would have provided some channels through which hot, liquid-phase glue could flow, just as they do for hammer veneering, but as that toothing did not continue to the edges of the joint, I'm not sure that we can assume that was a reason for the practice, as initial closure of the joint would have stymied squeeze-out along that easier route.

I'm curious as to whether Martin or other manufacturers engaged in toothing on joints other than the fretboard and bridge, and whether any woods besides ebony and rosewood were routinely prepped for glue-up with toothing or random scratchwork? It seems that - if Martin did not routinely tooth other joints such as rim to blocks or maple bridge plate to top - the rationale for the practice would be tied to specific materials versus general practice.

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For the times they are a changin'

- Bob Dylan

Justifying past practices of others can be difficult. The important thing is that we now know better.