microwave curing of hide glue?

[Bob Hammond]

Hi,

I thought about using Franklin Hide Glue (FHG) instead of HHG for a top to body bond, but I've always been skeptical about it. Maybe I'm biased. So I ordered a very fresh bottle of FHG and did a couple of test pieces. It's true that there's plenty of positioning/clamp time, but how long does it take to really set up and harden?

I prepared several test pieces about 2 sq inches of overlap between 1/4" plywood scraps, and also a 1/4" stick of pine onto another plywood piece. It was not difficult to push them out of position after 2-3 hours of clamp time. So I took one of the samples and gave it 25sec in the microwave at 1100W. The squeeze-out bubbled up like polyurethange. When I took it out and let it cool for a few minute and scraped off the bubbles, the glue was rock hard, and the pieces were stuck very firmly together.

Would this work in 'real life'? Has anybody else tried this? (By the way, I was thinking of gluing a ukelele top, which would fit in the microwave.)

[Jason Rodgers]

That's an interesting result. The only thing I'd be worried about is the microwave heating and forcing out water in the wood, which might lead to some funky dimensional changes that would mess with your alignment or lead to joint stability problems once the pieces reaclimate. But I don't know if the time you nuked your sample was long enough to do all that.

[Randy Roberts]

I would also have concerns that this would weaken the bond. Hide glue is basically protein. Heat will denature proteins (this is why you cook meat.).
If the squeeze out was bubbling, that would suggest the water in the glue was boiling, which would suggest you are also boiling the protein of the glue.
The recommended temperature of 145 F for hide glue is there for a reason.
I would recommend reserving the technique for dinner.

[Waddy Thomson]

I agree. If you are boiling the water in the hide glue, you are certainly weakening the bond. That's why we are not supposed to heat it above 145 - 150 degrees. If it's bubbling, I'd say it's around 212 degrees. Hotter than recommended, and certainly hot enough to cook the proteins in the glue.

[Bob Hammond]

Well, this was a first experiment.

At one time, I knew something about the molecular structures of the various types of collagen. The only thing that I remember is about Marfan's Syndrome, where there's a defect of the proline residues. All collagens will dissociate with prolonged heating, and that's the reason that hide glue should be thrown away after some (uncertain) time in the gluepot.

But if you think about cooking stew meat which has a large amount of collagen, it comes out much better when it's slow-cooked in a crockpot (gluepot) as opposed to a rapid brief heating to boiling on the stovetop. So I'm not so sure that a blast of microwave heatiing would destroy the hide glue's properties.

I'll think it over further. By the way, the freshly-bottled Franklin Hide Glue that was not microwaved is still sticky-icky.

[Alan Carruth]

I lost all faith in liquid hide glue a long time ago, when stuff I'd used it on started to move around due to the glue going sticky long after assembly.

I have used microwaves to warm up glue when the shop was too cold, and it works well. The key is 'warm'; you watch it like a hawk, and pull the pieces out as soon as the glue liquefies.

I was always told that 145 F was the MAXIMUM temperature for hide glue. That's when proteins really start to denature, which is why it's said to be a 'safe' temperature. Once you get a piece of meat, say, up to 145 nothing harmful can live in it. My little coffee cup warmer keeps the glue at about 120 F, and it lasts a fairly long time. Of course, at that temperature, you've got less time to get the clamps on before it gels, but there's always the hair dryer...

[Bob Hammond]

Perhaps we should not get caught up in a particular way of thinking and habits. That leads to stagnation.

[Bob Hammond]

I think hide glue will never be effective unless the jointing between the opposing parts is very close.

When the joints are done right, then hide glue is the super-glue for woodworking. It's perfect.

[Waddy Thomson]

The real truth is that that is true for all glues that are used in guitar making. The only exception I know of is epoxy. Most others don't work well if they have to fill a gap, or if there is a thick glue line.

[Todd Stock]

Hide does not cure - it dries - so anything that speeds up the drying process (zippier evaporation of water from joint) moves things along faster. I suspect that one-time overheat of the glue will not have as detrimental an effect on cohesive or adhesive performance as repeated overheating does, but not sure there is a good reason to risk the other issues associated with heating to well over the max temps we want to see with hide (e.g., as pitch flow in spruce or other resinous woods; steam expulsion of glue solids from the joint; color change in some woods).

[Michael Lewis]

The problem I had with the bottled hide glue was it would pick up moisture from the atmosphere and become soft and gooey. That's a problem for instruments assembled with this stuff, and why I don't use it. Maybe a fresh bottle of it will work well, but I'm not going to risk my work on it.

[Bob Hammond]

Michael, I have a theory that hot hide glue's hygroscopic properties are part of what makes it a superior glue for wood. I think it expands and contracts at rates that are similar to the lignin 'glue' within most woods. Again, it's just a theory.

Maybe the additives (urea?) in bottled glues makes it much more hygroscopic and therefore gooey. A cursory web search turned up some interesting results. Frank Ford discusses it in detail and recommends 5-10% urea (by weight). The Pianotek Supply Company gives specific directions for preparing hide glue, and state that up to 3% table salt can be added to extend open time. They also discuss adding urea (up to 15%), and go to state that bottled glues have about 15-17% urea, which accounts for the weakness.

By the way, the Pianotek Supply Co. has a very interesting tool catalog ( incl. Safe-T-Planer!). They are not far from where I live, so I might go see if they have a retail store.

[Bob Hammond]

I tried the Pianotek advice about adding salt. It seems to add a little more open time but does not make the glue permanently 'sticky' like the bottled glue.

By the way, I make up the glue in small batches of about 25-50ml, in orange-top 50ml plastic tissue culture tubes that have tight-fitting screw caps. When the glue is made in these tubes and 'slow-cooked' at 135-145F for a few hours with the caps loose and then tightened down, the glue remains useable for months without a need for refrigeration. I recently found a tube that had been on the shelf for 3-4 of years, and while it was solid as a Jolly Rancher hard candy, there wasn't any sign of bacterial or mold growth. I bet I could add some water and use it, which I might do for non-critical work such as jigs and fixtures.

[David King]

There are microwave curing guns for Titebond and other aliphatics that are used in the furniture industry. There's probably a lot of literature on those. I'm sure that once the hide glue is dried it becomes much less sensitive to heat. I guess the question is at what moisture content would it be safe to blast it? Easy enough to test some glue joints in a conventional microwave but how repeatable would your results be?

I could definitely see the usefulness of reheating a gelled HHG joint with a little microwave action if you could control the heat. That's where I would go first with this idea.

[Michael Lewis]

Hide glue cures by evaporation of the water in it. If you disturb a joint before it is completely dry it will not have any strength.

Microwaves are not good for you. That's why the industry is very careful to seal microwave ovens for the consumer market, so you don't get blasted with microwaves and receive cell damage.

[David King]

On the safety of microwaves, my dad remembered going to the 1939 Worlds Fair in New York and seeing a GE microwave stovetop. There was a man sitting on top of the range with a metal frying pan in his lap, frying up eggs. Fun times for sure. I don't own a microwave or I'd be experimenting with this.

Thinking about this a little bit more I realized that the glue curing presses and guns use R.F. to heat up the glues.

[Jason Rodgers]

There was a man sitting on top of the range with a metal frying pan in his lap, frying up eggs. Fun times for sure.

WOA! What kind of "eggs"?!

[Michael Lewis]

Don't do this at home!