Glue strength

[Dave Bagwill]

This reason was given on a glue site, don't know what to make of it:

Its (HHG) bond is molecular. It's a chemical bond primarily, and a mechanical bond, secondarily. PVA glues bond only mechanically.

From the same site:
Q: TiteBond, Elmers, and other white and yellow PVA glues seem to work just fine. They are stronger, they apply easier, they are more readily available, and they will glue a broader array of materials than will animal glue.

A: That's wrong on all counts. Stronger? Not hardly. A wood to wood bond, for example, will vary widely with the PVA and modified PVA glues, depending on the porosity, time of clamping, etc. Also, once they have fully dried, their hidden glue joint looks like an old-fashioned lace, because it must lose a percentage of its water content through evaporation or absorption before it can set. Therefore, it must be under pressure the entire time, since moisture loss results in adhesive thickness decrease. That weakens its bond. Hot hide glue isn't under those constraints at all. It sets by gelling momentarily, dependent on temperature and time to set. From that point, no clamping is even required, and it just keeps getting tighter and tighter, a 100%, perfectly conforming airtight joint that will not leverage apart its own bond, as does PVA glue in unexpected instances.


I have no opinions on this yet, just seeking to learn.

[Ken Hundley]

One of the biggest benefits for HHG is that it is the only glue that actually pulls the joint tighter

Please expand this thought - I have never heard such a claim as HHG singularly having this characteristic, HHG has water just like Titebond, Elmer's etc. all catalyze with oxygen and draw the porous surfaces together via evaporation.

A few of the luthiers in a previous site mentioned this, Rick Turner may be one of them? Also mentioned in "Properties" in Wikipedia.

http://en.wikipedia.org/wiki/Animal_glue

Look at the third paragraph under Properties

[ken cierp]

These glue discussions are always interesting. It is my belief however, that we must take care to pluck out the scientific facts. Of course boutique luthiers are always striving to pitch their products in the best possible light to give them an edge over the factory made guitars and each other. For instance I recently read a comment by a guy that stated it’s important to be able to make a dovetail joint not because it was better but rather its easy to convince the customer it is better. I’ll also sight a study performed by Tim McKnight where he did wood glue comparisons to determine hardness and frequency transferring properties of the various glues – his experiment used “metal slats” for the glue surfaces – I have read since that he indeed realized that the test of wood glues on metal had little scientific value. Lastly, who has not heard of the experiments were at the beginning of the year teachers were told that the least promising students were their brightest – sure enough at the end of the school year those children in many cases ended up at the top of the class. It is very difficult to conduct studies when you are hoping for or trying to prove a certain outcome at the offset. So what I am saying is that I highly respect all those in our peer group and their opinions, but I personally like to get chemical data from the scientists and engineers not guitar makers.

My favorite wood related scientific text book is “Understanding Wood” Authored by R. Bruce Hoadley I plan a review for the book section but here’s an interesting excerpt from the adhesive chapter. Take particular note his reference to “water as an adhesive”.

The general term adhesive includes any substance having the ability to hold two materials together by surface attachment. Those most commonly used for wood are called glues although materials described as resins, cements and mastics are equally important in the assembly of wood products.

No truly all-purpose adhesive has yet been manufactured and probably never will be. A general-purpose adhesive cannot hope to attain all the individual capabilities and attributes of closely designed ones. Although any of the standard commercial glues will do a satisfactory job if the moisture content of the wood is controlled and the temperature remains within the human-comfort range, there is an increasing trend toward development of special adhesives. Adhesive selection must therefore take into account factors such as species, type of joint, working properties as required by anticipated gluing conditions, performance and strength, and of course, cost.

One interesting adhesive is water. It is easily spread, wets wood well and solidifies to form a remarkably strong joint. It is delightfully inexpensive. However, it is thermoplastic and its critical maximum working temperatures is 32°F. At temperatures at which it will set is has a very short assembly time. But due to its temperature limits, water will never capture a very important position among woodworking adhesives.

A wide and confusing array of adhesive products confronts the woodworker. A common pitfall is the belief that some glues are better than others; the notion that simply acquiring “the best” will ensure success is careless and may give disastrous results. With certain qualifications, all commercially available adhesives will perform satisfactorily of chosen and used within their specified limitations. An important corollary is that no adhesive will perform satisfactorily if not used properly. Within the specified limitations, most woodworking adhesives will develop joints equal in strength to the woods being joined. Thus, the wood, rather than the glue or its bond, is the weak link in a well-made joint.

Glues made from natural materials have been used from earliest times and even today, hide glue (made from the hides, tendons and hooves of horses, cattle and sheep) and casein (primarily a milk derivative) are still in use. However, the bulk of modern wood glues are synthetic compounds. Perhaps the most versatile are the polyvinyl acetate emulsions (PVA), commonly called white glues. More recently the yellow glues (modified PVA) have emerged, which have greater rigidity, improved heat resistance and better “grabbing” ability These yellow glues are satisfactory for the bonding jobs of most craftsmen. They are easy to use and are more tolerant to unfavorable conditions than are white glues. Yellow glues also give less trouble in clogging abrasive paper.

[Dave Bagwill]

Very interesting. The ubiquitous Titebond has nothing to be ashamed of!! Nor does the LMI white, or Elmer's.

For my part, I'm happy to have a few very good choices that don't involve a glue pot :-) Though I will try a little HHG on a test project to see what the big whoop is. Plus I understand the smell is to die for - or it makes you want to die, one or the other...
Peace

[hikermike]

Hide glue is "harder".... more brittle hence may transmit sound better but most importantly it shrinks. The point being that if you've made
the perfect joint, it will draw it in tighter until it is almost invisible. If you haven't made the "perfect joint", then maybe PVA is better. Also PVA
"creeps" with time so bridges etc may shift with time.

[ken cierp]

Shrink, creep, and hardness -- common sales support claims used by the boutique builders for their use of HHG. However, after 175 years there is still no documented studies or evidence --- definitive proof, so the bottom line is most like to use modern PVA --- after almost 50 years of constructing musical instruments I am convinced that its a work-ability choice, nothing more.

Lets stop this here --- this of course is my opinion, there is no need to hash out which glue makes for a better guitar that will always also be an opinion --- and there are other places on the internet where one can read a mountain of that stuff.

On this forum the useful information would be one's method for using HHG.

[stevem]

Tony, you mentioned that you use epoxy for pore filling. I've seen it mentioned for sealing soundboards in Don Teeter's book and I was experimenting with some yesterday on some walnut. Would you mind explaining your process. Do you put it on raw wood or seal it first? oh yeah, my cutaway side turned out well!

Steve