Glue Issues

In early 2013, Earl became aware of a couple nyckelharpas in need of repair. When he looked at them, he asked himself what would have caused this failure. He concluded that it appeared that these might be glue joint failures and decided to look into the glue issue. This write up is the result of that investigation.

In late-winter/early spring of 2013, he shared this glue information with a couple well known builders in Sweden. He let them know that it was his opinion that hide glue should be used, but it was OK if they “agreed to disagree”. Both of these builders agreed to disagree and are continuing to use Titebond glue.

Earl is presenting this information so that nyckelharpa players can make an informed decision about the importance of this issue.

The failures he is addressing are not real common, but not that uncommon. The bass bar failure occurs more often than the back/base of neck. It may take 15 or more years for the failure to occur in the life of an instrument. He sees this as a long term life-of-instrument issue.

Glue issues with Nyckelharpas:

Many Swedish nyckelharpa builders use Titebond glue in building instruments. We have been told it is being used because it is the glue used to make guitars.

We have discussed this with a high level guitar builder who told us that Titebond is the most common glue used in guitar building. The guitar builder also explained that the neck to body joint in a guitar is a complex dovetail joint that is very well engineered to withstand the stresses in a guitar.

It has come to our attention that there are a couple failures that occur that may be linked to the use of this type of glue. The first is a bass bar separation from the top, at the key box end of the body, and the second is separation at the seam located at the base of the neck/top of the back.

The bass bar separation results in the instrument bending and the key box coming down and sometimes touching the top. The top may actually get a depression near the end of the bass bar. The standard fix for the separated bass bar is to replace the top (and the bass bar).

The separation at the base of the neck results in the neck tipping. The most noticeable initial symptom is that the A-string will be near the top of the tangents at key 20. If the bend is significant, the key 20 tangent will pass under the A-string. When Esbjörn was asked what to do about this, he suggested shimming the key box up to make the instrument playable. This is a temporary fix and does not really address the root of the problem. Earl is aware of an instrument where the separation is more on one side than the other, resulting in the neck twisting. Earl believes that the fix is to remove and re-glue the back, using a glue that can withstand the stresses involved.

What is Titebond glue and what are its properties? Titebond glue is a common trade name for yellow woodworking glue often used in woodworking. The basic glue chemical is Polyvinyl Acetate (PVA). The USDA Forest Products Laboratory publication, the Wood Handbook, chapter 10, tables 10-2 and 10-3 lists the following about PVA glues: recommended for non-structural use and that “joints will yield under continued stress”. On the label on the glue bottle under User Tips, it states: “Not for structural or load bearing applications.” The Titebond web site has a specification sheet on this type of glue that also does not recommend it for structural applications. An earlier specification sheet indicated that it will creep under high load conditions.

It is Earl's opinion, based on a US government publication and the recommendation from the manufacturer, that this type of glue should not be used in high stress locations. He understands that there is a broad range of PVA glues available. The differences in the glues are based on the specific chemical treatment of the glue by the manufacturer. So, some PVA glues really are much better than others. But they all have the same recommendation of not to use for structural applications.

The USFS Wood Handbook does not recommend Hide glue either. Earl suspects this is due to it being susceptible to heat and moisture. But, this makes it a glue that can be taken apart.

The violin family instruments use Hide glue. Hide glue is made from animal hide. If used correctly, it produces very high strength joints (in Earl's opinion, stronger then PVS).

Commercial glue makers have developed pre-prepared bottled hide glue. It is liquid at room temperature. Earl has no experience with this product. From the discussions he has had with violin luthiers, they are continuing to use the traditional old type of hide glue.

Earl looked into the comparisons between PVS and Hide glue. An on-line chat about glue for guitars gives some comparison comments that if you use PVS and have to fix the instrument, you must make new parts. It is extremely difficult to get old PVS glue off of the joint surfaces to do a re-glue. If you use hide glue, you can usually re-use the parts.

A benefit of Hide glue is repairability. You can warm up the joint and apply moisture and get the glue joint to separate. With some water, you can loosen the glue and all but completely remove it without damaging the wood. You can then re-glue the joint with all original parts. Hide glue will re-glue to itself with full strength. Luthiers have been doing this with violins for hundreds of years.

To take apart a PVA glue joint between the body and top (or back), you typically heat up a thin knife and force it into the joint. You work your way around the instrument. When you are done you still have old glue on both surfaces that needs to be removed. Old wood glue is hard to remove. Earl usually uses mechanical removal (scraping/sanding). Mechanical removal actually takes material off the parts, this may change the acoustic characteristics of an instrument. He understands that Acetone will soften it, but you will not get the glue out of the wood pores (required for full strength joint). Acetone will also damage the finish.

Back to nyckelharpas:

Try to visualize a general force diagram of a nyckelharpa body (Earl is an engineer, and this discussion is based on 1st semester college physics). The 16-strings exert a significant pull (tension) force across the top of the instrument. This must be countered by an equal tension force along the back and body of the instrument. These two tension forces result in a significant compression force along the center of the instrument through the top of the body. There is also a significant strain caused by the strings-nut-bridge-tailpiece that exerts a bending force on the body. The bass bar functions in part as a structural beam to counter this. But the significant force concentration will be near the middle of the instrument at the neck end of the bass bar. The glue joint here is under a lot of continuous stress to pull away from the top of the body.

The strain along the back will produce a high stress point at the point formed by the base of the neck and the top of the back. This is a sharp angle and is the furthest out location along the back. Both of these features contribute to this being a very high stress location.

It is Earl's opinion that both of these failures appear to be glue joint separations that may be caused in part by using a glue type that should not be used in location of continuous high strain.

The other issue here is that the design of the nyckelharpa body does not use any real structural joints to transfer load/force (the guitar neck uses a dovetail joint). The nyckelharpa design depends on simple glue joints for strength. As the nyckelharpa evolves, this might get addressed.

Common failures and repairs

Bass bar separation:

With PVS glue, you remove and replace the top (and bass bar). What Earl thinks may have contributed to the failure (incorrect glue) is not addressed if the repair is done using PVS glue. Since the top is the primary sound diaphragm, replacing it will probably noticeably change the sound of the instrument.

If hide glue is used to make the nyckelharpa, it should be fixable by just re-gluing. If the bass bar has a crack, you might need a new bass bar. There should be no need to replace the top, and the sound remains the same as it had been.

Glue Joint at base of neck:

If PVS glue was used, to fix the problem, Earl thinks you should remove the back. Due to the problems with removing PVS glue from an old glue joint, you may need a new back (and re-surfacing the bottom of the body for a good glue surface).

If the instrument was made using hide glue, you must take the back off and re-glue it. This can be done at any good violin repair shop.

As a result of this glue evaluation, Earl is now gluing bass bars, tops and backs using Hide glue. If he were building guitars, he would use Titebond. Since nyckelharpas are definitely not guitars, we need to base the materials used for building them on the properties needed to address the issues with this instrument.

Nyckelharpas are fairly expensive instruments. There are no factories making cheap ones. We believe that most people will expect an expensive instrument to be repairable and to have a considerable life-span. Using hide glue makes the instrument more repairable, and results in a very long lasting instrument. We like the idea of building instruments that may last long after Earl is gone.

So what if you replace parts? Through research and discussions with luthiers and musicians, Earl has found that as an instrument is played over a number of years, the sound will improve. The common opinion is that this is due to aging of the wood along with the effects of the sound vibrations. Basically, a luthier can’t build a 20-year old instrument. This also lends to some observations that old instruments often have better sound.

If you have a nyckelharpa that you have been playing for many years and the bass bar fails, you will probably end up with a new top. The top on the nyckelharpa body is the primary sound diaphragm. Unless you can get a replacement top that was cut from the adjacent slab off of the log your initial top came from, you will end up with a slightly different piece of wood (assuming the same sub-species & thickness). You will end up with a slightly different sounding instrument. It might still be a fine sounding nyckelharpa, but it will not sound the same as it did. The back is a secondary sound diaphragm and we would anticipate the affects would be less.

Earl is building instruments that can be repaired with original parts and will give you the best chance to maintain the original sound of the instruments. We think this is an important issue for the musicians.

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(Page last updated on 9/7/2015)