Introduction
One of the most essential materials for violin makers is animal-derived glue. Having battled for many years with glue a colleague recommended, I needed to know more.
I started testing hide, fish and bone glues, using various glue to water ratios, gluing and breaking many samples of spruce and maple, and recording the results on a spreadsheet. My sample size was too small to have any scientific value, but I did learn a lot about glue in the process.
Among the glues I tested were
- Kremer (fish, bone and rabbit, unfortunately I did miss their hide glue),
- Bjorn Industries (hide, diverse bloom strength),
- Holfter Geltac (technical gel),
- Schilbach restoration (technical gel),
- Dictum (hide and fish glue)
- Amanosan (hide glue, different bloom strength)
As a maker I need to be sure my center joint never open again. As a restorer I need very strong yet rather liquid glue, which I can work into tight cracks. There are some situations where the glue must be extremely strong, such as in repairing slab-cut cello ribs and backs, where tension of the wood can be very high and the gluing surface very small, and soundpost cracks in tops and backs. Many experienced violin restorers will claim that slab cut cracks always open after a while, but I am not so sure about that anymore.
I usually grind the glue to reduce soaking time . I measure the glue to water ratio by weight. The glue to water ratio will be very different for every type of glue: The decisive factor is the viscosity once the glue is warmed, I like my glue to be similar to cooking oil. I keep my glue in a plastic bottle with a dropper lid to maintain the water to glue ratio stable. After soaking I warm the glue to a maximum of 50C.
For this particular test I tried to dilute each batch to a similar viscosity. I warmed each glue joint, applied clamps, and after letting the glue cure for about 3 days I broke the pieces and examined if the break occurred in or next to the glue joint, which gave me an indication of the joint strength for each type of glue. This test could be repeated with more sophisticated measuring devices (if one has the time and funds!), however in most maple samples (Dictum, Bjorn Industries, Schillbach, Geltac, Kremer bone and fish glue) the glue joint broke clean.
Conclusions
The only way to understand glue is to test its breaking point. Relying on the manufacturer’s word that his glue is best or on a friends recommendation is not sufficient!
Not to mention that there are misconceptions in the violin makers community. High bloom glue does not necessary have the highest joint strength or is useful for all type of applications or types of wood.
I personally learned to stay away from high bloom strength glues as well as technical gelatin.
The only two glues that in most cases in maple samples had the break occur next to the glue joint were:
- Amanosan 3B, 140 gram bloom
- Kremer Rabbit skin
I do have to add that the Kremer rabbit skin glue is a little tricky to use due to the strong jelly it forms upon cooling, so my personal winner is Amanosan 3B, 140 gram bloom Strength.
A common misconception Though it seems logical that high-bloom glues create the strongest joints, that is not at all the case. The Bloom number indicates the strength of the gel, and has no correlation to the final bond strength. I is my opinion that although high-Bloom glue has excellent cohesion, it significantly lacks adhesion compared to lower grades once fully cured. High-Bloom glue is also notoriously difficult to use. On cooling, it will rapidly form a strong, stiff gel that can create serious problems, such as preventing a joint from closing properly. A crack might show gaps, or a neck might point in the wrong direction even if the mortise has been fitted perfectly. Lower-Bloom glue is much more forgiving; it tacks more slowly and can be squeezed out more easily with less clamping pressure. This prolongs the working time and therefore allows for better results. The ideal Bloom strength also depends on the type of wood used. It might be around 150 for maple, maybe a little more for spruce, and a little less for ebony.
More about different types of glue and their properties:
Bone glue Has low Bloom strength, penetrates deep into the wood – so deep that it can even bleed through ribs and thin areas of top and back when making new instruments. It is very brittle and considered an inferior glue, yet it is very adhesive on difficult materials such as ebony and Corène, the perfect glue to glue Fingerboards on necks. And maybe to glue tops to ribs.
Hide glue Made from cow hide, this is a very versatile glue and generally sufficient for all purposes. Nearly all brands of hide glue I tested probably have sufficient bond strength for general purposes. I prefer low bloom, unprocessed hide glue, since they are easier to work with and have a higher adhesion.
Rabbit skin glue The sample I tested had a very high Bloom strength and was very elastic, yielding a much higher bond strength than most hide glues, but the stiffness of the gel makes it difficult to use.
Fish glue Although generally said to be stronger than hide glue, I could not confirm this in my tests.
Technical gelatine This type of glue has usually bloom strength of 300 +. It created the weakest bonds in my tests. Industrially produced, it is also made from cow hide but has been highly processed. It is of the highest clarity and can also contain preservatives, for example Geltac stays fresh for month in the fridge. I found it the most difficult glue to use and the least suitable for luthiers’ purposes.
Japanese hide glue One particular Japanese brand proved to be much stronger than all other glues I tested. Amanosan 3B, which has been made specifically for restoration and without the use of any chemicals. This is by far my favourite glue in terms of both bond strength and ease of use. As a bonus, the strongest Amanosan glue has a Bloom strength of only 140, is easy to work into cracks, and forms a soft slow tacking gel.
Glue properties. Some of the properties I am interested in are: bond strength; gel or Bloom strength; tack time; viscosity; penetration; shrinkage; flexibility; gelling time; gelling temperature; clarity; acidity; and bacterial content.
Adhesion The force with that the glue sticks to the material (wood)
Cohesion The force with that the glue sticks to itself. There seems to be an correlation between bloom strength and Adhesion/Cohesion, Low bloom strength glue seems to generally have higher adhesion forces and lower cohesion, the opposite is true for high bloom. That might be the reason why fingerboards as well as ingrain joints are so weak when high bloom glue is used.
Clamping pressure I found that a properly clamped glue joint will be much stronger than a rubbed or loosely pressed joint, especially on hardwood like maple and ebony.
Shrinkage and deformation Glue can shrink when it dries, which can cause significant deformation. Over the course of my research, though, I could find no significant difference in shrinkage between the different types of glue. More research is definitely needed because shrinkage matters in restoration, especially when patches are present. In the case of a soundpost patch, long-term deformation is unavoidable, though not very significant since the area is small. It is beneficial to leave a freshly glued patch clamped in the counterform for several weeks, although even this cannot prevent long-term deformation.
To avoid plate deformation it is best to avoid patches at all costs. If a patch has to be inserted or replaced, it is important to keep in mind where the glue joint is located. If it is not exactly in the centre between old and new wood, deformation is more than certain. To minimise deformation when gluing patches, the best option is to minimise the pulling force by keeping the patch size as small as possible, the glue joint as thin as possible, and to have the glue joint at the centre (old wood and the patch have the same thickness).
Clarity In most cases it is not necessary to use high-clarity glues, but to focus on bond strength and viscosity, since regular hide glue in a well-fitting joint or crack will be invisible anyway. Clarity can come at a cost, the highest clarity glues I tested were the weakest.
Bond strength. This is arguably the most important property, defined as the combination of adhesion and cohesion. Most commercially available animal-derived glues will be strong enough to withstand the regular stress that a violin is exposed to, but they are often significantly weaker than the wood itself.
Bloom strength. Arguably the second most important property, and yet among violin makers the most misconceived. The Bloom test (named after Oscar T. Bloom, who invented it in 1925) indicates the strength of the gel t. Bloom strength usually ranges from approximately 80 Bloom (weak or soft) to 320 Bloom (strong or stiff).
Viscosity Warm hide glue should have a similar viscosity as cooking oil. Low Bloom glues such as bone glue might have the right viscosity at a 2:3 glue to water ratio while technical gel might require a ratio of up to 1:5.
Tack time Initially the glue acts as a lubricant, allowing the pieces to be slid into position. The time a glue joint sets is especially important when working on difficult joints such as levelling cracks. Some glues (especially technical gelatine) can tack after less than a minute, while others allow for more time to make adjustments and are therefore preferable.
Gelling temperature All the glues I tested had a very similar gelling temperature, usually just above 30C (warm to the touch). To prevent early gelling especially when glueing cracks or other repairs that need time for adjustment I use a heat lamp, taking care not to overheat the varnish.
A personal thought and point for discussion: Instead of discussing the right way to secure a crack with cleats or other reinforcement, shouldn’t we focus on achieving the strongest possible joint so no reinforcements are needed?’
Amanosan 3B can be purchased either from Cremonatools or directly from the manufacturer. I do not profit from the links and I am in no way affiliated with the manufacturer of any glue.
https://www.cremonatools.com/japanese-hot-glue-nikawa-100g-bag.html
Christian Schabbon 
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