A violin is made up of various parts and the stuff which holds them all together is animal glue.
There is a long tradition for this kind of glue. The first and earliest finds of pieces of furniture glued with animal glue and descriptions of how to glue pieces of wood with animal glue date back to 1400 B.C Egypt. Greek philosophers wrote about the preparation and use of this glue (e.g Homer towards the end of the 8th century B.C). Also the Romans knew about the techniques and practices. The Middleages left us manuscripts dealing with the making and useage of various animal glues (11/12th century)
For violin making and restoration the material is absolutely perfect and indispensable with any modern type of glue. The biggest advantage is its reversibility- once the glue is dry and set it can always be returned to gel with water. In practice it means you can for example reopen an old crack if necessary e.g when it became dirty with time or was not glued level. For glueing the back and front to the ribs a weaker kind or more diluted glue is used to ensure the instrument can be reopened for repair or more vital the plates can pop up at the seams when shrinkage of the plates occurs during the drier seasons of the year- this loosens tension in the wood and avoids the energy built up releasing in a crack further in the plate (dry-crack). Animal glue fairly colourless which is good for glueing cracks invisibly and it does not creep with time.
The strength of animal glues is very good too. Different kinds of glues are available and it´s strenght can easily be manipulated by how much it is diluted with water. Types of animal glue are Bone glue (made e.g from Cattle bone), Skin glue (e.g from Cattle-skin), Rabbit glue (made of rabbit skin), fish glue (fish skin) and techical gelatine (made of cattle skin and comparing well to gelatine from the food industry).
Glue in granular form is soaked with water for 1-2 hours or even overnight, heated up to about 65C, taken out the water bath to cool once and than reheated for ideal strength. I store the glue in the fridge if not used to make it last longer and not getting mouldy- it is a natural material after all. It is liquid when warm and sets when cold.
The process from raw material to the glue-granulate we use takes about 90 to 100 days. The animal parts are being cleaned, cooked and degreased, cooking releases the collagen from bone or skin into the cooking water. Filtering, steaming down and drying brings it into the customary form.
The secret of the glue are the extremely though fibres of the collagen. Collagen is protein found in human and animal fascia. Our cartilage, skin, tendons and ligaments are mainly consisting of collagen.
One “measure” for sorting animal glue is the so called Bloom number. According to Dr. Kremer (www.Kremer-Pigmente.de) the bloom number is defined by the flexibility of the glue. The granulate is cooked up with a certain glue to water ratio and poured into a plastic cube of a set volume. Once cooled and gelled the glue-cube is taken out and set under pressure with a metal ball pressing down on the cube until it starts to tear/disrupt. This measured pressure is the Bloom number. The higher the bloom number the more flexible the glue. A flexible glue can be good e.g for glueing a patch in a repair- since the glue bond between original wood and the new wood is constantly vibrating when an instrument is being played.
Glue adhesiveness is greatly affected by its fat content. Less fat means a better bond.
I like to use Körnerhautleim in Würfeln (cattle-skin granulate) and a new Kremer product Rabbit skin glue for stronger bonds.
Here are some photos to show what the granulate can look like before it is soaked in water and cooked up and a picture of the prepared glue ready to use. (First picture: cattle-skin glue on the right and a japanese glue Harima S1, very light in colour on the left)