Hide glue and gluing

In the construction of violins only high quality hide glue should be used. Other glues, such as the white, yellow glue the cabinetmakers use, should be avoided. The main reasons for hide glue are reversibility /the joint can be taken apart and re-glued again/ and specific toughness of the joint with no creep and good acoustical properties.

Hide glue

It can be in the form of small flakes, pearls or powder. Usually offered are strengths varying from 250 to 400 grams. Stronger sorts give you less working times so take the middle road and buy something of around 300 grams of strength. Usually small amounts of glue are used during building so you don't have to buy more than 100gr of dry glue to get you started. Avoid the hide glue that is offered liquid and premixed.

A glue pot/kettle

It should be rather small, ideally 8-10 cm in diameter, allowing you to prepare small amounts of glue quickly. Hide glue temperature must never exceed 70C, so you should be able to regulate the pot's temperature or its power output should be in the range of 10ths of watts. A regulated, double bottom pot with an insulating water-bed is perfect. See Fig. 1 and Fig. 2 for an example of the pot and hot plate.

If you are pressed for money, or love to DIY, you can build a small glue pot yourself. It will work even without the water-bed, but you must make sure the wattage of the heating element is not more than 60 watts - a small halogen light bulb, such as the G9, can serve well here. The upside is that the heat up times are short and you will have a batch of hot glue ready in a minute. The downside: You will have to watch the pot like a hawk, keep stirring and turn it off before it overheats. It may seem complicated but when you get used to this, you can prepare small amounts of glue very quickly. See Fig. 3 for an example of such a setup.

This glue pot is made of a round wooden base and a sheet of metal turned around a bell, which serves as a pot. The heating element is the halogen light bulb mentioned above. Use your imagination.

The bottom line: You don't need anything fancy or regulated but a good thermometer before you get the feel for this is a must.

A set of brushes and knives

One smaller 5 mm wide all-purpose brush, one larger 20 mm wide for bigger jobs. A palette knife will be used to work the glue into the seams when gluing to top and bottom plates to the ribs.

Preparing the glue

1. For a start, put a teaspoon of hide glue in the glue pot and add about 100ml of fresh water.
   
2. Stir and let stand for 1-2 hours. When you see the glue has gelled and grown in size considerably, it is time to heat it up.
   
3. Turn on the glue pot and make sure the temperature never exceeds 70C for longer periods of time. Should your hide glue start to boil, don't use it, and prepare another batch.
   
4. Keep stiring until you are sure there are no undissolved pieces in the pot.
   
5. Check for the thickness of the glue. Visually, the glue is hot and thick enough if a film starts to form over its surface. You will learn to guess the right thickness by letting the glue drop from the brush. The medium thickness hide glue should feel oily between your fingers. The thin hide glue used for glue sizing should almost be the consistency of milk. Make sure the glue is about 70C when testing for thickness.
   
   

Application

1. Always try to work in a warm room with no droughts so that you have more time to make the joint.
   
2. Preheat the critical joints with a hair dryer, ie. center seam joints.
   
3. Make sure everything fits right without the glue first.
   
4. Have everything to hand, you need to act quickly and precisely.
   
5. Don't use more glue than necessary and always remove the surplus with a damp brush or towel. Sometimes it may be better to wait for the glue to gel a bit for easier removal. Be especially careful with the areas that will be varnished. Areas with hide glue blotches will not absorb the stain properly, leaving marks.
   
6. When you are using the palette knife, always have it preheated in hot water, otherwise the glue gels fast when in contact with the cold knife.
   
   

Drying times

Under normal humidity and temperature the hide glue should be left to dry for about 12-24 hours. Usually overnight should be enough time. More structurally important joints, such as the center joint or the neck joint, should be left undisturbed for the full 24 hours.

Hide glue shelf life

The dry hide glue should be stored somewhere... dry. Properly stored, it has an almost infinite shelf life.

The shelf life of the prepared hide glue depends on many factors: humidity, room temperature, possible contamination, thickness. It all boils down to how bacteria friendly your workshop is. The freshly prepared glue is the strongest, so for critical joints, ie. when joining the billets for the top and bottom plates, I would always use fresh glue. On the other hand, many violin makers may leave their glue in the pot for days without apparent loss of strength.

Some people prepare the glue, let it cool down, cut it up into small cubes and put them in a fridge.

The signs of bacterial degradation are the change in color, often turning brown, tiny watery pits on the surface and a bad smell.

So if you want to stay on the safe side, always prepare a fresh batch of your glue.