Hide glue preparation
How to prepare Hide Glue
By: Leif Luscombe
I have been asked several times how properly prepare hide glue. Hide glue is the standard glue that is used by violin makers, and is prized for its organic nature and ability to be dissolved to remove a plate or other part of a violin and perform necessary repairs.
These very qualities are responsible for many of the small violin repairs – humidity and dryness can weaken the glue joints, making re-gluing of loose joints necessary. However, on the same note, the strength of a good hide glue joint is almost unmatched – it can easily be stronger than the wood itself. Spreading a thin film of hide glue on a plate of glass will damage the glass – often times breaking it (I have had clear glass baby food jars and ceramic pots broken while it is shrinking during drying).
Hide glue is available in a powder, granular or flake form. It needs to be mixed with water to dissolve.
Below is my glue set-up. This is working equipment – so is well used.
To heat the water for the glue I use a rather ordinary laboratory hot-plate with a dial heat adjustment on the front. An alcohol lamp, old clothes iron (upside-down), or other heat source could also work.
Hide glue is made in a double-boiler to prevent the glue from over heating or becoming scorched on the bottom. Half of a soup can or other metal container could be suitable. I use a copper glue pot from an alcohol-lamp heated glue pot assembly that I purchased some time ago.
This is the outer jacket that holds the (hot) water. The glue container is placed inside this.
This is the copper pot that I use to actually hold the glue. Again, it is from the pot that I purchased. It is placed inside the water jacket (image 3) so that it is suspended in the (hot) water.
I have also used baby food jars, held with a simple wire frame to keep it from tipping. A tin can smaller than the water jacket would also be suitable.
The glue pot holds about 4 ounces. I usually start with about 2 ounces of cold water, and put one common teaspoon of dry glue into this. The glue should be fully dissolved while the water is cold.
The glue should be heated to about 70°C, or 165°F. I must admit that I don’t use a thermometer. The water in the jacket should be quite hot, but not near boiling point.
When the glue is ready, there should be a slight skin forming on the glue – but beyond this the glue will become too thick rather quickly. At this point I usually put a couple of teaspoons of water into the glue. Between the fingers it should feel oily, but not thick. If it feels watery, it is likely too thin.
If you refer to Image 1 (above), you will see a selection of tools that I use for applying the glue. A 1” brush (dollar store variety should be adequate) is great for applying glue to fingerboards, nuts, saddles, and other flat surfaces requiring glue.
I use a knife (the black handled artist’s knife with the aluminum blade is one of my favourites) to apply the glue into an open edge seam (there is no way of getting a brush into these tight cervices). Be careful to leave the metal knife blade in the hot glue for at least a few seconds before using it, or it can quickly cool and gel the glue on the knife. Once hide glue has gelled, it must not be clamed, as it will create a “cold joint” that will fail to obtain maximum strength.
For gluing cracks on the top or back of an instrument, rub the glue into the crack (this can be done with a brush or clean fingers) until it starts to come through the other side. Have appropriate clamps ready, so that it can be clamped while the glue is still warm and thin.
Make sure that all glue is wiped off of varnish, and washed from unprotected wood. Glue or glue-water can damage varnish that it is allowed to dry on. Unvarnished wood that absorbs glue or glue water will not accept stain or stained ground varnish as well as the surrounding wood, so will leave light spots under the varnish!
I recommend the medium strength (315 gram strength) glue. All strengths of hide glue are capable of making a joint stronger than the wood. The major differences in the glue is the working time. The strong glue (380 gram) tacks faster, and the weaker glue (195 gram) gives more working time. In my opinion, the 315 gram gives the best of both worlds, and IS rated stronger than the weak one.
An excellent plastic glue pot, with 2 sections (one for glue, one for hot water) can be found at www.violins.ca/product/3813
Dry hide glue can last indefinitely. Once the glue has been dissolved in water, however, it does not last long. In the summer (here in SW Ontario, Canada) when it is hot and humid, the glue will typically only last one day. The second day it starts to show very small pits in the surface (mould). By the third day it can be green and furry. In the cooler, winter weather it will usually last several days easily, but I usually make new glue each time to be safe (it is a small investment).
I have heard that refrigerating the glue can extend its useful life. This is probably true, though I have never found it necessary.
Ready-to-use hide glue can be obtained. It contains a preservative (formaldehyde?) that keeps it useful for up to several months. I have, however, heard of one maker who used it exclusively on his first several instruments with disastrous results – so I would not recommend its use, even if there are those who have good experience with it. Don’t say I didn’t warn you!
Below is an article about hide glue that I have had around for a while, that may touch on some points that I have overlooked:
Preparing Dry Glue for Use
Where really good mechanical stirring is available, powdered glue may be dissolved directly in water maintained by a steam jacket at 70-76°C. This method is commonly adopted in large scale operations but where equipment is not available to dissolve glue in this way the under mentioned procedure should be followed:
- Ground or powdered glue should be first soaked in clean water (not above 21°C) for 1 to 2 hours or more. The glue should be added gradually and stirred to avoid formation of lumps.
- After soaking, heat the glue in a jacketed kettle or double boiler but do not allow the temperature to rise above 75°C. DO NOT BOIL or subject the glue solution to excessive heating which will cause deterioration of adhesive strength and reduce the glue to a lower grade.
- If practicable, only make up sufficient glue for the day’s work. If more is required during the day make sure that the additional glue has been properly soaked before adding to the glue solution.
- All glue pots and brushes should be cleaned at the end of the day’s work by scalding and use of a good disinfectant as a safeguard against bacterial infection. An infected glue need not have and objectionable odour, but whether odour is present or not bacteria will bring about a chemical change, and destroy the essential properties of the glue.
Concentration of the Glue Solution
The proportion of glue to water in preparing a glue solution depends in the main upon the class of work for which it is required. For general jointing purposes the following percentages of glue to water will serve as a guide:
- Russian glue 25 to 30 % of glue by weight
- Match glue 30 to 33 1/3% of glue by weight
- Cabinet glue 30 to 35% of glue by weight
- Joiners glue 35 to 40% of glue by weight
- Box glue 40 to 50% of glue by weight
Some “Dont’s” to Remember
- Don’t buy glue on price. Buy the grade most suitable for your work.
- Don’t use unclean water for soaking glue: bacteria may be present. Use clean water not above 21°C.
- Don’t heat the glue solution above 75°C. To boil glue is a sure way to reduce its adhesive strength.
- Don’t add dry or unsoaked glue to that already in solution.
- Don’t use glue left over from the day before. There is always a risk of decomposition, particularly in warm climates.
- Don’t use unwashed glue pots or brushes. All utensils should be cleaned by scalding and use of a disinfectant at the end of the day’s work.
- Don’t guess the temperature of glue – use a thermometer.
Destructive Action of Bacteria
If a fresh supply of glue is not made daily and glue posts are not thoroughly cleaned, preferably at the end of each day’s work, deterioration of adhesive strength and viscosity of the glue will often be observed. This may also be accompanied by darkening of colour and development of an unpleasant odour. It is now common knowledge that such effects as glue solutions. The bacteria secrete small amounts of complex substances known as enzymes, the chief function of which is to facilitate the chemical decomposition of the medium, and it is to the action of these enzymes that most of the destructive effects of bacterial action can be traced. Bacteria are minute living organisms that cannot be seen with the naked eye (50 million could be placed on athree penny piece) yet multiply so rapidly and the by-products of their life processes are so potent, there Is no limit to the damage that may result when a glue solution becomes infected and remains favourable for their growth.
When conditions become unfavourable, for example, when decomposition of the media is complete, or when glue in an uncleaned glue-pot has dried, or partially dried out, bacteria are able to form highly resistant “spores”. In this form the organism can remain indefinitely in suspended animation ready to revert to a state of active growth again and infect a new supply of glue added to the uncleaned pot. fortunately the growth of bacteria can be prevented by the use of a dependable preservative and by thoroughly cleaning all glue pots and brushes with scalding water to which a reliable disinfectant has been added. The need for cleanliness in these respects cannot be overemphasized.
There is excellent information on our Violin Making and Restoration Forum that discusses the making of hide glue, which includes some images.