Below is a condensed version of a paper that I wrote for the Drostdy Museum a few years ago, when I was introducing traditional hide glue's preparation and usage methodologies to conservators, students and visitors to the museum.
In my experience, hide glue still remains the best choice for furniture making for a plethora of reasons. This article might seem a bit of a heavy read, however woodworkers who have persevered, tell me that it was worth it...(I think they were just being nice!).
Below, I attempt to present many of the facts and some of the fictions surrounding the most commonly used furniture glue in history...
The History of Collagen Glue
Mankind's use of animal collagen glue has been traced back over 8000 years, with the discovery of artefacts in caves near the Dead Sea, which contain this material as an adhesive. The Egyptians were also using collagen glue some 4000 years ago.
Laminated or "Plyed" wood artefacts from the Egyptian Pharaonic eras show that they were assembled using hide glue!
Historical records show that large specialised glue factories were established in Europe around the end of the 17th century and in the USA at the beginning of the 19th century.
Hide Glue, also known as Hot Glue, Scotch Glue or "Horse Sauce" in Australia, was used exclusively by furniture makers right up until the start of the 20th century, when the development of synthetic adhesives began to change the market for the product.
Making Glue in the 17th Century
The Americans abandoned the use of hide glue quite early on, but in Europe and Britain, the use of traditional glues continued until well after World War 2.
Today, traditional animal glues are generally used by museum conservators, antique restorers, top end cabinetmakers, reproduction period furniture makers and luthiers.
French Marquetry Specialist Paul Miller with his amazingly intricate Marquetry Jewellery Chest built with Hide Glue.
Animal & Vegetable Glue
The are two basic types of natural glues which are commonly used: animal and vegetable. An extremely wide variety of vegetable glues are derived from starches, gums, cellulose, bitumen and natural rubber, and they all have specific applications. Animal glues are derived either from casein (a milk protein also used in paint), blood albumen (used in plywood), and collagen (used in woodworking).
A massive positive for these glues in our modern world is that the products are completely organic in nature, are entirely non-toxic to humans and completely sustainable.
The Chemical Make-Up of Animal & Fish Glue
Animal glues are essentially high polymer proteins derived from hydrolysed collagen. These organic colloids are comprised of complex proteins found in animal hides, connective tissues and bones. The proteins essentially have two elements that define their characteristics: chondrin, which gives the glue it's adhesion strength, and gluten which gives it gel strength (gelatin).
These glues are made using a very simple process, which has not changed much over thousands of years. The raw material is first conditioned in a water and lime (Calcium Hydroxide) solution. Thereafter, the PH value is adjusted by adding diluted mineral acid and then it is rinsed with water.
It is at this stage that the cooking can begin. While the material is being cooked, the water/protein solution is extracted and filtered. The protein that is collected by the filtration is then dried and ground up to form the final product.
The resulting glue is then tested for it's viscosity (fluidity) and gel strength (stiffness of gel formation) and is then graded in a scale from 50 to 512.
Lower grades dry slower and are more flexible and higher grades dry faster and are harder.
Glue chipped glass is an artistic technique made using hide glue with a strength of 135 grams, which allows the glue to actually tear the surface off the glass as the glue sets! Show me a PVA adhesive that can do that!
Example of artistic Chipped Glass using Hide Glue
Animal hide and bone glues set in a two part process, which first begins by the cooling process from the optimum heat of 63º Centigrade (145º Fahrenheit) down to room temperature and then by completely drying by evaporation during the next 12 to 24 hours.
This process allows the traditional woodworker to use this glue to tremendous advantage, since "Hammer Veneering" and "Rubbed Joints" both require a glue with a rapid initial "Tack" or "Grab" capability as it cools. In addition, the strong initial hold of these glues allows the clamps to be removed way earlier that with PVA adhesives. So,... you can either use Hide Glue, or keep buying more clamps!
Protein Glues form a chemical (Molecular) bond as well as a mechanical bond. This is extremely important to note, as it means that fresh animal glue will re-activate and bond to previous animal glue surfaces as well as forming an incredibly strong mechanical bond with wood surfaces and other natural fibres.
Hide glue sticks to surfaces by means of electro-chemical attraction or "specific adhesion". It is also one of the few truly reversible glues, which can be changed from liquid to solid and back again by the addition or subtraction of heat and moisture.
A remarkable property of hydrolysed collagen glue is that it is easily modified to suit specific applications. It can be mixed with all water soluble materials such as sorbitol, glycol, sugars, syrups, metal salts and sulfonated oils.
It can be made more waterproof by adding 1% aluminium sulphate, alum (aluminium potassium sulphate), tannins and formaldehyde fumes. Adding salt and potash prevent brittleness over long periods of time and a 5% glycerine addition makes the glue flexible enough for the canvas backing on tambour doors.
Adding 5-10% urea or more by weight extends the gel time of the glue and also increases flexibility, producing a form of hide glue that is liquid at room temperature. (This is how the excellent "Old Brown Glue" is formulated).
Any of these additives will fractionally reduce the strength of the glue, but the final result will still be an adhesive bond that is stronger than the wood surface being glued and stronger that any PVA (polyvinyl acetate) adhesive.
Traditional Uses & Techniques
All pre-industrial woodworking processes that required adhesives were designed to maximise the working characteristics of hydrolysed collagen glues.
In the Northern Hemisphere cold climates in the 18th century, workshop stoves and open fires were used to preheat the wood surfaces and during the 19th century, furniture factories used specially heated rooms to enable the cabinetmakers to make use of longer assembly times.
The rapid setting action of the hot glue as it cooled is used to great advantage when "Hammer Veneering", as it allowed the craftsman to lay the veneers directly onto the final surface, cutting the joints as the work progressed.
The simple technique of "rubbing-in" glue blocks was universally used from clockmaking to cabinetmaking and it allowed rapid construction without the need for nails, screws or clamps!
Using clamps to merely keep the position of the five section coopered lid correct and square after rubbing the joints...
Tape is then used to hold the joint together...yes: tape!
During the 18th, 19th and early 20th centuries, hide glues were modified with additives to allow veneering of turned columns, which became a very popular and cost effective form of furniture decoration, and one which required a reversible glue to accomplish the task.
Hide Glue in the 21st Century
There is a wide variety of traditional animal glue applications that continue to be used by us as modern 21st century craftsmen. One interesting example is Rabbit Skin Glue which is essential for laying down Golf Leaf properly.
Musical instrument builders (luthiers) and antique restorers have a wide variety of applications that depend on the use of Hide Glues. For example, the fact that these glues can be tinted and mixed with many components allows the addition of Plaster of Paris to the glue for the laying of Ivory piano keys.
Marquetry workers add different colours to the glue when restoring "Boulle" tortoise shell components and to make their mastic.
A Boulle Brass & Red-Tinted Tortoise Shell Marquetry Cabinet, Louis XIV
Fish glue has certain properties which make it perfect for using with exotic materials such as tortoise shell, horn, leather, shark skin, cloth and metals. Fish glue is a liquid glue with a very strong "Cold Tack" grip and it's main use is to glue brass, pewter and copper in Boulle marquetry. Interestingly, fish glue adhesion can be further strengthened by first rubbing the metal with a fresh clove of garlic!
In this absolutely incredible Contra Partye pair of tables, master cabinetmaker and marqueter Aaron Radelow used Nguni Horn that he specially imported from South Africa. These are magnificent reproductions of the originals that were commissioned by Louis XIV (The Sun King) and made by the Royal Cabinetmaker, Pierre Gole.
You can see the story behind the tables and more of Aaron's astounding work at www.aaronradelow.com
Animal bone and hide glues are used individually and mixed together for all types of woodworking. Diluted animal glues are used for "sizing" and flattening veneers, as well as for "sizing" end grain and porous woods before sanding.
Brooklyn Tool & Craft Hide Glue Granules in 192, 251 and 315 Gram strengths
Problems with Synthetic Glues
The are a few characteristics associated with synthetic glues which can make these adhesives unattractive to furniture makers. These problems are usually overlooked in favour of the generally perceived "ease of use" features that tend to make a "ready to use" product handy within the workshop.
One of the most overlooked issue is the lack of "reversibility". Most furniture makers of today do not consider the future problems that synthetic glues create when the time comes to repair their creations.
It is an undeniable fact that all furniture will be subjected to abuse and damage at some stage in its life, and therefore it stands to reason that all furniture needs to be repairable, if it is going to survive through the generations.
Synthetic glues cure by means of catalytic conversion from one chemical state to another and are thus irreversible. This means that to take furniture apart that was made with synthetic glue requires destructive intervention and the physical removal of all of the pre-existing adhesive prior to the repair being executed.
Modern adhesives only have a mechanical bond and require tight fitting joints and very even clamping pressures. They do not bond to themselves, and set up unevenly: remaining wet in one area of the joint whilst setting dry in another. These adhesives all seal the wood surface and prevent dyes, stains and finishes from penetrating evenly.
These adhesives are also notoriously difficult to plane, sand, scrape and remove from the wood surface when dry.
One of the biggest problems of synthetic adhesives is the lack of resistance to sheering forces, which allow the wood to "creep" along the glue joint. This "creep" allows veneer joints to open up and solid wood joints to move during the clamp up process and over time, as wood movements continually occur relative to humidity and temperature fluctuations, as well as natural wood shrinkage due to ageing.
In Part 2 of this article I will explain how to go about using hide glue in your workshop and on your projects.
Hide glue perfectly ready to use.
Until next time, let's make shavings, not dust!