Certain things in life are as ancient as dirt. Or as ancient as hide glue. Archeologists confirm hide glue use at least as far back as ancient Egypt. Hide glue was utilized by Egyptian carpenters to construct sarcophagi, and artists used it as a pigment binder to paint the surfaces of these elaborately ornamented burial boxes for the pharaohs.
Ancient as it may be, animal hide glue has certain properties that make it extremely desirable, even in the 21st century. One of its chief beneficial properties is that guitars assembled with this glue can be repaired more easily, and with less damage than the ones assembled with synthetic adhesives.
To prepare a batch of hide glue for use, it is soaked in cold water, allowed to swell, and then heated until it is dissolved. Lately, I’ve been using antique (late 1800′s) cast iron glue pots. They are somewhat collectible, fancier, decorative examples fetching a pretty penny on ebay. A glue pot is basically a double-boiler, meant to heat the glue gently, without cooking it, and destroying its adhesive properties.
Hide glue consists of rendered animal collagen, extracted from skins and tannery byproducts. It is similar in nature to food-grade gelatin. With proper technique, wood joinery with hide glue is strong, with nearly invisible glue lines. When needed, glue joints can be released cleanly, without tearing-up the wood and destroying delicate surfaces of guitar soundboards, braces, etc. Most modern synthetic adhesives lack this quality of reversibility.
Traditional glues have been largely supplanted by modern synthetic adhesives, although not entirely. Some luthiers, museum conservators, period furniture restorers and woodworkers, along with a few other specialty trades continue to rely on the time-tested animal glues.
I’ve done a number of experiments over the years, trying to better understand the nuances of hot hide glue.
Hide glue has a pleasant, honey-like consistency. Even the way it flows off of the brush reminds me of my childhood, working with my dad, grandpa and uncles at the family apiary. As I use the glue, I constantly monitor its consistency and flow. From time to time, additional water needs to be added to replace the moisture lost to evaporation.
A good friend who is a talented ceramics and clay artist helped me make some ceramic inner pots to compliment the original cast iron ones. The original inner pots were made of cast iron and had an enamel coating inside. Over the years, the force of drying glue stripped the enamel, exposing the cast iron below. This rusty iron oxide has a tendency to stain the glue. While slightly brownish glue may not be problematic for darker woods, spruce glue lines would benefit from clean, unstained glue. This is where this new ceramic inner pot comes handy!
Here is an example a couple of different grades of glue, and the way the look in dry form, before they are mixed with water and heated. On the left is pearl glue, and ground hide glue on the right.
If I were to keep only one glue in my shop….it would be the good stuff–hide glue!
The following is for lutherie geeks only (since you are still reading)!
Over the years, I’ve experimented with different glue-heating setups, from larger commercial pots, to DIY, re-purposed contraptions. The one below was made from a yard sale special, a water heating pot. After all, we just need a warm water jacket around our glue vessel. Not the fanciest set-up, but it worked okay. The temperature control (rheostat) on this tea pot was not very sensitive, and maintaining stable temperature was a bit tricky.
I also used the same pot with a baby food jar, installed in the original teapot lid (modified with an opening to accommodate the glass jar).
Here is a hefty, cast aluminum large capacity glue pot, with a solid copper liner. It was manufactured under Sta-Warm brand in the USA. I thought this pot was a higher-quality, sturdier version of a better-known Hold-Heet glue pot, which is still available through woodworking stores and luthiery supply houses. After using it a bit, I sold my Sta-Warm behemoth as it was too large for my needs. Otherwise, it was an excellent example of a plug-in type pot.
Currently, I use a laboratory hotplate to heat my glue pots. I like the absence of electrical cords around my bench. These antique glue pots are the original cordless equipment! Cast iron + water have enough mass and sufficient heat capacity to allow me to glue for 15-20 minutes before the pots need to return to the hotplate.
Slightly off-topic, but still related to hide glue….the glue brush. Traditionally, they are made without metal, in order to prevent the glue from becoming stained or discolored. A traditional glue brush can be purchased from a violin supply house, or even better, it can be made in house! All that’s needed is a few minutes and some bristles.
Glue…the difference between a guitar, and a pile of kindling.