Sunday, December 5, 2010

Why Use Hot Hide Glue?

Years ago in elementary school I remember being warned not to eat my glue because it was made out of horse’s hooves. I thought this was an odd thing to tell someone since I couldn't imagine any reason to eat glue in the first place regardless of what it might be made of. Many years later I learned that my classmates weren’t quite correct.   We used good old Elmer’s white glue, or polyvinyl acetate (also called PVA), which is a chemist’s concoction and contained no hooves. However, there was a time glue was indeed made from hooves, skins and other bits of connective tissue so I suppose my classmates weren’t completely off-base.

Today I use traditional hot hide glue almost exclusively both because it’s use is historically accurate to the 19th century and because it has several advantages over modern PVAs, polyurethanes and epoxies. It is reversible, cleans up easily, is non-toxic and essential for repairing antique furniture.

Hide glue is rendered from connective tissue and other parts of animals that contain lots of the protein collagen. The treated hides and other parts are boiled in large vats of water, the nasty scum skimmed away and the reaming accumulation collected and dried. During the 19th century the glue was usually shipped in small blocks but today is broken up and sold in granular form. I keep mine sealed in an air-tight container: an old mason jar.

Here is some granular hide glue up close. Since hide glue can be re-heated and re-used it once was the responsibility of the shop apprentice to gather up all the waste bits of glue and return them to the glue pot so nothing would be wasted.

The granular glue is soaked in cold water over night until the water is absorbed. The usual ratio of glue to water is one part glue to two parts water. Thicker or thinner glues can be cooked for certain occasions. I use a cast iron glue pot which is actually two pots in one: a smaller pot which holds the glue/water mixture and an outer jacket pot that holds water and functions like a double boiler.

I place the glue pot on our shop's wood stove until it reaches about 150 degrees. Despite being made from the skin and tendons of farm animals the hot glue has almost no odor.

Once the glue reaches the correct temperature it will become viscous, have the consistency of egg whites and can easily be applied with a brush.

One disadvantage of hide glue is its short open time. That means it sets up quickly so large glue-ups must be planned and executed quickly. Here I am doing a simple glue-up of a small table top. I apply the glue liberally to both surfaces with a glue brush. It is important to use plenty of glue and not to clamp so firmly that all of the glue is squeezed out. Sometimes urea was added to the glue to increase the open time (it is also added to liquid hide glue which you can buy in bottles) but this can weaken the glue if too much is used.

I sometimes clamp when using hide glue (especially when its cold) because it can thicken and gell so you don't get a tight joint. In this case I rubbed the joint until it just began to stick and then quickly clamped the pieces. As you can see it can be a messy process but excess glue can easily be softened with a damp cloth and and then scraped away with a knife. I have dedicated an old, dinged-up chisel to the task of scraping away the gluey mess. Be sure the surfaces are warm. If they are too cool the glue will shock, or gell, and the joint will not be tight.

So, why use hide glue? Its greatest advantage is that it is reversible. Heat and warm water remove spills and messes. Reversibility is particularly important when restoring or conserving antique furniture because any work that you do can be undone later on with no damage to the piece. Epoxies, PVA and and polyeurathane glues often cannot be satisfactorily removed and can leave residue. This residue cannot be dissolved by fresh glue meaning later glue-ups might not adhere to the old glue and result in a weak joint. Old hide glue can be scraped away quite easily or reactivated with heat or fresh hot hide glue. It is very strong and (depending on the grade of glue) creates a bond that rivals that of modern epoxies. It is also compatible with stains and finishes. Since modern glues can leave residue in wood pores there are often spots and blotches when finishing. Hide glue can even adhere to glass!

Like all glues, it does have some weaknesses. It is not resistant to water, although some additives can be mixed in to make somewhat more resistant. It does not fill gaps well, meaning close-fitting joints are essential. I am convinced that most of hide glue's poor reputation comes from failures attributable to poorly fitting joints rather than problems with the glue itself. Left-over glue will deteriorate quickly if left in the pot so it is best to make just enough for your project and throw the remainder away if not used within a few days.

Even though it is all-natural and non-toxic I cannot recommend eating hide glue. But, if on a dare, make sure it is hot hide glue and not the stuff in the bottle. I hate to think where the urea might come from that is added to keep it liquid. Bon appetit!

1 comment:

Anonymous said...

Good piece!