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In contravention of all the rules of web page design, this page is long and all text. That's why I'm putting this blurb here. If you'll bear with me, I think you'll find it's worth it. This article does two things: it compares hide glue to yellow carpenter's glue and highlights the strengths and uses of hide glue. Second, it explains how to buy and use hide glue without putting you in the poor house. As there is enough text below to choke a horse, I'll stop right there.
Back when young men were dressing like Don Johnson, and I was still trying to make sense of Boy George, I made my wife a Colonial style coffee table of pine. She still loves it although the table has received its fair share of bumps and bruises since the cancellation of Miami Vice. A few summers ago, the air thick with humidity, I stripped and sanded the clear finish from it – and sweated off more than a few pounds. As I applied the stain she selected, light-coloured blotches appeared around the joinery where the yellow glue squeezed out during the original assembly. Unfortunately, this is one of the characteristics of PVA glues that woodworkers have to work around. Had I used hide glue, the blotches would never have showed.
A few months after refinishing that table, I attended a woodworking class where I learned about hide glue. What an eye-opener. Like most woodworkers, I had been led to believe that hide glue is difficult to work with, requiring careful mixing and heating - and expensive equipment. However, hide glue is safe, natural, non-toxic and has been the mainstay of wood glues (at least) since the time of the Pharaohs. Advances in 20th Century chemistry have provided us with the ubiquitous yellow carpenter’s glue, also known as PVA or polyvinyl acetate. Unfortunately newer glues have some characteristics that can be a real pain in the glue pot.
Hide Glue and PVA Compared
PVA is plastic - an artificial, organic polymer made from petroleum. Yet other glues contain highly toxic solvents but hide glue is still made the same way it has been for centuries. While the odour may cause you to hold your nose, it won’t hurt you. The smell lends an air of authenticity to the workshop, conjuring thoughts of craftsmen and projects long forgotten. However, there are many other, more important benefits to consider.
Homeowners email me to ask why loose chair legs re-glued with PVA frequently come apart again despite their best efforts. All wood glues bond mechanically by penetrating the surfaces they are applied to, the bulk of the holding power created when glue is carried several cells deep into the wood. As yellow glue is inert once cured, it cannot be reactivated so the woodworker is left but one option. For the leg repair to endure all traces of PVA must be removed from the wobbly joint or dried glue will remain embedded in the wood cells, displacing the new glue and preventing it from penetrating. Hide glue is completely reversible, offering a less demanding solution to our broken chair problem.
While PVA requires a host of chemicals to cure, hide glue needs only heat and moisture. Being reversible, an old towel damp with hot water from the kettle can be wrapped around the joint to reverse the bond, releasing the leg. This is especially true of antiques because, as long as the piece dates before 1900, it is guaranteed to be joined with hide glue. The best part of using hide glue is you don’t need to scrape any of the old glue from the joint. An application of fresh glue will rejuvenate the old, even the stuff that has penetrated the wood cells, making the joint as strong as the original. This property can be helpful on new projects too.
If you’ve ever had a glue-up go awry or discovered an oversight during assembly, you may appreciate the ability to take everything back apart and start afresh. Apply a little hot water or steam then wait until the dismantled parts are dry again and you get a second chance at perfection. The characteristics of hide glue that make this possible also provide a solution to the light coloured blotches on my coffee table.
Carpenter’s glue is yellow because it was coloured. PVA can be coloured as a monomer but once cured, it becomes an inert, solid plastic and cannot be dyed or hold pigments. Hide glue can be stained and coloured so a finish cannot be ruined by a little squeeze out caught in the pores of the wood. Actually, when mixed with a little matching sawdust it can be used as an effective filler that can be finished to blend in with the rest of the project. However, while reversibility and its ability to accept color are probably the biggest pluses when working with hide glue, it has many other positive attributes.
As the moisture evaporates from curing hide glue, the polymer chain bonding the wood shrinks to less than a third of its original size. This makes the glue rock hard so the longevity of the joint is affected only by your skills as a woodworker. The better the fit, the longer it lasts. PVA adhesives are subject to creep – changes in the shape and size of the cured glue over time. Creep occurs because yellow glue is pliable after it dries. If you’ve ever cleared a blocked nozzle on a glue bottle, you know that the blockage is easily bent. PVA manufacturers add plasticizers, like dibutyl phthalate, for just that purpose (it can’t be good for the environment either – I can’t even pronounce it). In fact, they make yellow glue more flexible than white glue so the bond won’t fail when working with softwood framing lumber, which is worked at a much higher moisture content than hardwood used for furniture. The extra movement may lead to joint failure even with tight joinery. Yet, there are things you can do with hide glue that are impossible with PVA.
Hide glue has a naturally high tack (it won’t skate around during a glue up like yellow glue can). For joints where strength is not important you can skip the clamps creating a rubbed joint. Apply hide glue to the edge of two boards and bring them together with a rubbing motion. Hold the stock in place and in a short time you have a solid bond. Caution should be exercised as clamping pressure is required in most applications so the glue penetrates the wood for maximum strength.
The high tack of hide glue is very useful for many tasks where you require a quick bond with no more than a few seconds of finger pressure for clamping. Small pieces of wood can be used as low tech stops for drawers and cabinet doors. A couple of dabs of hide glue will temporarily affix clamping cauls to the sides of a dovetailed, solid wood carcass: they can be tapped off with a mallet after the clamps are removed and a couple passes with a plane will remove all traces of evidence. And don’t be concerned over how long the glue will last.
If you are worried about hide glue joints being weaker than PVA, you can set your mind to rest. Testing shows that, under shear stress, the wood around the joint fails before the glue line. This is true of both adhesives. PVA and hide glue bonds are usually described with the oft used phrase, “stronger than the wood itself” which means that neither glue will create a weak point in a properly executed joint. Beyond that it matters little which glue is stronger. Aside from strength, you may hear that hide glue is not suitable for projects susceptible to high humidity. Both hide glue and PVA are degraded by moisture but for your fine woodworking they are perfectly acceptable. If you are building a picnic table, I would avoid both.
It’s interesting to note that both hide glue and PVA require water as a wetting agent to help them flow across and penetrate the bonding surface. PVA glue is purchased with the water already added, placing a finite limit on its shelf life. Hide glue is sold as a dried powder and, as long as you keep it in an air tight container, will store indefinitely. While you can grab a bottle of yellow glue off the shelf and dive right in to your next assembly, using hide glue will take a little more preparation.1
Using Hide Glue
Hide glue does require planning ahead but it’s as easy to use as any other adhesive. And it doesn’t have to be expensive. Once you get it mixed and heated, I find hide glue spreads faster and covers better than PVA, making my glue-ups go much smoother. After preparing the glue once or twice you’ll find that it’s simple and fun and won’t take much of your time.
The first item you will require is a glue pot. Look at any woodworking catalogue and you will find them retailing for around one hundred dollars - I imagine this is one of the key reasons why hide glue is overlooked. That sum of money will buy a pile of sandpaper, a case of yellow glue or some tools. However, you don’t need to spend next month’s rent on a glue pot. I found an interesting alternative on the internet (www.spurlocktools.com) in the form of a thirteen dollar hot pot with a jam jar to hold the glue. (The hot pot is a Rival model 4071.) A baby bottle warmer will also work. The commercially available glue pots will hold enough glue for a woodshop full of woodworkers while my little hot pot keeps a more manageable amount of glue at a consistent temperature and has proven very reliable. However, a new glue pot must be calibrated before use.
Before use, I placed between one and two inches of water in my glue pot along with several cooking thermometers I found in the kitchen. I recorded the temperature over a period of two to three hours, adjusting the thermostat until the thermometers averaged out at 140º. Finally, I marked the position of the knob on the outside of the hot pot so if anyone played with it, finding the correct temperature again would be easy. After calibrating the glue pot, it’s time to check the recipe.
Not wanting to make a mistake with my proportions, I did extensive reading and testing to find the perfect ratio of water to dry glue crystals. After much research and several calculations I found that my alleged perfect proportion was identical to the manufacturer’s simple instructions. I can safely say that hide glue is very forgiving and finding the right ratio is not complicated. I use 1 part water by volume to 1 part glue crystals by volume. I placed two equally sized scoops of water in my jam jar using a small container, and I marked the level on the jar after each scoop with an indelible marker. This serves as a quick way to measure out equal volumes of glue crystals and water.
To mix, I let the glue and water sit until it turns to jelly – stirring once or twice in the first ten minutes. After a half hour to forty five minutes I set the jar into the preheated water in my hot pot and let it come up to temperature. You could begin heating the glue immediately but it will need to be stirred occasionally. Once the glue is ready it should flow off your brush like pancake syrup and can remain, ready to use, in your glue pot all day. You can even reheat it the next day but after that throw the rest in the flower bed – your roses will love you. That’s all there is to the process but keep in mind that there are different glues available.
Hide glue is graded by its gram strength and this can vary from under 100 gm to over 500 gm. Gram strength is measured in grams (gm) and is equivalent to how many grams of force needed to push a half inch plunger down into a solution of glue, where the protein level in the solution is 12.5% by weight and the temperature is 10° Centigrade. That’s more information than you wanted to know but you’ll need it when choosing what strength to buy.
As you decide on a grade of hide glue, remember this important generalization; as the gram strength of the glue increases, its bond strength increases but its working time is reduced. Hence a balance must be struck between strength and open time. Although the hide glue you find in your average woodworking store may have a gram strength of 250 gm, I prefer a glue closer to 200 gm because I like a little extra working time before it gels. This is nice when you have a complicated assembly, or when working with dovetails. Anything less than 150 gm is too weak for practical woodworking purposes. Being a natural product, some variation should be expected so when you purchase new glue, make a test batch in case you need to adjust the recipe.
In Conclusion
For years I have worked to learn time-honoured, traditional methods of building furniture. While I may eschew some forms of gadgetry, I am not one to overlook a better way of doing things – especially if it came out of the past. I won’t stop using yellow glue. It is a great product and is indispensable in the shop. Yet, hide glue solves more of my woodworking dilemmas every day. Don’t be intimidated by those who say how hard it is to mix, or how expensive the equipment is. Be creative. You can be set up to use hide glue for under $20 – easily.
Once you discover how hide glue can help you innovate and save time you’ll be as hooked as I am. Hide glue may not be a product you use straight off the shelf but the time taken to prepare it is offset by its wide variety of positive attributes and, with the glue pot gurgling away, the scent of fresh glue in the air will render you wistful and nostalgic. Hide glue is a valuable product that definitely has a place in the contemporary woodshop.
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