#26 - World's First Hide Glue Composite

Sep 08, 2020

“Maxim of the pig: ‘Use everything and make sausage of the rest.’ ”

— F.E. Kleinschmidt

“Maxim of the sausage: ‘Use everything and make hide glue of the rest.’ ”

— S. Singh

Hooves and hides and bones, oh my! If you’re anything like me, you want to pair those ingredients with composites. You know, the cutting-edge technology of Formula 1, of supersonic jets and spaceships.

If you’re not like me, hide glue and composites probably don’t seem like a good match. A typical composite is made with a fiberglass or carbon fiber cloth and an epoxy glue. Epoxy is a remarkable material for its mechanical properties and its resistance to heat, moisture, and chemicals. The fact that it’s so widely used across industries should clue you in to its all-round performance. I only have one complaint: I hate working with it.

Previously, I’ve grumbled about having to sand off some of my table to clean errant drips. Worse than that, though, is the itchiness. Getting any on your skin leads to rashes; the more you’re exposed, the worse the reaction gets. I haven’t worked with epoxy that much but epoxy and I have made up for the infrequency of our encounters with intimacy. Long story short, in college, I spent a few hours with my whole upper body inside a mold that was essentially an epoxy cavern. I had three trashbags as protection -- one on each arm and one over my head. At the end of the night, my forearms and thighs were soaked in epoxy from leaning on the mold, and no amount of scrubbing could save me. I’m at the point now where even catching a whiff of epoxy sends phantom itches spreading across my arms.

Enter hide glue, a substance so safe that it’s apparently used as a hand soap for clingy oil and grease. In fact, the “glue” part of hide glue comes from collagen, which people are rubbing on their faces for youthful skin. Maybe if I work with hide glue enough, I can resurrect my dreams of a hand modeling career.

Given how safe it is, it’s surprising that hide glue has been relegated to its current niche of antique furniture restoration and violin-making. I only learned about hide glue two weeks ago, but that won’t stop me from speculating about its decline. I’m guessing there are concerns about its longevity. Natural materials seem to have an undeserved reputation for decay. I say “undeserved” because hide glue has been found in intact furniture from ancient Egypt, like this chair from 1300 BC.

Another reason for its lack of popularity could be the time and skill it takes to prepare it. With epoxy, you stir together the resin and the hardener and that’s that. Hide glue, as I found out, is more involved. Luckily though, I didn’t have to collect hides; I bought a bag of hide glue off Amazon. The glue came as pellets/granules that surprisingly didn’t smell of anything. Then, following a woodworker’s instruction on YouTube, I mixed equal parts of granules and water in a glass jar.

Once the glue combined into a gelatinous mass, I popped the jar into a pot full of water and brought the water to a boil. The glass rhythmically clinked against the side of the pot as bubbles formed and were trapped and then released, like a scene at some pleasant old apothecary. Soon the glue melted down to the consistency of a light syrup, and I was ready to start.

But I haven’t even mentioned what the glue’s for. A few weeks ago, I made a fiberglass panel for my Saab 96. This time, I planned to make the same panel again, but with hide glue replacing the epoxy. For a fair comparison, I kept my methods on this fiberglass/hide glue part as similar as I could to the previous fiberglass/epoxy part.

While the glue was melting down, I had cut the fiberglass cloth down to size and released the mold with the same combination of wax and PVA as before. My PVA skills are improving; it dried to a nice even film, which told me the mold was ready to go.

I took two approaches to starting the lay-up. On the right half of the mold, I brushed on a coat of hide glue directly onto the mold. I’ve done this before with epoxy -- the idea is that a thin layer of epoxy ends up on the outside of the part, functioning like a clear coat. This time, I don’t think it was such a great idea. With the first brushstrokes, I could see that I was disturbing the PVA. Either the thicker-than-epoxy hide glue dragged it out of place, or the water in the hide glue dissolved it, since it’s water-soluble. Whatever the reason, the layer of hide glue was lumpy, which as I found out later, would create air bubbles on the panel surface.

I laid down the first layer of fiberglass -- directly on top of the PVA on the left half, and on top of the hide glue on the right half. My next step was to wet out the dry fabric. As I painted, I could see that the top of the fabric was getting wet, but I couldn’t be sure how far the glue was penetrating. I learned as I went. The glue on the brush would cool and thicken fairly quickly, so returning frequently to the glue pot for hot glue was the way to go. A more liquid glue had a better chance of fulling penetrating the fabric. Leaving the brush in the glue pot during breaks also allowed for the glue in the brush to reheat and liquify.

The contrast to epoxy was interesting. With hide glue, I felt that it was easier to avoid introducing bubbles into the part. Epoxy stays liquid for much longer, so pulling on any part of the fabric brings a lot of fabric along for the ride. At my skill level, whenever I tried to manipulate the fabric in one area to get rid of a bubble, it would shift the fabric somewhere else and introduce a bubble there -- a constant game of whack-a-mole. Because hide glue becomes tacky in about a minute, I was able to avoid that issue. I could work one area and not worry about it again, confident that it would stay put when I moved on to a different area. The fabric wasn’t fully locked down, but it was a lot more stable.

There’s always a tradeoff -- I found the downside of that quick tackiness when I added the second layer of fiberglass. Excess glue from the first layer had pooled and dried in lumps. Maybe heating the mold (or making the part inside a sauna) would keep the hide glue liquid longer. In any case, I soldiered on with the second and third plies.

I did have to stop a few times to wash my hands. As the glue stuck to my fingers and dried, I transformed into Edward Fiberhands, with limp strands of fiberglass dangling from my fingertips. Though I’m used to manipulating epoxy composites with my fingers, hide glue is probably best dealt with a brush. Thankfully, it washed off easily.

The next morning, it was finally time to see if this wacky idea of mine had worked.

Actually, I wasn’t too worried about what I’d find. I knew what to expect based on a test piece I’d worried over two days before. During that trial, I’d checked on the piece a few minutes after applying the glue, thinking that it was supposed to be fully dry by then. It wasn’t. Dejected, I wrote in my notes, “Let it sit for a while to see if anything changes. Don’t think it will.” After letting that test dry for another twenty four hours, I was surprised to find that it had stiffened up and could hold its shape. If this were a Disney movie, there would be some sappy life lesson about believing in yourself. But this is real life, so the lesson is to pay attention when you’re reading about hide glue so that you know it takes a day to fully cure.

But let’s return to the story and pretend I didn’t already know what to expect.

That fateful morning, I approached the part with trepidation. Fingers shaking in fear of what I would find. Would it be sad lump of wet fabric? Would it still be sticky? Would it work at all? I inched closer, stomach doing somersaults, heart in mouth, and (naturally) mouth in heart, kidneys performing a triple axel followed by a double salchow, and other organs all akimbo. I extended my hand to make first contact. Solid! I was saved!

Alright, I’m done being dramatic.

The part had a nice sepia tone to it, but I could see one small issue straight away. The short sides of the part had lifted away from the mold. I’m guessing that, as the panel dried, the glue contracted and pulled the fabric along with it.

Ah well, not a big deal to me right now, but something to keep in mind for the future. It did give me somewhere to insert my popsicle stick and pry the part from the mold. Thankfully, this was pretty uneventful, with just one small issue: on the right half of the mold, where I had added that preliminary coat of hide glue, I could see the popsicle stick deforming the part. When I pulled the part free, I confirmed the issue. The right half of the part was still soft. The solution was easy -- I just left the part to dry for another day.

The part looked good overall. There were a few issues, but that’s not surprising considering it was my first time.

I somehow managed to make three different zones in one part. On the right side, there were air bubbles from the disturbance to the PVA.

In the middle, there was a dry patch where the glue didn’t penetrate fully.

And on the left side, I found my Goldilocks zone with just the right amount of glue.

Not too shabby at all. I didn’t really have any expectations coming in, but combining fiberglass and hide glue ended up working out surprisingly well. This is a glue that’s been around for at least three thousand years. And I combined it with fiberglass and it worked! I’m not much of a chest-thumper, but I’ve been told that I need to give myself more credit. So well done me.

All that was left to do was trim the part down to size. I whipped out my Dremel-like rotary tool and the diamond-coated engraving bit that had worked on the previous epoxy/fiberglass part. This time, when I started cutting, there was a fair amount of burning glue. I didn’t have any better ideas though, so I kept on burning/cutting my way around the panel.

I also noticed that the edge was fraying. I think this was because of a lack of glue, which either was melting away from the edge as I was cutting, or just hadn’t penetrated all the layers fully.

I now had two versions of my panel. (The more translucent one is the epoxy version.) Compared to the epoxy, the hide glue was a little less stiff, but not by a huge amount.

Here’s the panel mocked up on the Saab.

With all its issues, this panel wasn’t going on the car permanently, so I took the opportunity to experiment. The articles on hide glue all mentioned its reversibility -- apparently, you can undo a glued joint by applying heat or steam. I figured I could use that ability to change the shape of my composite even after curing. So I steamed the panel over a pot of boiling water.

And bent it.

The panel dried stiff in its new shape. How cool is that? To the uninspired, it looks like I ruined all my hard work. To the inspired, it looks like I unlocked a world of possibilities. I’ll definitely keep hide glue in my back pocket for prototyping, where being able to adjust the final shape could be useful.

I’m not sure how a hide glue composite would fare in a more permanent application. Hide glue is traditionally used in internal joints, which are fairly protected. In an exposed composite, it might deform from heat and rain, like it did from steaming. But before I commit to hide glue, I have one more natural glue in mind. That’ll be next time.

Corrections? Questions? Comments? I’d love to have your input. Leave a comment, email me at surjan@substack.com, or find me on LinkedIn.

An Engineering Self-Study

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