Waterproof hot glues

IMPORTANT NOTE (3/22/21):  Since publishing this article, I have learned that pine rosin fumes have been found to cause asthma.  The process described below should always be done outdoors.  If weather doesn't permit, then postpone it.

This year I've been trying to eliminate any period-inappropriate materials from my designs.  One that's stuck with me for a long time is rubber cement, which I've used to attach leather facings to wooden scabbards.  I use it primarily because in a real military campaign you couldn't reasonably expect to stash away your scabbard every time it rained, and rubber cement is waterproof, while for a long time the only alternative that came to mind was hide glue, which isn't.  (I understand traditional hide glue is less readily water-soluble than the modern kind that I've been using, which has urea added to keep it liquid at room temperature - traditional hide glue must be kept warm to work with it.)

Anyway, I've read years ago about making varnish from pine rosin by dissolving it in alcohol.  This can be used to protect sinew wrappings on arrows from softening and coming loose.  It recently occurred to me to ask if this varnish would be sufficiently sticky to use for scabbard facings before it dried.  I was in the process of making a new letter opener, and it seemed like the right opportunity to try it out.

If you've ever touched a sawn-off stump of a pine tree, you know that pine resin is incredibly sticky and impossible to wash off with normal soap and water; you need highly-concentrated alcohol (70+ percent hand sanitizer gel works well for this purpose) or some other nasty organic solvent like acetone (nail polish remover).  Resin consists largely of rosin, the solid component, and turpentine, a natural solvent that keeps the resin liquid.  Turpentine takes a long time to evaporate out of fresh pine resin, so resin is processed commercially by heating in a still.  Those who aren't trying to collect the turpentine just cook it off in an open pot.

I ordered some rosin from Creekwood Naturals for experimentation.  This product arrives as a coarse, sandy powder.  It becomes sticky when wet, but it seems to be waterproof.

My first experiments went thusly:  I dissolved some rosin in rubbing alcohol and let it evaporate to a thick, syrupy consistency that could be painted onto a scrap of sanded poplar plank.  For comparison, I also smeared some fresh pine resin on the wood, alongside rubber cement.  On top of all three adhesives I pressed a tiny swatch of chamois, then let the substances dry.

The fresh resin took the longest to dry.  After several days, all three swatches peeled off easily.  The resin was still slightly sticky under the leather; the rosin brittle; the rubber cement, well, rubbery.

I'd heard that beeswax is often used in rosin-based pitch glues to reduce the rosin's brittleness, so my next test used a mix of alcohol-rosin varnish reduced to a syrup and a bit of Sno Seal, which is emulsified beeswax used to waterproof shoes.  This did not work out at all.  The mix took forever to dry, and wasn't sticky enough.

Giving up on emulsions, I decided to try a hot glue.  I shaved some beeswax into a metal bowl and, working by eye, added about two to three times its volume in rosin, then slowly melted the bowl's contents with a heat gun.  At first, the beeswax melted while the rosin turned into a sticky mess, but with prolonged heating and stirring with a brass rod, the mixture became uniform.  (It also released fumes that stung my eyes, and when hot enough, began to smoke.)  Then I smeared a little onto the poplar and quickly stuck the chamois swatch onto it.

I repeat:  Wow!  Once cool, the glue bonded the chamois to the wood with immense strength.  When I tried to peel it, the leather itself ripped up before the glue failed.  I can dent the glue with my nail, but it's not sticky, and even though it's mostly rosin, it more resembles very hard, dark beeswax.

It seemed perfect, but now was time for the real test.

Earlier, I'd ordered a piece of pitch glue to try to glue the wooden halves of a scabbard core together.  It was tricky.  I warmed the glue, pinched off thin bits and laid them on the edges of one half, then melted them with the heat gun and quickly pressed the halves together.  Then I had to heat each edge in turn and press it more tightly, and scrape off or press down the lumps of glue sticking out of the seam.  I also had to shoot the heat gun into the scabbard, then stick the blade in to force any inward-protruding lumps aside and keep a good fit.

When it came time to apply the facing, I melted the entire bowl of rosin-beeswax mix, let it cool to a consistency sort of like room-temperature corn syrup, and smeared it as evenly as possible on the core with the brass rod in one-inch stages.  I melted it again with the heat gun, then pressed on the leather.  When each half of the facing had been applied, I heated the whole face evenly and pressed it against a flat surface to try to smooth out the lumps of glue that had built up underneath.  The result was still somewhat lumpy.  Finally, I stitched up the facing with linen thread in a double-running stitch, and trimmed off the excess.

My heavy reliance on a heat gun does call into question the practicality of this kind of glue in period.  Trying to do the same thing over open flames might well be difficult, as may trying this method on a larger, more complex scabbard such as one for an akinakes.  Nonetheless, when the time comes, this will be what I try.

I have no doubt that this glue will prove waterproof.  Even if it becomes slightly sticky in the rain, the idea of it bleeding through the leather is inconceivable to me.  To my mind, the real test will be whether it softens after a few hours in the hot sun, for which purpose I'll have to wait for the right weather.

As for the letter opener itself:  The blade is another full-tang dagger blade from Atlanta Cutlery, with maple scales and steel pins.  I did reshape the tang to be a little more like a classic Luristan dagger, but while it affects a Near Eastern Iron Age look and I'm using it as a test bed for period materials, I don't believe this object bears sufficient resemblance to any Achaemenid-period dagger to warrant actually using it at reenactments.  (I would, however, love to be proven wrong.)

As an aside, I've bought four Windlass Steelcraft blades this year, and their geometry seems to have worsened compared to my previous purchases, with slightly thinner spines and much thicker, squared edges that will take forever to sharpen.

The scabbard, with its central ridge, straight lines down the edges (here interpreted as facing seams), and rounded tip without a chape, is modeled after those shown in Neo-Assyrian art such as the Lion Hunt of Ashurbanipal and at the Palace of Sargon II.  Its core is made from American commercial poplar (tulipwood), probably yellow poplar, which is simply the low-end hardwood we usually get in my part of the country.