The X1 Leather Hardening Method

The first and second drinking vessels I made using the X1 method

This article details specifically the “X1” leather hardening technique I developed in late 2018 as an attempt to discover a method of creating hardened leather with the best balance of qualities including strength, cut resistance, pierce resistance, water resistance, and resistance to extremes of temperature, while minimizing brittleness and expense. The X1 method was the best of over 16 methods attempted. “X1” was the designation given to the first of several “cross” methods I attempted. Find the full article here. This hardening technique is perfect for a wide range of applications including drinking vessels, armor, tool sheaths, cases, and others. Click here to jump to the 6/25/21 update, investigating the use of carnauba wax using the X1 method.

We’ll begin with a full step-by-step “how-to” for achieving X1 hardened leather, followed by further notes to consider for leatherwork projects, and troubleshooting. This method has been tested only with vegetable-tanned cowhide, and I can’t vouch for results when using other leathers.

Materials needed

Your leather project made from vegetable-tanned cowhide
Stearic acid- enough to fully submerge your project. I buy mine here. (No affiliation)
A metal container large enough to melt the stearic acid and immerse your project
Hot plate or stove for melting stearic acid
Thermometer- infrared is handy. Needs to measure molten stearic acid temps from 130F – 200F+
Method to achieve 150F dry temperature- I use a toaster oven
Rags or paper towel

Step 1: Ensure your leather project is absolutely bone dry.
I use a digital moisture meter to be sure. Any moisture remaining in the leather will cause warping and shriveling of the leather. Stitching, tooling, and dyeing should be done prior to hardening. Be aware any adhesives such as Barge, Weldwood, and others will fail completely during hardening, so all pieces should be well stitched together. The hardening process results in significant darkening, which tends to weather/lighten considerably and quickly with handling, and results in a beautiful color pattern of a somewhat darker shade of the dye used.

The ONI logo stamp after hardening. This is how I learned the importance of fully drying the leather.

An un-dyed fire tool showing age after just a few days of handling.

Step 2: Preheat your leather to 150F. I had surprising difficulty finding a way to hit 150F precisely, and currently use a toaster oven. I set the project in the oven with a remote probe thermometer on the “keep warm” setting, and nudge the dial back and forth until it settles into an acceptable range of +/- 5F of 150. Too hot, and the leather will begin to plasticize and prevent full saturation of the stearic acid. Too cool, and the acid will soak into the leather too slowly and create an inferior end product. Keep your leather at 150F while you prepare the stearic acid.

A piece of scrap leather is left inside the oven to set items on. This prevents lines from the metal rack.

I also use the toaster oven to dry my leather. The dial is cranked up a little higher, door propped open, and a fan aimed at it to circulate the air. Just be sure to keep it under 150F.

Step 3: Fill your melting container with stearic acid, place on your heat source. Melt the stearic acid and then turn off the heat. Allow it to cool to 150F, +/- 5F.

Step 4: Immerse your preheated leather project into the stearic acid. Observe for bubbling as the leather soaks up the acid.

An eyeglass case soaking up stearic acid

Step 5: When bubbling ends, turn the burner up to high. When the acid temperature nears ~200F, remove the pot from the heat. It’s very important to continually and gently stir the acid while you raise the temperature, to ensure an even distribution of heat and therefore, of hardening.

Step 6: Allow the pot of stearic acid with your project immersed, to cool to about 145F. Remove your leather item.

Step 7: Immediately begin wiping off excess stearic acid from the leather with a paper towel or rag. Work quickly, as the stearic acid will begin to haze in less than a minute. You have a brief window of time to make any adjustments to shape as well.

Molten stearic acid is shiny as it oozes out. Wipe it off gently and the item will appear dull.

Step 8: Allow the leather item to cool. I sometimes place the item in the freezer for ten minutes, or outside in cold weather.

Step 8: Once cool, the leather will be fully hardened. Finish edges by sanding from 220 up to about 3000 grit. After sanding, use dye to match the edge color to the rest of the piece. Rub briskly with canvas to buff. Then soften a bit of beeswax over a flame, run it over the edge, and buff again with canvas. This process results in a beautiful, durable, high gloss finish. The entire surface of the item can be buffed with canvas as well, to give added gloss.

The beautiful high gloss is easy to restore as needed with just a scrap of canvas.

Additional notes when working with X1

I use a belt sander for the early stages of edge finishing. You’ll find that sandpaper very quickly gets loaded with stearic acid and leather, and I strongly recommend a belt dressing device to get much more use out of your belts.

This block of rubber from Harbor Freight quickly removes buildup on belts

The flesh side of X1 leather will also burnish well, with no compound needed. Obviously, since the surface is far coarser, it will need more sanding at a coarser grit to begin, but polishes up beautifully.

Dressing up the exposed areas of armor for a bit of shine

Do not use a heat gun to soak up excess stearic acid after hardening as was mentioned in the original article. This will leave an inferior, chalky finish. Instead, repeat the immersion in stearic acid. Bring the acid up to 190-200F just briefly, and remove the leather. Wipe off excess thoroughly, to avoid pooling.

It’s difficult to adhere anything to stearic acid treated leather, but I have been successful using coad to glue snake skin to a hardened bracelet. This is a mixture of 50/50 pine rosin and beeswax and is easy to make at home. Google will yield plenty of methods.

This is then stitched down to the hardened piece.

X1 leather will take up dye to a limited degree. This is useful for covering scratches or redoing edges.

Layers of leather can be glued and stitched together to greatly increase thickness. These pieces should be given extra time to preheat and extra time to soak up the stearic acid. I also recommend raising the temperature much more slowly, to ensure that the thermal hardening effect is as uniform as possible, throughout the thickness of the leather.

If the leather is intended to be shaped around a rigid form, then wet-forming the piece prior to hardening is often unnecessary and can be skipped entirely. Since the leather remains pliable for a few minutes after removal from the stearic acid bath, that time can be used to form the leather around the desired shape before final hardening occurs. This can be done by simply holding it with the hands, suggesting the shape using blunt tools, wrapping with plastic wrap, vacuum sealing, or other methods. Keep in mind that the finish may be ruined by attempting to alter the shape of the piece while it’s too cool. If extra time is needed for shaping after hardening, remove the leather from the stearic acid bath while it’s a little warmer than you usually might, to allow longer working times.

Troubleshooting

The most common mistake I’ve been seeing in feedback from others attempting the technique, is a failure to follow the temperatures and times recommended. Understand that thinner leathers and larger pieces will remain somewhat flexible. The most rigid items are those made from 10 oz leather or thicker, and kept in small pieces or made as cases or containers, lending stability due to their geometry.

I’ll update and add to the troubleshooting section as I receive feedback. I also plan to write another article with new developments on the X1 technique, including torture testing with arrows and bullets, comparison with kydex and sheet steel, and others.

UPDATE 6/25/21

It recently occurred to me to give carnauba wax a try using the X1 method, given that it’s the hardest natural wax. As with the original 2018 study, I created three small, domed disc samples of original, stearic acid and heat, “X1” hardened leather, as well as three samples using 100% pure carnauba wax, but otherwise following the same method for combining this with a thermal hardening effect. I performed all tests as similarly as possible to the originals, and compared results. I found that carnauba wax is indeed very hard, more like a plastic or resin than a wax. It also melts at around 180F, while stearic acid melts at ~150F. Carnauba’s melting point is very convenient, as it’s basically the same temperature at which vegetable tanned cowhide begins to plasticize on it’s own. This, however, does pose a little trouble in preheating the leather- I put it in the toaster oven at 165F, warm enough to encourage the molten wax to flow into it easily, but not so warm that the leather would begin to harden thermally before immersion, preventing full saturation with carnauba. This preheating temperature worked well and the leather readily wicked up the wax, and test findings showed moderate improvements in some categories. (X1 hardened leather using carnauba wax will be referred to as “X1C” going forward).

X1C on left, original X1 on right. The carnauba sample is just as shiny with a less chalky appearance.

In water resistance testing, the X1 sample absorbed 0.08 grams of water in ten seconds, while the X1C sample absorbed 0.04 grams. This is half the water of original X1, though I’d still describe both as “very water resistant”.

Puncture resistance was interesting. The X1C sample required, on average, 2 kg more force to puncture with a stitching awl to 6mm than the X1 sample, over three trials. In strength/bend testing, the two samples were exactly the same, both requiring about 11 kg to bend once, then 5 kg to bend again, perpendicularly to the first.

Neither sample showed any brittleness at room temperature, nor any loss of strength after exposure to 110F for half an hour. This isn’t surprising given the melting point of both stabilizing agents is well above 110F. For cold testing, each sample was held at ~4F for an hour, then bent in half to observe for cracking. Of note, a vise grip was needed to bend both samples for this testing, I couldn’t manage it by hand.

X1C did show a very modest increase of brittleness at cold temperatures, but I’d still consider it pretty cold-resistant. It certainly didn’t snap and create huge fissures in the leather as some other methods did.

Again in cut testing, both samples were basically identical. A heavy weight was placed over a sharp blade and the blade briskly pulled out. The cuts on both samples were pretty shallow and I’d rate them equal in cut resistance.

In cross section, the X1C sample appears considerably darker and somewhat more dense than X1, with a distinct band of dense, dark material on the grain side, and a secondary band of darkening as well. I’m curious what makes this layering occur.

In the end, X1C did turn out to be equal or superior to X1 in all tests. By far the biggest downside to this method is cost. Carnauba wax at roughly $22/lb, is considerably more expensive than stearic acid at about $3/lb. Given this vast price difference, I’ll continue using the X1 method for most applications, at least until I can find a source for carnauba that’s much more reasonable. The increased performance simply isn’t worth the cost.

End update.

Following are photos of a few different items I’ve finished since developing this method. Thanks for reading!