A comparative study of leather hardening techniques- 16 methods tested and novel approaches developed
Jason F. Timmermans 12/11/2018
Note: The following information is free to be used, shared, copied, pasted, and further developed. Please credit the author when appropriate.
I’ve been studying and practicing leatherworking for some years, and have always been interested in new and novel techniques. Lately I’ve tried my hand at formed and hardened leather items- bags, pouches, bottles, flasks, and similar. I set out researching the various methods of hardening mentioned across different forums, wanting to find the method resulting in the most durable product, preferably employing traditional techniques. I scoured the internet for any mention of a method to harden leather. I found discussions in detailed, scholarly articles by historians of leatherworking, random forums otherwise unrelated to leather, and everything in between. I wanted to test as many methods as possible- those with cited sources to support their claims, vague rumors floating around on obscure forums, and a couple of new ideas I thought might be worth looking at. Some methods had highly detailed information with several different approaches for a process, others had little or no guidance. I have a deep fondness for traditional methods, and so I chose to focus on those that used materials available during the period that hardened leather was common. A few techniques were tried using modern materials. I didn’t document every mention of every technique I saw.
Please be aware that I have no formal scientific training whatsoever. This experiment was done solely for personal interest, information, and entertainment. You’re likely to spot errors or awkwardness in math, measurements & units, conversions, and more. You’ll find variables unaccounted for and methods poorly designed. I tried to give it a good shot and answer my questions, and really had fun with the process and ended with information very useful to me. I value your feedback and suggestions for improvement on future projects.
In part one, I’ll describe the different hardening techniques that I made samples of, and some notes and findings on those processes. In part two, I’ll describe the testing methods performed on each sample. In part three, I’ll present the results of testing. In part four, I’ll review a few promising methods and attempt adjustments of those processes to improve the final result. In part five, I’ll present some new & improved techniques I developed based on the results of prior testing. In part six, I’ll present conclusions and closing thoughts.
There are a lot of methods tried, and a lot of data to present here, so I’m going to use a lot of abbreviations for brevity. I’ll do my best to remain clear. Going forward, I’ll use the abbreviation WF to mean “conventionally wet-formed”. This is the method that I and most leather workers use when constructing rigid bags and cases, allowing the leather to hold a shape after drying. It’s not generally considered a ‘hardening’ technique but does result in notable stiffening compared to untreated leather. This is also the technique used for the control to maintain consistent geometry among samples. For conventional wet-forming, the leather is immersed in room temperature water (measured in my testing as 65F) for roughly ten minutes or until bubbling has stopped, to ensure saturation. The leather is then formed by hand or clamped to a wooden mold in the desired shape, and allowed to dry until stiff. Samples stating “WF” are already dome shaped and fully dried. A couple other abbreviations I use here and there are “U/A” for unable to assess, and “RT” for room temperature.
Following is a list of abbreviations for the different hardening methods used, which you’ll see used at times:
180- 180F water hardening method
W- beeswax immersion
G- gelatin immersion
A- ammonia immersion
V- vinegar immersion
ET- denatured alcohol immersion
AG- agar immersion
EM- Elmer’s white glue solution immersion
P- “Pitchwax” blend immersion
S- Stearic acid immersion
Br- brine immersion
Ac- Acetone immersion
EP- Epoxy immersion
Several sources recommend drying under low heat after applying the hardening method, from 120–150F, to enhance results. I’ll omit this step from all methods to remove that variable, and test room-temp immersion followed by baking at plasticizing temps as a stand-alone method.
The majority of conversations online refer back to one of the following articles. Several methods mentioned here were tested [PDF WARNING]:
I often use the phrase “plasticizing temperature” or similar. I’ve gathered from online research and kitchen experimentation, that above approximately 180F, leather begins to undergo structural changes. The heat seems to melt certain structures or substances in the leather, which solidify when cooled and result in a much harder leather. These “thermal hardening techniques” usually result in some degree of shrinkage and deformation of the leather, and when employing these techniques it’s recommended to closely watch the leather and remove from heat as soon as or shortly after this change is observed.
PART ONE: MAKING THE SAMPLES
The first step in creating the samples was figuring out how I’d keep them consistent, and how I’d create a regular domed shape that would allow for the samples to support the piercing and crushing forces of the tests to follow. For a mold, I found a deviled egg tray.
Which I cut into strips and drilled holes for ventilation to create a mold for the samples.
Hard plastic is tough to drill, and I cracked quite a few of these. I ended up with enough to mold six samples at once, which worked out well.
After applying the hardening technique, the sample was pressed onto a lower mold, and an upper mold pressed on top of that. The samples were clamped in place and left overnight to set.
I was happy with the mold. Above, wet-formed samples are removed and inspected. I ended up with consistently sized and shaped leather samples that almost always looked delicious.
A lot of discs were cut. In fact, this was a factor limiting continued experiments- for consistency, I wanted all samples cut from the same piece, and this project started to seriously eat into the half hide I planned to get a few other items out of. All samples were made from 10–11 ounce (~3mm thick) vegetable tanned cowhide. I made three samples of most methods, since several tests destroyed the sample.
The first sample made was the control “C”. Instead of a completely untreated piece of leather, I needed a sample without significant hardening that could still maintain the domed shape required for certain tests. Again, for this purpose I used a simple wet-formed piece of leather. The leather disc was immersed in RT (65F) water for ~10 minutes, then clamped in the mold and allowed to dry.
The second sample employed the “180F” technique. The leather is first soaked ~10 min in RT water, then immersed in water at 180F. The leather is closely observed for darkening and shrinkage indicating thermal changes. When this reaction is noted (roughly 45–60 seconds after immersion), a timer is started for 30–45 seconds, and then the leather removed and clamped into shape. Of particular note, I learned in an early hardening project that pre-soaking the leather in RT water is vital. If the piece is immersed in 180F water while dry, the leather will take time to soak up the water, and will begin to plasticize at the edges and surface long before the process begins deeper. The result is a misshapen, shriveled piece with hardening at different stages throughout. Below are two leather flasks- both started looking identically. Same pattern, same leather, same dye job. The lower flask, I forgot to pre-soak. The upper flask was soaked properly. The difference is clear.
The sample discs required much less time in the hot water than is typical, due to the small size. A decent amount of shrinkage and darkening was noted. The samples were blotted dry and clamped in the mold.
The finished samples were rigid and dry to the touch.
The next process was gelatin immersion (G). I followed the recommendations of a source linked above. Gelatin can be considered basically identical to “hide glue”, a common adhesive of older times. 27 grams (4 envelopes) of Knox unflavored gelatin were dissolved in 200 ml hot tap water. This was allowed to cool to 93F, then WF’d samples immersed x 20 minutes. Agitated regularly. The gelatin had cooled and set considerably by 20 min. I thought a heating pad might maintain gentle heat to allow longer immersion times, but wondered if that temperature recommendation was necessary or even optimal. I wondered if a warmer immersion would speed up absorption of the gelatin… After immersion, the gelatin was wiped off, not rinsed. The samples were dried in air and didn’t need re-forming. The completed samples had a slightly scaly and slick feeling. There was slight darkening and the leather had a glossy sheen from the gelatin.
Next was hammering (H). Some sources noted tooling and hammering increased hardness of a final product, and my experience making wet-formed bags and cases supported this. I wondered just to what degree the hammering did harden the leather. I cut three discs, soaked them in RT water, beat the hell out of them with a ball-peen hammer, and clamped in the mold to dry.
Ammonia immersion (A) was next. Sources mentioning this method recommended ratios ranging from 1 cup ammonia in 1 gallon of water (~6% solution), up to a 25% solution. I decided to try 20%. 40 ml of 10% ammonium hydroxide solution (household ammonia) was mixed with 200 ml tap water. Samples were immersed 5 minutes and agitated frequently, as the leather would not sink.
Darkening was immediate and rapid. When removed from solution, the leather was slimy, especially on the flesh side, and difficult to clamp. Semi-dried, samples had a plastic-like feeling and a pen dragged across them looked like soft wax. It’s clear this process alters the leather chemically. Some sources mention hardening with lye or urine, which I’d bet both function via a similar chemical process. Of note, ammonia is not fun to work with. This stuff burns the eyes and airways if you get anywhere near it. Do it outside or use lots of ventilation. I wore a respirator and goggles. I haven’t played much with concentrated piss, but I doubt it’s much more pleasant.
Finished samples are hard and dry. A cross section reveals the ammonia has created two distinct layers within the leather:
There’s an outer layer of leather that’s darkened similar to the outside (which has a range of colors- brown, orange, green, gold, dark brown) and then an inner core of lighter fibers that look somewhat more dense than the control. When fully dry, the waxy/plasticky feeling is gone and samples are smooth.
Next is the boiling method (Bo). As you’re aware after reading the earlier cited links, a common term for some types of hardened leather is “cuir bouilli”, a french term translated as “boiled leather”. Now, there’s a lot of debate on whether boiling was ever actually used for hardening leather historically, or if this is just not a good interpretation for one reason or another. I’m not calling anything I do “cuir bouilli”, because after all this research I still don’t really know what it is or isn’t. I’m making hardened leather. Anyway, I boiled this leather. I first soaked it in RT H2O x 10min, then immersed in boiling water x 20 seconds, then blotted dry and clamped to the mold. My elevation is 617', if that matters. Darkening and shrinkage was significant. Plasticizing reaction was nearly immediate and progresses rapidly. Sources suggest clamping the leather between two hard, flat surfaces before molding to a shape may mitigate some shrinkage.
The final samples are significantly shrunken, but did flatten out a bit in the forms. They are hard, dry, and brittle.
Vinegar immersion (V) is next. Samples were immersed in RT household white vinegar. Bubbling is vigorous. Immersion time ~10min. Samples blotted and clamped to mold. Surprisingly little darkening.
I didn’t take photos of every method. Sorry.
Next was beeswax immersion (W). This is definitely one of the most popular methods mentioned, especially for creating waterproof flasks, and armor for medieval style combat arts and re-enactment. I used Country Lane 100% Beeswax Pastilles. I intentionally kept all temperatures below plasticizing. Sample discs were preheated 30 minutes in the toaster oven. I think, if you drop cold leather into molten wax, the wax will momentarily harden at the surface. After a moment it melts again and begins absorbing, then slows down again as it meets cooler leather, continuing this slow advance. Preheated leather allows the wax to saturate quickly and evenly. I needed to use a remote probe thermometer and kind of nudge the dial on the “warm” setting back and forth until it bounced within an acceptable range. The temp got to 147F at the highest during preheating. Beeswax was melted and then allowed to cool to 140.5F. Samples immersed 4 minutes. No bubbling noted. Excess wax wiped off easily after removing. Significant darkening, and samples harden quickly. Finished samples feel and sound like plastic when struck together. Slightly waxy texture. Note, paraffin has been offered as an alternative to beeswax, which is somewhat harder. I did not test this material.
Next up, acetone (Ac). Used Jasco brand, RT. Immersed x10min. Swirled occasionally. Didn’t need blotting, acetone dries fast. Felt slightly stiff coming out. Clamped and dried. No or very little darkening, no obvious shrinkage. Chalky, white residue after drying.
Alcohol method (ET): Jasco brand used, RT. Immersed 6 min 30 sec. Slight bubbling immediately. At 4 min 30 sec, noticed edge curling. Slightly stiff coming out. Little to no darkening. The next day, darkening noted where the leather was exposed to air at the ventilation holes in the molds.
Pitchwax (P): This was a recipe I came up with. Beeswax is such a popular hardening agent for leather, and I often see pine pitch used as an internal lining on hardened leather vessels. I wondered if it was possible to mix the two and create a better single-dip solution to hardening the leather and ensuring a durable seal against moisture. At the very least I’d learn if pitch was an additive worth investigating more. I got “brewer’s pitch” from Townsends and used a 2:1 pitch:wax ratio. I had trouble figuring out the melting temperature, somewhere around 180. Probably hotter. Samples were WF, then preheated to 165F x 15min. Pitchwax temp was 170F at immersion. Agitated regularly, no bubbling noted. Immersion time 4 minutes, pitchwax was 140F when samples removed. It was beginning to thicken quite a bit and had the consistency of uncured epoxy. Easy to wipe off with paper towels immediately after removing, but this stuff cools fast and becomes extremely sticky and thick. It also stinks when melting. Cooled, hardened samples are glossier than beeswax samples. I can immediately feel they’re slightly pliable and remind me of a very stiff rubber.
Stearic acid (S): Mentioned in a source cited above. Country Lane brand was used. This is a fatty acid derived from animal or plant sources. I can only describe it as “waxy” but harder than other waxes I’ve worked with such as beeswax or paraffin. I’m not clear at what point in history stearic acid was made in quantities sufficient to harden leather in. Stearic acid melts at 156.7F according to wiki, which seemed accurate. Boiling point is 681.8F. Samples were WF, preheated x10min @ 144F. Stearic acid was 158F at immersion x 6 min. Bubbling immediate and vigorous, stopped bubbling @ 1 min. Stearic acid 135F when samples removed. It looks like water freezing as it cools. There is significant darkening of the samples. Stearic acid is very flaky and it’s difficult to remove excess before it cools and hardens into a thick, chalky, white layer. I would later learn this excess is easy to remove with a heat gun and towel. Cooled, finished samples are very hard and plasticy.
Elmer’s white glue (EM): I saw this mentioned enough that I thought it was worth a shot. I guessed this would yield a result similar to hide glue. I found three sources discussing this method. Two didn’t mention any glue to water ratios at all, and one recommended 1/4 cup glue to 2–3 gallons of water. For 200ml water that equals 1/3 tsp. That seems too light. I used 1 tsp glue to 200 ml water to give it a fighting chance. The solution is milky, looks like Calpico. Solution RT, samples immersed x5min. Bubbling seen at surface, can’t see the samples. Swirled occasionally. Samples removed, blotted, clamped, and dried. Looks to me the same as a plain water immersion.
Brine (Br): One source says brine soak, one says boiled in brine. I’ll do a warm/hot brine soak. Dissolved 32 grams Morton’s natural sea salt in 100 ml water. Looks like a good ratio, just a few crystals not dissolving. As an afterthought, I looked up recipes for brine. I thought brine was supposed to be a super concentrated saltwater solution. I read that I should have used 7.2 grams salt for that much water. Whoops. Immersed 5 min, “brine” started @ 140F, ended @ 121F. Vigorous bubbling which stopped @ 30 sec. Swirled occasionally. Clamped and dried. Not much darkening, typical for plain water soak. Next day, the leather has swelled significantly, from 3 to 4 mm thickness.
Baked (Bk): Discs immersed in RT H2O ~ 10 min. Clamped to the form, then placed in oven. Baked ~190F x 101 min. That was too long. The leather is scorched. This method definitely bears watching the leather and removing it when it starts to brown. At any rate, the end sample wasn’t too terribly shrunken or misshapen, and was extremely rigid.
Agar (AG): When researching the gelatin method, I found mention of agar, a seaweed derived thickener I was familiar with from my mushroom growing. Agar melts at 185F and solidifies at 90–104F, a property called hysteresis. I thought this may make a hardened leather superior to gelatin, which would allow for cooler working temperatures but a product less susceptible to high temperatures. I used NOW brand agar, 10 grams dissolved in 200 ml near-boiling water. Samples preheated to 120F x 20 min. Agar cooled to 125F. Immersed 5 minutes, 114F at removal. Slight bubbling, stirred regularly. Slightly slimy, hard to clamp.
Epoxy (EP): Sample immersed in East Coast Resin crystal clear epoxy resin & hardener, 1:1 by volume. Immersion 5 minutes. Significant darkening. Sample removed and clamped to form. Full hardening apparently takes 2–3 days. The next day, samples are dry and extremely hard. I had a feeling these might be a top contender. The problem with epoxy is that it’s expensive, and certainly not a traditional technique.
I had initially planned to test dye resistance of each sample, but after making the samples I realized the vast differences in colors and finishes would render that question kind of moot. If you’re using wax, ammonia, or similar methods, the leather is darkened so much that dye will offer pretty subtle color differences. The flask pictured further up was hardened with the 180 method, and you can see it still looks decent after hardening. In these cases, I strongly recommend dyeing before hardening, as any hardening process limits dye absorption to some degree.
Pictured below are front and back of most samples made. Refer to the abbreviations listed earlier. The number after the abbreviation is just the sample number of three, and can be ignored.
PART TWO: TESTING METHODS
Samples were tested for the following attributes: Water resistance, puncture resistance, cut resistance, strength, brittleness, and resistance to changes in extreme temperatures.
Water resistance was tested by first weighing the finished sample. The sample was then fully immersed in RT tap water for ten seconds. It was removed and given two quick shakes to remove surface water, and weighed again. The difference was calculated as a percentage of total dry weight absorbed in water. A completely untreated disc of leather was tested as well for an additional control. Interestingly, it started at 2.9 grams and absorbed 2.9 grams of water in ten seconds, giving us a nice convenient control of 100% weight absorption.
Samples were next tested for puncture resistance. I placed the sample on top of a postal scale and used my phone to record the scale output so I could focus on the sample.
To puncture the sample, I used a sharpened leather stitching awl. The awl was stropped on a leather surface with green polishing compound, then degreased with denatured alcohol. This was performed between each test. A 5 mm mark was made on the awl blade. The awl was held perpendicularly to the scale surface, and the awl pushed into the sample to the 5 mm mark with as consistent a motion as I could manage. I then reviewed the video of the scale display, and recorded the highest weight registered. I repeated this three times for each sample, and averaged the results.
Next up was cut resistance. I had a lot of trouble figuring out how to test this with an acceptable maximum of variables. I came up with a method that still leaves a degree of interpretation, but I think at least gave a good ballpark idea of the ranking of each method.
I screwed the sample to a piece of wood, and held a fresh, degreased utility knife blade to it. I then set a weight on top of the knife- in this case, I used my quartz tooling slab, and a heavy piece of railroad track on top (my makeshift anvil). I then yanked the knife out, and visually inspected and compared the resulting cuts.
It was difficult to place a number on the resulting cuts, but it was fairly easy to at least rank them from most to least cut resistant.
The next test was strength/hardness and brittleness. The same scale was used again, and the sample was placed edge-on to the scale surface. I used a wooden block to push down on the opposite edge until the sample bent approximately 180 degrees. The highest weight registered on the scale was noted, as well as any cracking present.
This test revealed a lot about the usefulness and practicality of several methods. For instance, it was quickly obvious why truly “boiled” leather is not a preferred method:
Remember, this sample was exposed to boiling water for about twenty seconds. Some sources I read recommended boiling leather up to thirty minutes. I can only assume these folks are passing on secondhand knowledge they haven’t actually tried themselves, or are doing something fundamentally different with their hardening process. I can’t see how boiling leather for thirty minutes would result in anything but a shriveled, brown mess.
Following this, samples were tested for resistance to extremes of heat and cold. They were first squeezed in a spring clamp at RT, and the amount of deformation measured. The samples were then held at 110F for ~10 minutes, squeezed in the same clamp and measured again, with the additional deformation noted, if any.
Cold testing was first done by holding samples at 35.4F for 8 hours. Then the samples were flexed by hand to observe for new or worsening cracks. Samples were next held at 3.1F for 30 minutes and flexing/observation repeated.
Following testing, samples were also cut in half to observe cross sections and determine to what extent the hardening techniques changed the inner structure of the leather. I’ll post those results in part three.
PART THREE: TEST RESULTS
You’ll notice that some samples are omitted from some tests. This is because for one reason or another, I chose not to continue testing a certain sample once it had failed a certain test badly enough to no longer be a top contender.
We’ll begin with results of water testing. Below is a list of methods in descending order of water resistance. The number listed represents the amount of water, as a percentage of the sample’s dry weight, that was absorbed in ten seconds.
- Stearic acid- 2.17%
2. Beeswax- 2.2%
3. “Pitchwax”- 2.3%
4. Gelatin- 6.2%
5. Boiled- 22%
6. Baked- 26%
7. Ammonia- 37%
8. 180F- 48%
9. Brine- 52%
10. Agar- 62%
11. Hammered- 71%
12. Control- 74%
13. Elmer’s- 75%
14. Vinegar- 78%
15. Ethanol- 92%
16. Untreated- 100%
17. Acetone- 107.4%
It’s obvious that methods that saturate the leather with wax or waxy substances offers the most water resistance. Gelatin also did surprisingly well, though I would bet this method’s water resistance would drop considerably with prolonged exposure, compared to wax methods. I was surprised to see acetone actually increase the leather’s permeability. I wonder if pre-treating with acetone would affect the dye characteristics of leather? An experiment for another time.
Next up, the results of puncture resistance testing. Again, in descending order from most puncture resistant to least. The number listed is the average force in kg required to push the awl to 5mm depth in the leather. Of note- several samples flattened to the surface of the scale before the awl could reach 5mm. I considered the data unreliable, as I couldn’t tell when I was no longer pressing on the sample and instead pressing on the scale surface. I’ve concluded these methods are not true “hardening” and listed them at the bottom here as unable to assess.
- Epoxy- 8.5
2. Baked- 8.1
3. Gelatin- 6.13
4. Boiled- 5.533
5. Stearic acid- 5.532
6. Ammonia- 4.87
7. “Pitchwax”- 4.48
8. Beeswax- 4.45
9. 180F 4.06
U/A- C, H, V, ET, Ac, EM, BR
There was discussion in more than one forum with some folks suggesting that beeswax hardening techniques, while popularly used for modern armor recreations, may actually provide a lubricating effect and decrease puncture resistance compared to other methods. I think those folks are correct. I’ve pushed an awl through leather quite a few times, and could easily feel the smoothness with which the awl slid through the waxed leather. As seen here, it placed almost dead last among samples that were able to be tested.
Strength testing results are presented below. In descending order of strength, with the maximum weight required to bend the sample to approximately 180 degrees noted in kg, and any cracking noted with an asterisk.
- Epoxy*- 24 (severe cracking)
2. Stearic acid- 22
3. Beeswax- 20
4. Ammonia- 19
5. Baked*- 17
6. Boiled*- 14
7. Gelatin*- 10
8. 180F*- 8.2
9. “Pitchwax”- 7.4
10. Brine*- 7.4
11. Elmer’s- 6.8
12. Acetone- 5.9
13. Vinegar- 4.9
14. Ethanol- 4.3
15. Control- 3.8
16. Hammered- 3.5
So, confession time, I couldn’t wait the 2–3 days for the epoxy sample to cure fully, and wanted to play around with it. I had a few samples to mess with, anyway. I noticed the sample did extremely well with puncture resistance and expected it to pass the rest of the tests with flying colors also. And indeed, it was very strong, requiring 24 kg of pressure to bend. But, when it did…..CRACK!
Strength is great, but cracking is not great. I think I can do better.
I also noticed that all high heat hardening techniques result in a strong yet brittle leather, all of which showed cracking.
Cold temp testing results are as follows:
Samples that did not exhibit cracking at low temperatures:
- Control, Beeswax, Hammered, Ethanol, Stearic acid, Brine, Acetone
Cold resistance in descending order among samples showing cracking:
8. “Pitchwax”- Major cracking @ 3.1F
9. Ammonia- No cracking until 35.4F
10. Gelatin- Cracking @ RT, additional cracking @ 3.1F
11. 180F Cracking @ RT, much deeper cracking @ 3.1F.
12. Boiled- Cracking @ RT, deeper cracks @ 35.4F, additional, deeper cracking @ 3.1F.
High temp testing results are as follows:
Samples unaffected by heat testing:
- 180F, Ammonia, Boiled, Stearic acid
Heat resistance in descending order of those weakened by heat testing:
5. Gelatin- 2mm change
6. Beeswax- 9mm change
7. “Pitchwax”- 20 mm change
Samples C, H, V, ET, and Ac were fully bent in half by the testing clamp at room temp and deemed U/A.
I was amused to find my beloved “pitchwax” blend crack at low temps and didn’t pursue that much further given how well other samples were doing. It also didn’t handle heat very well at all, and it seems my original impression of “very hard rubber” was actually pretty accurate. I was also surprised to find sample A crack with cold temperatures. All samples hardened with heat showed increasing brittleness with lower temperatures.
Cut resistance testing results required some interpretation. I’ll list the samples below in descending order of cut resistance, with a note on observations.
- Baked- Short, very shallow cut
2. Stearic acid- Short, shallow cut
3. Ammonia- Long, shallow cut
4. “Pitchwax”- Medium length and depth cut
5. Beeswax- Medium length and depth cut, slightly deeper than sample P
6. Gelatin- Cut is nearly as deep and long as sample C
7. 180F- Cut edges curled inward, cut is moderate to deep and fairly long
8. Control- Cut is deep, about 75% thickness and nearly across the sample
Samples H, V, ET, EM, Br, and Ac are basically identical to sample C
15. Boiled- Deeper cut than Control, full length of sample
Some clear winners are beginning to emerge. It’s interesting that results are kind of all over the place for this method. A thermal method at the top, followed by a stabilizing method, followed by a chemical method. A better, more controlled cutting test would yield better results I’m sure.
Examining cross sections was interesting. Wax immersed samples were clearly fully saturated, the cut surfaces glossy. Gelatin sample was also fully saturated.
Several samples showed little to no change from the control or untreated samples. Most of these samples also had little to no hardening.
Thermally hardened samples looked surprisingly normal inside, aside from the baked sample which had darkening throughout.
PART FOUR: ADJUSTING PROMISING CANDIDATES FOR BETTER RESULTS
There were several methods I expected to perform well, but two that took me by surprise were gelatin and ammonia immersion. Gelatin showed particular talent in puncture resistance, and ammonia was the only chemical hardening method that resulted in anything one could call “hardened”. I wanted to take a second look at both of these methods and see if they could be adjusted for improvement. Also, I was a bit surprised by results of ethanol immersion. I saw more than one person insisting alcohol submersion results in a very hard leather. Reviewing those claims I noticed folks mentioning specifically rubbing alcohol, or isopropyl. I’ll try that too.
Since sample A turned out so brittle, I tried halving the concentration. These would be called A2. Samples were immersed for 5 minutes in a 10% ammonia solution, then clamped and dried. Samples darkened but not as significantly as sample A.
A2 scored 34% in water resistance, just ahead of the original method at 37%. It scored 4.75 in puncture resistance, slightly less than last time at 4.87. It took 19 kg of force to bend the sample, identical to the previous test. Nearly identical brittleness was seen, with the sample cracking deeply at 35.4F. The results of this adjustment were not particularly encouraging, but I had one last idea to test with ammonia…
For G2, my second attempt at gelatin hardening, I wanted to test my previous theory. I wondered if preheating the leather, and a warmer gelatin bath, would help the gelatin incorporate into the leather better and maybe give improved results. I used the same gelatin ratio as last time. I did not WF the samples first, but just used plain, untreated sample discs. They were preheated to 155F and the gelatin brought to 157F. Samples were immersed 15 minutes with regular agitation. No bubbling was seen. During immersion, I noticed an initial, rapid darkening of the edge of the sample, and then a second line of darkening, not quite as dark as the initial reaction. This second line crept toward the center until the entire disk had darkened at 5 minutes. The samples were removed, wiped off, clamped, and dried.
G2 scored 8.8% on water resistance, slightly behind its predecessor at 6.2%. On puncture resistance it scored 7.17 kg, a decent improvement over the previous 6.13 kg. Strength testing put it at 20 kg, tying for third place with beeswax, and well ahead of the first gelatin sample at 10 kg. Unfortunately, cracking did occur at 35.4F. It seems this adjusted method did provide some significant strength and pierce resistance benefits, at the expense of some water permeability. I had one more trick up my sleeve for gelatin as well.
Isopropyl (IS) was fairly straightforward and inexpensive to test. I used Quality Choice 91% isopropyl rubbing alcohol. Samples were immersed 4 minutes @ RT. Some bubbling noted which stopped @ 2 min. Some darkening. Swirled occasionally. Clamped & dried.
I’ll save you the suspense- IS, like ET, did not result in any significant hardening. Now, I saw knife makers in their forums recommend isopropyl dip for their sheaths more than once, and seemed confident enough in its effectiveness. Maybe I’m doing something wrong here, but as yet I’ve been unable to reproduce those results with either ethanol or isopropyl. I could try methanol I guess, but I don’t have high hopes.
PART FIVE: DESIGNING IMPROVED METHODS BASED ON TEST RESULTS
I set up this experiment to discover the best all-around technique for hardening leather. I think after considerable time and energy researching, experimenting, and reviewing results, I was successful.
It occurred to me that the various methods of hardening can be broken up into four groups:
- Thermal hardening methods- 180F, Boiled, Baked
- Chemical hardening methods- Ammonia, vinegar, alcohol(s), acetone, brine
- Mechanical hardening methods- Hammering
- “Stabilizing” methods- Beeswax, Stearic acid, “Pitchwax”, Gelatin, Elmer’s
The last group which I’ve chosen to call “stabilizing”, involve saturating the leather with a substance (“stabilizing agent”) that hardens by itself and offers rigidity and strength. Stabilizing methods don’t necessarily change anything about the structure of the leather itself.
After reviewing test results and my subjective impressions of the different methods, I wondered if it might be possible to combine two or more hardening mechanisms for a “stacked” effect, a method that would provide better test results than either one alone. Chemical and thermal methods both impact the structure of the leather fibers themselves and would likely simply compete against each other. I was concerned any stabilizing agent in a sample would be compromised by a strong ammonia solution, and hammering a stabilized method would probably just squeeze out the stabilizing agent.
The most likely candidate for a new, better method seemed to be a combination of thermal and stabilizing techniques. The problem I saw was, if I thermally harden the leather first, the rigid fibers would inhibit thorough, proper absorption of the stabilizing agent. And if I stabilized first, it might melt out of the leather when being heated up for hardening.
I wondered, though, would it be possible to use the stabilizing agent itself to transfer heat to the leather to provide the thermal effect?
This turned out to be extremely effective and led to the creation of a new method that basically blew the rest out of the water. But before I reveal the culmination of this study and the final technique, let me review some other methods attempted.
“X2” is an abbreviation given to the second of a set of novel methods I tried and tested. A sample was preheated to 150F and a container of beeswax melted and cooled to 150F (you may notice by now I’m generally no longer wet-forming samples prior to any hardening- I found that using pre-formed leather or plain, untreated leather doesn’t seem to matter. Using flat, unformed leather works fine for all methods as long as you can get it clamped into shape quickly). The samples were immersed in the wax, and the container was returned to the heat.
My thinking here is- if I first introduce the leather to the wax at a temperature below plasticizing, that will give the wax a chance to saturate the sample as much as possible. Then, as heat is raised, a thermal hardening effect can take place while the leather is still submerged and not lose any stabilizing agent.
The wax with leather immersed was raised to 200F and held there about 40 seconds. Total time above 180F was about 3 minutes. The sample was removed and clamped to the form.
With this test, I discovered something very interesting. The leather sample, despite being held at or above plasticizing temperatures for three full minutes, showed no shrinkage or deformation at all. None. In fact, I next brought a sample up to 265F under molten wax, with little to no shrinkage noted. I’m not sure exactly why this happened, I expected it to fry like a pork rind and turn into a crispy, brittle chip. As it turns out, leather can be heated much higher under this medium than under water, while retaining its original size and shape.
And it worked out very, very well.
X2’s puncture score was 5.97, well above beeswax alone at 4.46. Surprisingly, strength was lower at 18 versus beeswax’s 20. I assumed water resistance would be similar, but didn’t continue testing after results from strength tests.
X3 was an attempt to combine Gelatin stabilizing with thermal hardening. The same gelatin ratio was used as before. The sample heated to 150F, gelatin cooled to the same. The sample was immersed, and I attempted to raise the temperature to 200F. And then, another surprise. The leather began rapidly shrinking at about 165F, and by the time I’d realized what was happening, it was a shriveled little ball of failure. I cut another disc and reflected on the fact that gelatin seemed to have the opposite effect of wax, and actually lower the temperature at which plasticizing begins. I tried the process again, this time slowly raising the heat and carefully watching for thermal changes.
X3’s puncture result was 7.77, taking third place behind the baked and epoxied samples. I took it to the crush test where, at 16 kg, the sample failed spectacularly with numerous, very large cracks. Well. We already knew gelatin makes leather brittle, and thermal methods make leather brittle. Is anyone surprised combining the two makes an even more brittle leather?
Preparation of X4 was identical to X2, except a blend of 50/50 beeswax and stearic acid was used. Puncture score was 6.0, good for beeswax and better even than X2, but not stellar. Strength testing scored 19.3, and some minor cracking occurred. This strength rating was below several others, and I didn’t continue testing this sample.
X5 was kind of a shot in the dark, and not something I wish to try again. I decided to give a shot to a chemical & thermal combination. A solution of 10% ammonia was prepared, and a sample immersed at RT x 90 seconds. The sample was then moved to a 10% ammonia solution @ 180F for 90 seconds, or about 20 seconds after thermal changes were seen. The sample was removed, clamped to the form, and dried. Let me say here, that simmering ammonia is not fun.
X5 did not even make it to testing. After drying, I removed the sample from the form and squeezed it gently between my fingers. Right away, I heard the gentle popping sound of the leather beginning to crack. I squeezed harder and it snapped like a twig.
I did not continue testing.
Once again, I wasn’t too surprised. Ammonia and thermal techniques both impart brittleness. Taking both to the extreme gives us something way too brittle for use.
So, finally, we come to the method that I think takes the first place trophy for best overall result. As you’ve probably gathered from the testing, Stearic acid did very well in many categories and warranted more investigation. As with the other stabilizing & thermal techniques, the sample was preheated to 150F, Stearic acid melted and cooled to the same. Sample immersed, and the temperature raised to 200F for about a minute. The leather was then removed and clamped to the form. This new method, which was actually the first I tried, was called X1.
From the outside, X1 doesn’t look noticeably different from a lower temp Stearic acid sample. As you can see, they do clean up pretty well, and easily, with a heat gun and a rag.
In cross section, the difference between the two is more obvious. X1 is notably darker inside, and the fibers seem to be denser and more compact. Cutting the sample to view the cross section was like carving wood. The leather is very rigid and strong.
Test results for X1 were outstanding. At 2.17%, it tied with Stearic acid for first place in water resistance. Puncture resistance scored 7.27 kg- not at the very top of the pack, but far better than Stearic acid alone at 5.35 kg, and behind only epoxy, baked, and high-heat gelatin samples, all of which were too brittle for use. In strength testing, X1 came in at a whopping 24 kg- tied for first place with epoxy…but it didn’t crack!! The sample took enormous force to bend in half, and when it did, simply bent without breaking at all. X1 was unaffected by high temperature testing. In cold testing, no brittleness was noted at 35.4F. However, at 3.1F, some cracking of X1 did happen as the sample approached a 180 degree bend.
This was not exactly what I’d hoped for, but I would still call this sample “highly crack resistant” given that these cracks are somewhat minor compared to those in other samples, and they didn’t occur until well below freezing temperatures.
X1 tested very well and I was confident I’d come up with a new spin on an old hardening technique that provided the best overall results for all methods tested. X1 is extremely strong, yet flexible and resistant to cracking. It isn’t affected by warmer climates and tolerates cold quite well. It’s highly resistant to water, cuts, and punctures. And, stearic acid is cheap. Again, I’m not sure exactly when in history stearic acid became commonplace, but in a source cited above, Rex Lingwood mentions it as being a hardening candidate for some historical pieces. Interestingly, Mr. Lingwood also states the leather is blackened by the stearic acid, something I did not see. He also proposes pure pine pitch as a hardening agent. I thought of testing this as well, but felt confident the result would be far more brittle than stearic acid methods.
Going forward, X1 will be my hardening method of choice. It’s inexpensive and creates a very durable leather. I may continue to use some other methods for specific applications where different properties are desired. As stated earlier, if you decide to use the X1 technique, you can pre-form your leather to shape using conventional wet forming and then apply the hardening method. Or, you can use untreated leather and then apply hardening, but be aware the leather will become rigid within a few minutes after removing from the Stearic acid, and will need to be clamped into shape quickly.
PART SIX: CONCLUSIONS
This experiment took several weeks of research, preparing samples, testing, organizing data, re-testing old methods, developing new methods, etc. I actually debated for a while whether and how to release my findings and new technique after what I’d invested in it, but in the end decided to embrace the free exchange of information and allow others to benefit from what I’d found.
A few methods were debunked as I was unable to demonstrate any significant hardening effect. Some were slightly stiffer than the control, but none offered what I considered true “hardened” leather. I remain open to the possibility I didn’t prepare the samples as intended, and a different result may be achieved with different approaches. Hammering, ethanol, isopropyl, brine, acetone, Elmer’s glue, vinegar, and agar all failed testing pretty miserably and I would never recommend them over plain, conventional wet-forming as a means to form leather to a desired shape.
Thermal hardening methods seem to invariably lead to brittleness of the leather. The 180F method attempts to minimize brittleness by keeping temperatures just at plasticizing, but the result is still very prone to cracking with flexion even at room temperature. These methods may be useful for items where vibrant color is desired, since the finished hardened leather doesn’t become permanently darkened and will still show the effects of dye quite well. Again, I recommend dyeing before hardening, and if using the 180F method, don’t forget the pre-soak! I would definitely not recommend full boiling for any purpose. Baking could be useful as well, just be sure to closely watch the leather and remove from heat before scorching happens. I would reserve use of these methods to applications not likely to cause any flexing of the item. Even a flask dropped on a hard floor could crack with thermal techniques.
Ammonia was the only chemical hardening method with any notable effect. The result is very hard and cut resistant, but also brittle. Adding any degree of heat to this method seems to only increase the brittleness without providing much additional strength. Also, given the very significant darkening and discoloration, I can’t think of any application I’d use ammonia, lye, or urine hardening unless I for some reason had to make hardened leather and those were the only options available.
Gelatin worked surprisingly well, and given how common hide glue was, I was really interested in this and will likely play with it more in the future. The puncture resistance offered by gelatin was really surprising and I’d be interested to know why this method happened to be particularly good at that test. Gelatin is also cheap and I think this method could have some useful applications.
My “pitchwax” and agar ideas were both a bust. Agar made a sample somewhat stiffer than control, but nothing to write home about, nothing worth using as a hardening method. I was hoping pitchwax would offer the strength and rigidity of pitch with the flexibility of beeswax. Unfortunately, instead it seemed to combine the worst aspects of both. It turned into a limp mess at high temps, cracked at low temps, and didn’t do anything else very well.
Beeswax methods worked quite well, and likely would be fine for most uses. I do feel the wax lubricated the blades during testing and made cuts and punctures easier, and despite some reports to the contrary, beeswax hardened leather will in fact soften at elevated temperatures. 110F is definitely a hot day for most places, but certainly not unheard of especially in the south, or after sitting in a vehicle on a warm day. Also, beeswax is kind of expensive.
Epoxy is very strong and very puncture resistant. The problem is, it’s expensive, it takes a long time to cure, and it’s brittle. Given that X1 turned out to be just as strong, that Stearic acid is much, much cheaper than epoxy resin, that X1 treated leather takes a few hours to fully harden versus a week for epoxy, and that Stearic acid is at least certainly more traditional of the two, I just can’t think of any reason I’d soak leather in epoxy.
Some final notes- a few tips for working with hardened leather I picked up during the experiment. One of the issues needing to be worked around is the degree of shrinkage caused by hardening. As mentioned before, the leather could be pressed between to hard, flat surfaces to help enlarge it again before clamping. For an application like scale armor, the leather could be first hardened and then cut to shape. Regarding tooling of hardened leather, some sources suggest stabilized methods may be carved after applying the hardening technique and shaping the leather to the desired shape. It would likely be necessary to use a heat gun or other source of heat to keep the leather soft and workable while tooling. I wouldn’t recommend tooling before hardening, as you’ll likely see a significant loss of detail, and the stabilizing agent will pool in your cuts and impressions. Carving before using any thermal hardening techniques will likely result in near total loss of detail, and would likely be difficult to perform after tooling.
I think it’s possible to perform the X1 technique without a thermometer. Preheat the leather until it’s warm, but not hot to the touch. Melt the stearic acid, then remove from heat until you see it begin to solidify at the edges, and immerse the leather. Return to heat and slowly warm it up again. As the leather reaches plasticizing temperatures, you will notice some changes, but it’s far more subtle than with other thermal methods. You’ll have to experiment and use your judgement for when to remove the leather, but in my experience this is much more forgiving than water or dry thermal techniques.
That about wraps it up. This was a lot of fun, a lot of work, and at times I found myself hunched over the kitchen table looking at piles of various colors of little leather discs, wondering if I was losing my mind. In the end, I gathered lots of really useful information, and a new technique I’ll be using frequently from now on, and I hope you give it a try as well. Thank you for reading, please give me some feedback and if you’re so inclined, please continue testing and follow up with your results!
Jason F. Timmermans