Leather under a microscope

[spectre6000]

Last weekend, I promised that I'd make this thread pursuant to some comments in the Ask a Question thread. I was sick and it was snowy, so I figured I had the time. Turns out, I was sicker than I realized, so I apologize for the delay. At any rate, here is the promised thread. Various leathers under a microscope for curiosity's sake.

The microscope a low power stereomicroscope for watchmaking and electronics. I don't have a memory card for the camera currently (and the camera has limited optical control anyway), so I just took photos through the eyepieces with my phone. Each photo set will consist of a macro view of the object being imaged, then maximum internal zoom (2X Barlow lens X 4.5X magnification in the head) with 10X, 20X, and 30X eyepieces. That works out to 90X, 180X, and 270X total magnification respectively. Focus is a bit of a challenge with three dimensional objects, plus I keep my head with a bit of a tilt for tool clearance and didn't want to mess with that adjustment given the three dimensionality of everything making it difficult to keep the sample in plane.

If there's interest down the road, I can scrounge up additional samples for imaging. Otherwise, this is what I was able to work up on a snowy Saturday morning.

 

[spectre6000]

First up is a 1960s vintage Hartman briefcase (clean section):

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The pores in the leather are very well defined. There's some compression at the very surface, but the light fluffy internal structure is visible in the pores. I believe this is undyed full grain veg tan cow, but I don't know for sure.

 

[spectre6000]

Same '60s briefcase, but a more worn section with some color.
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What I find noteworthy about this one is how the leather is compressed, and made translucent with hand oil. You have the yellow of the less compressed/saturated areas, and red/brown where it's more compressed and saturated. Very deep, complex color that can only come from natural wear. Additionally, when you consider that leather is substantially comprised of collagen, it's interesting to think of it as a sort of super dense, dried gelatin. It's more complex than that obviously, but it sort of makes the translucency and texture at this level of magnification make a bit more intuitive sense.

 

[spectre6000]

Prototype Moleskine notebook cover in full grain veg tan cow w/ oils/waxes and lots of sun:

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The oils and waxes combined with the intense and intentional UV damage has made this leather very dark without the use of any dyes or pigments. The oils take the place of compression to make the matrix translucent, and the UV damage prevents it from reflecting shorter light wavelengths. This is full grain veg tan cow, and you can see the pores similar to the clean briefcase sample above, but with a lot of beneficial damage. Neat.

 

[spectre6000]

Shell Cordovan shoes.

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This particular bit of shell cordovan started out #8 (the actual name of the color people often refer to as "cordovan"), but when I had them resoled a few years back a colorblind cobbler polished them brown... You can see both colors and a bit of my fury in these images. "Shell" is an inside out cut of horse leather, and "Cordovan" is the oil/wax/glazing finishing treatment. It's basically a super duper dense, tight grained suede that's been oiled/waxed and polished to death, typically with very rich dye. This particular set of shoes hasn't been worn since before the pandemic, and has been collecting dust in my closet all this time. I brushed them off really well with a horsehair brush, but the brush is also old, and was just shedding hair... So, this is a pretty dry and dirty bit of leather... Imagine I had the time to get them cleaned up and conditioned, and it's easy to imagine all those white cracks gone, and the surface smooth like it should be with all that deep and variated coloration.

 

[spectre6000]

Upholstery leather laptop bag

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I made this bag... 15 years ago or something like that from leather I stripped from a couch I found by a dumpster in college. It's chrome tanned cow that was abused when I found it, and hasn't had any better luck since. You can see past the pigment surface treatment to the pale chrome tanned fibers below. Chrome tanning is very aggressive relative to veg tanning. If you think of the leather fibers like little filaments of hot dog, chrome tanning is like cooking the hot dogs/fibers over a jet exhaust to a veg tan's broiler or something. Much more gentle method. The result is like the outside is charred before the inside gets warm, but you can't NOT cook it, so the outside just gets wrecked. You can see this in this sample via the finer fibers with all the space between them. I recently had a sample of leather that was VERY AGGRESSIVELY chrome tanned, and I've never seen anything like it. I guess you can tan leather in an hour, or whatever these guys were after, to save money, but the charred remains barely qualify as leather.

 

[spectre6000]

Italian veg tan glasses case flap

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This was billed as Italian veg tan leather from an Etsy seller in Eastern Europe (I forget where exactly). The way they advertised it was clearly an attempt to make it look like THEY were based in Italy rather than just sourcing one of their materials from there. I have no way to prove their marketing one way or another. It's not as wrecked as the chrome tanned upholstery leather above, but the fibers look like they've seen better days (I've only had this a few months). I kinda wonder if this isn't a combo tannage. Not sure what the grit is. Looks like fine sand.

 

[spectre6000]

Italian veg tan glass case flap - flesh side

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See the previous post for the majority of the comments on this. The variable focus is due to the three dimensionality of the sample with the fibers shooting up out of plane. Provides another view of what I'm pretty sure are over cooked chrome/combo tanned fibers.

 

[spectre6000]

Heavily worn cow wallet

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I've had this wallet nearly as long as the upholstery leather laptop bag up there. Has lived in my pocket any time I've gone anywhere for that entire duration. It started life a dark brown, and is now definitely black. Very very very worn. The surface is nearly as smooth as the shell cordovan, though it's much softer and more flexible. Not as durable feeling. It's stretched a lot over that time. Still. It's crazy just how compressed that surface is. No light in or out either. I wish it had started out a lighter color so the effects of that wear on color could be assessed.

 

[spectre6000]

Sheep skin glove

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This is an example where the pictures fail. Stereoscopically, this is a very different picture. The sheep leather is way more fluffy than anything in the above posts. Very spongey. Very three dimensional surface texture, but not in a porous way. It's very easy to imagine the surface finish getting worn down in a hurry. There's a reason capeskin doesn't last very long. Too bad I can't take/post sterescopic video... This was an interesting sample. The darker spots are shinier/less reflective (not a paradox) from whatever finish is applied. The light that is reflected, goes in one direction and isn't scattered; think mirror. That mirror-ness makes it darker. You can see the more textured parts in the valleys, so to speak. Combine that variability and fluffy surface texture with the softness of sheep leather, and you start to get the picture... Hopefully.

 

[spectre6000]

Watch strap (light side)

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I believe this is a corrected calfskin. It's smooth, but not in a suede-y way. You can see the chrome tanned fibers pretty clearly, but there's still a little bit of the pores visible. I think I only wore this watch strap a few days. Came with the watch, and I didn't care for it. Immediately made something else in leather, then decided I wanted to just stick with my usual steel bracelet. That is to say, there's potential for a little coloration from skin oil, but very little, if any. If this had some sort of waxy treatment or top coat, it'd be a pretty typical corrected leather.

 

[spectre6000]

Watch strap (dark side)

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Reverse of the same watchband. It's some sort of waxed suede finish meant to emulate a vintage something something... I dunno. Not my thing. Not really sure what to make of this. The finish is weird, and it's confusing my eye. I see some three dimensional coloration, and it's only on the surface like the dirty section of the briefcase way back there, but this is a mass produced product. Maybe a really gummy oil/wax blend with a surface wipe of a weak dye? Not sure. Then there are those deep fissures with the visible and sparse fibers. Maybe it was chemically eaten up a bit as part of the "patina" process? It's interesting. It'll continue living in the box in case I ever decide to sell the watch it came with.

 

[spectre6000]

That's all I have for today. If there's interest, I might be able to find more. Maybe even something from an actual jacket depending on what I can get my hands on.

 

[TREEMAN]

Interesting ….

 

[Khilij]

This is a fantastic thread idea. I love seeing things from new perspectives and this is certainly one I hadn't considered.

 

[tuanhng614]

Wow, it’s crazy to see the pores under microscope.

And the difference between veg tanned vs chrome tanned too.

Can you elaborate more on the chrome tanned leather being charred? Maybe the cause and effect of chromed tanning process.

Please give the explanation so that fifth grader can understand if possible.

Thank you very much for doing this.

 

[spectre6000]

Can you elaborate more on the chrome tanned leather being charred? Maybe the cause and effect of chromed tanning process.

Please give the explanation so that fifth grader can understand if possible.

The comparison to a charred vs. well cooked hot dog was intended as a relatable analogy. Might be a bit American-centric. I'll do my best on the fifth grader level... I have zero experience with that age level, so hopefully I get close.

If you're cooking something, you're using heat to chemically change the food. The food takes time to heat up, and it takes time for the heat to make its way from the outside of the food to the inside. More heat won't make it cook faster, because that time requirement is a physical property of the food being cooked. The time it takes the heat to traverse the medium of the food itself (hopefully that's not too 6th grade) is a pretty limiting factor. If you cook the food at too high a temperature, you'll burn the outside before the inside gets up to temperature.

Maybe instead of a hot dog, think cake. If you bake the cake too hot, the outside gets dry, and even burned before the inside sets. It turns out, bread is a pretty good insulator. Carbon foam (burnt bread) is an even better insulator. As a result, you're pretty well limited to baking at 300°-400°. Too cold, and it dries out before it gets cooked just because it takes so long and the water evaporates and things just don't quite get as hot as they need to get to do their thing (I'm not a food chemist). Too hot, and you burn it (also a function of the water evaporating, but then sugars caramelize, etc. as well) while the inside is still liquid with the burnt outside insulating it further.

Tanning leather is a similar process, only instead of cooking with heat, you're cooking with chemicals. Think ceviche, only all the way through. Vegetable tanning takes months, and is like baking your cake at an appropriate temperature. Chrome tanning only takes days, and is like baking your cake at a super high temperature to where the outside of the cake is mostly palatable, and the inside is just barely done. Chrome tanning is much more aggressive. It's like the heat is turned way up. There are still limitations to the rate at which the "center" of the leather can be "cooked", so "cooking" it faster means you burn the outside a little.

You can see this in the photos in the fibers. The vegetable tanned fibers are full and tight. They're fatter and less dry looking... with less frizziness like frayed rope. They're also in physical contact with one another, and support one another like a dense felt. In chrome tanned leather, the fibers are much looser. They're reduced in width, frayed, and more separated from each other. In cheap chrome tanned leather, the tanners get as aggressive as they can with the chemicals, and can reduce the three days down further (I don't know the limits here), and save time/money. Doing so is much more aggressive, and like turning the heat up even hotter, and the fibers are more "burned". This means they're reduced in width, and they no longer support each other physically, etc. as described above. The most extreme example I've personally seen of this had thin white fibers that were practically held together solely by the top coat; think old school vinyl table cloth type fake leather, only actual leather.

In hybrid tanning (think Horween's ChromeExcel), they start the process off with a chrome tan, and "cook" the leather some percentage of the way through in a manner that doesn't "burn" the fibers as badly. Then they finish the leather off with a gentler vegetable tanning process. Kinda like starting your cake off at a really high temperature, then dropping the temperature partway through. The outside is going to be a bit over done, but it might not be all that noticeable. It saves time/money without too much detriment to the end product. It's a decent compromise, I suppose. I'd still prefer a straight vegetable tan for my money, but that's not a very common option these days and is more expensive when it is. It just takes longer, and time is money, and there's really nothing to be done about it. March of progress and all.

Now, a note to be aware of. One of the hallmarks of chrome vs veg tan is that the veg tan is more firm like for like as discussed above. It is entirely possible to have veg tan leather be softer than chrome tan by a process called milling. To continue the cooking analogies, milled leather is like beating it to death with a meat tenderizer. The fibers are physically broken up, reduced, and separated. It's a very common treatment for garment leathers.

I believe you can also get softer leather if the hide is allowed to rot before being tanned. I've never handled these leathers, as they wouldn't make it through QC anywhere I've ever purchased leather or leather goods from, but it's easy to imagine the whys and whatfors there.

 

[handymike]

Thanks for this thread! Very interesting.

 

[torfjord]

Very interesting to see different leathers up close!

 

[Observe]

Interesting stuff, thanks for sharing. The briefcase leather appears to be the most natural and highest quality.