Experiment this Weekend: Carta Boili

[Russ Mitchell]

Looking at "poor man's modern equivalents" for hidden armor and "get them on the field" armor that aren't plastic, I'm going to see if the tannin theory works on its own, and soak paper and cardboard in strong tea.... then bake the stuff at 180 for a while and see what happens.

It's weird, but it occurred to me that to my knowledge, nobody's done a falsify test on the "it's the tannins that polymerize" theory.

[Russ Mitchell]

A roll of paper, soaked in tannins (strong tea similar to what I've used before for tanning leather), and a control, of the same paper, simply wet, were heated at 180. There was absolutely no appreciable difference between the two.

Which is what was expected, since tea-soaking and baking is a known means of creating artificially-aged paper. But in *our* context, this is important -- it means that a significant amount of the basic theory for how cuirboilli works (i.e., polymerization of the tannins) is inadequate for the purpose.

Which begs the question: would soaked UNtreated rawhide polymerize in the same heat? Chrome-tanned leather does not make cuir-boilli, thus giving rise to the "tannin theory." But something's missing.

[Mac]

Russ,

I would have expected that if "water hardening" of leather involved polymerization, that it would be the collagen, rather than the tannin that actually polymerized. Am I "out to lunch" here?

Mac

[Russ Mitchell]

That makes some sense, except insofar as it's experimental "received wisdom" that chrome-tanned leathers don't water harden. And they have plenty of collagen. Theory I've been hearing is that the polymerization involves the tannins -- if it doesn't, it MAY simply be that the chrome provides temperature stabilization beyond what a pot of water will overwhelm (such tests are standard among leather chemists). Thus the question: can you make cuir boilli with.... cuir? Straight rawhide? I gave mine away to Eidolon and some other folks, and don't have any on-hand to test, sadly. I'll poke about and see if I can find some.

[Russ Mitchell]

It's ALWAYS bugged me, Mac: ask any tanner (I mean, ANY tanner), and they'll tell you that the hardest part of making leather is softening the hide. So why go through all that damned work, if you're just going to harden it back up again, especially in a semi-industrial society where they don't have vast inventories of the stuff just hanging around waiting to be delivered?

[Mac]

Russ,

First of all....Are we sure that chrome-tan won't harden?

Second.....Perhaps the collagen will only polymerize in the presence of tannins.

Third....If the above are both true, can chrome-tan be made to water harden by adding tannins? As a first approximation to this, you could see if the sort of re-tan that begins with chrome and finishes with tannins will harden.

Mac

[Russ Mitchell]

Chrome tan should not harden. A "boil test" puts one in boiling water -- it's supposed to be stabilized through that temperature so that no chemical changes occur. I don't know about a retan under those conditions, because in theory it would depend on the degree to which the chromium salts were removed.

Whereas if untreated hide DOES harden (any archiver with real "not a chew toy" rawhide and an oven could test this easily), we know it's not the tannins at all. What I've done is demonstrate that the tannin oils aren't doing anything notable. I do suspect it's the collagen, and have always been struck by the fact that thin vegtan doesn't harden for squat -- you need a bare minimum of belt-thickness, and preferably at least 8 ounces, to have any kind of hardness that would be useful for armor (and optimally in the 12/13 range).

I don't have any rawhide, but I DID find a scrap of old alum-tan (true alum-tan from the guys who used to do Rawling's baseballs, w/o all the extra crap Siegel's tanners put into theirs). I've got that in now, though I don't know to what temperature alum's supposed to be stable.

[Kel Rekuta]

Ah, Russ. Your timing is impeccable. I'm working on the quarter and year end reports for my business right now and you have to dig into something interesting...

I haven't read this stuff lately but IRRC it is the collagen in the presence of sufficient tannin and moisture that polymerizes at 167F. Too much heat and the polymer shrinks and becomes brittle. Dig out your copy of John Waterer's "Leather and the Warrior" for a brief description of the process.

As an aside, tawed leather (alum "tanned") isn't really tanned. Alum stabilizes the hide while it remains in the fibre. Sufficient wetting and drying will wash it out. A lot of the "crap" added is to prevent this happening. Face it, about the only use for tawed leather these days is sports equipment and that gets wet frequently.

Chrome tanned leather doesn't have tannins present per se. Chromium salts used in by 98% of the worlds' tanneries are stabilizers to the same effect as vegetable tannins or alum or oil/heat processes (chamois) in that they preserve animal epidermis from putrification in the presence of ambient moisture and fungi/ bacteria. None of them are directly comparable processes.

Now before you dismiss my comments, give me a couple days to get out from under the heavy hand of the tax department. I'll dig something out of my leather industry library to support and clarify this.

[Russ Mitchell]

That part's old-hat, Kel, but what does "in the presence of" mean? Chemically speaking, that's pseudoscience.
But I'll hold on and see what you can dredge up.

[coreythompsonhm]

It would be nice to know what untreated rawhide would do. I have a feeling that the collagen and tannin react to each other when soaked with water and heat, creating the hardened polymer. Possibly without tannin, the leather will not polymerize. I currently do not have a source of true raw hide, otherwise I would help you out with this experiment.

[Kilkenny]

That part's old-hat, Kel, but what does "in the presence of" mean? Chemically speaking, that's pseudoscience.
But I'll hold on and see what you can dredge up.

mutter. It's not scientific to dismiss information due to the form of presentation.

Kel's telling you that it takes three things to harden leather. The leather, tannins and water.

You've tested whether it can be done without the leather, with the tannins and water and found it doesn't work. Can't say that was a surprise.

I can tell you that I've tested a different pair of the ingredients - Tannins and leather, without the water. Doesn't harden. I've baked dry leather for four hours at over 200 degrees (required curing process for the food safe liner I use in my jacks and bottels) without any impact on the behaviour of the leather.

Leather with tannins, damp and heated - you get hardening. It takes all three, and not in any magic ratio, since you can do it with boiling water immersion, or with barely damp leather in the oven.

"Presence of" is probably as precise as it needs to be for a reaction to occur...

[Russ Mitchell]

Respectfully, you're missing the point. What I've done (and all I've done) is confirm that the tannins themselves aren't doing anything b/c of the heat. If all you want is end product, then this discussion is irrelevant to you. Feel free to disregard.

However, for those who actually want to understand why it's happening the "scholarly commonplace" needs to be tested, not assumed: are the tannins actually necessary for hardening*, and why? Not simply for armor, but for a dissection of the technology behind it.** Why is hard (an acidic effect on the collagen which alters the chemical chain, encouraging polymerization? Something specific to tannic acid? All tannins do is coat the collagen fibers, preventing them from "gluing up" and getting rigid when their moisture content drops under 23%: there's no non-reversible chemical reaction which happens, and the "standard disclaimer" against alum-tawed leathers, that they wash out, applies just as readily to "veg-tan," as any Tandy shop owner telling you not to moisten too much when tooling unless you want stiff, dried-out tooling-faces can tell you), but necessary we ought to be able to confirm it with a simple experimental test to see whether wet rawhide hardens the same way. As an example, I have an archer's bracer from Dobson's workshop. It's thin leather, thin enough that in straight veg-tan I would expect some hardening, but not a whole lot. On the other hand, this thing is hard as brickbats -- and just happens to be a scabbard butt with a rawhide core. Since rawhide is no harder (and arguably softer) than cuir-boilli, why the difference?

*hardening to cuir-boilli as we know it, rather than merely being "rawhide stiff," which will eventually soften with moisture.
**Example: how did the German crossbow-making guilds waterproof their composite prods? If presence-of-tannins is sufficient to harden and permanenly alter collagen, then one ought to be able to mix them into a batch of hide glue, glue it up, and see it harden into a non-reversible bond.

[Russ Mitchell]

It would be nice to know what untreated rawhide would do. I have a feeling that the collagen and tannin react to each other when soaked with water and heat, creating the hardened polymer. Possibly without tannin, the leather will not polymerize. I currently do not have a source of true raw hide, otherwise I would help you out with this experiment.

Thanks, Corey. I gave Eidolon quite a bit of it -- I may lean on him to try to harden a scrap if his oven's precise enough to allow.

[Mac]

**Example: how did the German crossbow-making guilds waterproof their composite prods? If presence-of-tannins is sufficient to harden and permanenly alter collagen, then one ought to be able to mix them into a batch of hide glue, glue it up, and see it harden into a non-reversible bond.

Russ,

These prods are generally (always?) covered in birch bark. Birch bark is also used on the undersides of saddle trees as a moisture barrier. I offer this as a point of information, and not as contradiction. If there is also something going on to make the glue more water resistant it would be very interesting to know.

Mac

[Russ Mitchell]

Mac,

Thanks. Sorry if I've come off touchy, folks -- I got this off the ground with a running commentary rather than waiting until I'd done everything and then summarizing, as I usually do, so any miscommo about what I'm up to is definitely MY fault.

[Kilkenny]

Respectfully, you're missing the point. What I've done (and all I've done) is confirm that the tannins themselves aren't doing anything b/c of the heat. If all you want is end product, then this discussion is irrelevant to you. Feel free to disregard.

However, for those who actually want to understand why it's happening the "scholarly commonplace" needs to be tested, not assumed: are the tannins actually necessary for hardening*, and why? Not simply for armor, but for a dissection of the technology behind it.** Why is hard (an acidic effect on the collagen which alters the chemical chain, encouraging polymerization? Something specific to tannic acid? All tannins do is coat the collagen fibers, preventing them from "gluing up" and getting rigid when their moisture content drops under 23%: there's no non-reversible chemical reaction which happens, and the "standard disclaimer" against alum-tawed leathers, that they wash out, applies just as readily to "veg-tan," as any Tandy shop owner telling you not to moisten too much when tooling unless you want stiff, dried-out tooling-faces can tell you), but necessary we ought to be able to confirm it with a simple experimental test to see whether wet rawhide hardens the same way. As an example, I have an archer's bracer from Dobson's workshop. It's thin leather, thin enough that in straight veg-tan I would expect some hardening, but not a whole lot. On the other hand, this thing is hard as brickbats -- and just happens to be a scabbard butt with a rawhide core. Since rawhide is no harder (and arguably softer) than cuir-boilli, why the difference?

*hardening to cuir-boilli as we know it, rather than merely being "rawhide stiff," which will eventually soften with moisture.
**Example: how did the German crossbow-making guilds waterproof their composite prods? If presence-of-tannins is sufficient to harden and permanenly alter collagen, then one ought to be able to mix them into a batch of hide glue, glue it up, and see it harden into a non-reversible bond.

Russ, any particular reason you're being a condescending snot at the moment?

I think I'll just absent myself from this thread.

[Russ Mitchell]

A general apology has already been tendered: I'm more than happy to make it personal in your case.
The intent is not to be snotty, but to get to the bottom of how cuir boilli actually works -- the general theory glosses over some serious shortcomings, and I'm trying to figure those out.

Meanwhile, I think it's safe to say this thread really ought to just be deleted. I'll circle back when it's all over. Mods?

[Andeerz]

However, for those who actually want to understand why it's happening the "scholarly commonplace" needs to be tested, not assumed: are the tannins actually necessary for hardening*, and why?

I've been doing some reading on what is known in the chemistry literature (which is not as much as I thought... it's a lot of speculation) but I feel I can safely assume that the tannins are not necessary for hardening.

Why is hard (an acidic effect on the collagen which alters the chemical chain, encouraging polymerization? Something specific to tannic acid? All tannins do is coat the collagen fibers, preventing them from "gluing up" and getting rigid when their moisture content drops under 23%: there's no non-reversible chemical reaction which happens

Tannins do more than just that. They DO coat fibers and essentially prevent them from gluing up (at least I think...). Tannins are hydrophobic and they bind to collagen through hydrogen bonds. They can "coat" collagen fibers and make them unable to solublize in water and stick to other collagen fibers. However, they also crosslink fibers by reacting within the nooks and crannies of the individual fibers, which are made of three strands a piece in a triple helix, and bridging together these strands through hydrogen bonding. This makes the fibers very strong and resilient to being denatured (by heat, bacterial/fungal enzymes, etc.) They can also do irreversible covalent bonding as well, which I think can aggregate collagen fibers and/or further bind together collagen strands. The cross-linking can sort of be reversed sort of, but only if the heat is high enough (cuir-boilli!!!) and/or certain solvents are used. This is what gives tanned leather its durability and a large part of its resistance to rot (though there are other mechanisms I can talk about if you want!). What gives veg-tanned leather its flexibility (to my understanding) is the swelling of the leather and separation of fibers the tanning process does in general, and the keeping-apart of these fibers by the hydrophobic tannin coat of tanned collagen fibers.

That said, here's what I think the cuir-boilli process does and why it hardens: Heating in the presence of water imparts enough energy to the cross-linked collagen fibers and strands to break the hydrogen bonds formed by tannin. This allows the collagen fibers to become soluble in water as well as undergo denaturation. The collagen fibers denature in that their component strands unwind from being a helix and then become randomly coiled. The random coiling causes entanglement of various collagen fibers and strands that then settle in place after drying. The coiling also causes the leather to shrink. This process makes the leather very hard. However, it doesn't break all of the crosslinks and a lot of the tannin remains, continuing to confer protection against rot as well as some degree of flexibility if not cooked to high-hell.

but necessary we ought to be able to confirm it with a simple experimental test to see whether wet rawhide hardens the same way. As an example, I have an archer's bracer from Dobson's workshop. It's thin leather, thin enough that in straight veg-tan I would expect some hardening, but not a whole lot. On the other hand, this thing is hard as brickbats -- and just happens to be a scabbard butt with a rawhide core. Since rawhide is no harder (and arguably softer) than cuir-boilli, why the difference?

We DO need to test this, I feel. And I have a feeling that the ultimate answer will be that they both harden the same, but that rawhide will require MUCH less heat and probably is hard by default without heat (since the collagen fibers would stick together through extensive electrostatic interactions and tangling of fibers whereas veg tanned fibers would require cooking to remove crosslinking).

BTW, here's what I read: http://www.sciencedirect.com/science/ar ... 2207005663
http://books.google.com/books?id=Og7OwD ... ng&f=false
http://books.google.com/books?id=a8lm4I ... &q&f=false

[Russ Mitchell]

Hey Andeerz,

The gents at the ALCS (American Leather Chemists' Society) confirm the "prevention of gluing up" part. The rest is JUST the sort of detail I was hoping for. Thank you. Any idea from what you've read as to why the process seems to work best on thick pieces rather than thin ones, and why it can be mechanically short-circuited? (For example, and this drives me NUTS: if you put a piece of lightweight leather on a last, it'll harden just fine...but wrap cloth strips around it tightly so that the leather's compressed even a little bit, and BAM! Vastly less hardening). Surely the crosslinking's not happening on such a macro-scale that simple compression can prevent de-linking?

[eidelon]

Thanks, Corey. I gave Eidolon quite a bit of it -- I may lean on him to try to harden a scrap if his oven's precise enough to allow.

let me know what you want me to do and i will see what can be done

or i will just carve off a chunk and bring it by for ya to play with.

[Russ Mitchell]

Well, given a chance, I'd much rather get a piece from you, talk armor, and drink a beer. But otherwise, we just need to soak and then bake a piece at controlled temperatures, and see what happens to it.

[Andeerz]

Hey Andeerz,

The gents at the ALCS (American Leather Chemists' Society) confirm the "prevention of gluing up" part. The rest is JUST the sort of detail I was hoping for. Thank you. Any idea from what you've read as to why the process seems to work best on thick pieces rather than thin ones, and why it can be mechanically short-circuited? (For example, and this drives me NUTS: if you put a piece of lightweight leather on a last, it'll harden just fine...but wrap cloth strips around it tightly so that the leather's compressed even a little bit, and BAM! Vastly less hardening). Surely the crosslinking's not happening on such a macro-scale that simple compression can prevent de-linking?

Hey there!

I have no idea... But, just because something is on the "micro scale" doesn't mean that things like mechanical pressure can't affect things. I could totally see pressure changing how things work on the molecular level in this case. A good example is how water boils at different temperatures depending on pressure. Perhaps the mechanical pressure exerted by the cloth strips actually restrict movement of fibers just enough to significantly impact how well they coil with each other when denatured with hot water.

As for why thin leather seems to not work as well for hardening... I don't have a clear idea. I wonder if it might have to do with how thoroughly the thin stuff might be (emphasis on the "might") tanned compared to thicker stuff. I'd imagine that the more thoroughly tanned the leather is (more thoroughly collagen is coated with tannin and maybe the more hydrogen bond crosslinks and irreversible covalent crosslinks there are), the harder it is for the collagen fibers to dissociate, coil up, and stick together with a given cuir boilli treatment. This is PURELY speculation and dependent on if there is anything qualitatively different in how thoroughly tanned thick vs. thin leather is.

On another possible relevant note, I remember another thread on this forum talking about period leather for armour and other applications likely often being "half tanned", or tanned mostly on the outside and having a relatively raw-hide-ish center (like leather used in Scandinavian knife sheaths). I've been looking for some here in the states, but it is extraordinarily hard to come by. Supposedly you can get things even harder than with through-tanned stuff commonly available.

[Russ Mitchell]

That's true: I'm one of the guys who was driving that a while back, and it's possible I'll half-tan a moosehide I've got this spring. "Scabbard butts" as they call them are very hard to get stateside, though they're in wide currency in Scandinavia for knife sheaths. Dobson believes that this was the leather used for making leather armor, and the bracer I have from his workshop is notably harder than anything I could achieve with leather of the same thickness. On the other hand, the method appears to actually require the use of a last for hardening to occur properly -- another skeevy mechanical issue causing my brain to hurt.

[Andeerz]

OMG. If you do end up half-tanning that hide, and you have enough and are willing, I would totally buy some from you! Depending on how close you are to me, I could even help! (I am currently in Tucson, AZ)

[Russ Mitchell]

Usually I just do a hungary-tan (hot-stuffed alum-tawed hide: relatively easy to make, though labor-intensive, strong and ridiculously durable), but if I'm feeling motivated, it could definitely happen. Wouldn't mind some help for the first stage, though -- you get used to it eventually, but cadaverene doesn't mix well with coffee.

[eidelon]

Usually I just do a hungary-tan (hot-stuffed alum-tawed hide: relatively easy to make, though labor-intensive, strong and ridiculously durable), but if I'm feeling motivated, it could definitely happen. Wouldn't mind some help for the first stage, though -- you get used to it eventually, but cadaverene doesn't mix well with coffee.

well yah know how to get ahold of me

[Kel Rekuta]

Andeerz - excellent job digging. Well done!

As my earlier comments were for the general reader and were dissed anyway...
Russ, you clearly know everything you need to know at this point and can do just fine without my input.

I will add one thing, again for the general reader. The previous comment to the effect that "the most difficult problem facing modern tanners is making leather soft" is more than a little misleading. By far the most difficult problem facing modern North American tanners is making a profit under the ferocious restrictions of EPA regulation, high energy and labour costs and an historically high cost of green hides. Competition from Asian tanneries, both for raw materials and low priced product forces N.Am tanners to rush as much product through the pits as possible in as short a time as manageable. This necessity creates the problem of keeping leather soft because "too fast and too harsh" pit treatment creates stiff, burned, brittle product.

The problem of poor quality output isn't a process problem. It is a competitive financial problem N.Am tanners are coping with, barely. Presenting this as some sort of chemical or process issue is ill informed and frankly misleading.

[Russ Mitchell]

Mac,

Thanks. Sorry if I've come off touchy, folks -- I got this off the ground with a running commentary rather than waiting until I'd done everything and then summarizing, as I usually do, so any miscommo about what I'm up to is definitely MY fault.

I had thought this was a sufficient apology for poor tone. Apparently that is not the case. Please allow me to be more explicit.

My apologies to anyone who has been offended by the tone of my posts in this thread -- at no time has offense or disparagement been intended.

[Kel Rekuta]

Russ, I was writing you a PM while you responded. Have a look.

Don't worry, be happy mon.

[Russ Mitchell]

Yay. Oh - no argument on the economics, btw, vis-a-vis modern tanneries (about which I know relatively little). Torvaldr edumacated me on that a bit ago - hopefully the drought-forced spike in early slaughters that's driving beef through the roof right now will at least give those guys a momentary silver lining.

[Kelby]

You get hides soft by 1) shaving the skin down thin enough that it will be flexible once tanned 2) properly oiling then stretching and breaking the skin as it dries over the course of days/weeks.

[Arrakis]

Hey Andeerz,

The gents at the ALCS (American Leather Chemists' Society) confirm the "prevention of gluing up" part. The rest is JUST the sort of detail I was hoping for. Thank you. Any idea from what you've read as to why the process seems to work best on thick pieces rather than thin ones, and why it can be mechanically short-circuited? (For example, and this drives me NUTS: if you put a piece of lightweight leather on a last, it'll harden just fine...but wrap cloth strips around it tightly so that the leather's compressed even a little bit, and BAM! Vastly less hardening). Surely the crosslinking's not happening on such a macro-scale that simple compression can prevent de-linking?

Hey there!

I have no idea... But, just because something is on the "micro scale" doesn't mean that things like mechanical pressure can't affect things. I could totally see pressure changing how things work on the molecular level in this case. A good example is how water boils at different temperatures depending on pressure. Perhaps the mechanical pressure exerted by the cloth strips actually restrict movement of fibers just enough to significantly impact how well they coil with each other when denatured with hot water.

As for why thin leather seems to not work as well for hardening... I don't have a clear idea. I wonder if it might have to do with how thoroughly the thin stuff might be (emphasis on the "might") tanned compared to thicker stuff. I'd imagine that the more thoroughly tanned the leather is (more thoroughly collagen is coated with tannin and maybe the more hydrogen bond crosslinks and irreversible covalent crosslinks there are), the harder it is for the collagen fibers to dissociate, coil up, and stick together with a given cuir boilli treatment. This is PURELY speculation and dependent on if there is anything qualitatively different in how thoroughly tanned thick vs. thin leather is.

On another possible relevant note, I remember another thread on this forum talking about period leather for armour and other applications likely often being "half tanned", or tanned mostly on the outside and having a relatively raw-hide-ish center (like leather used in Scandinavian knife sheaths). I've been looking for some here in the states, but it is extraordinarily hard to come by. Supposedly you can get things even harder than with through-tanned stuff commonly available.

Mechanical effects of small pressure gradients can even influence residual stresses in thin films (on the sub-micrometer scale). I would certainly say that mechanical factors cannot be ruled out as affecting the process(es) at work here.

PS: Why doesn't someone with access to a decent SEM or ultra-high-power optical microscope just look at the cellular and granular structure of unhardened veg-tan and hardened veg-tan cut sections? That should reveal any structural or granular changes in the leather, for sure.

[Russ Mitchell]

Great idea; anybody got one?

@Kelby: yeah, and that's a lot of work! It's why I only tan hides every other year or so (and only when I can get a hide cheap enough for it to be worth the bother, of course -- as a suburbanite, my access to processor-skins is low).

[Arrakis]

I'll see if my buddies in the microscopy labs can do it if I give them samples.