spectre6000

It's not come up so far in my readings. Heat and water in the liming phase would likely convert the collagen to gelatin. I could see the gelatin either dissolving away, or mayhap it's the dried (and hardened) gelatin that gives rawhide its mechanical properties... I'm also not sure the role the high pH from the lime has in gelatinization... Do you have something specific in mind or were you referring to something specific? Just in case I didn't mention it above, TinkerTailor, I have the book you linked to open in another tab, and it's next on my reading list. I think your breakdown of the neet/neatsfoot preference is very apt, and I lean mostly toward #5. My dad calls that sort of thing 'tribal knowledge', and I think the concept applies very well in this case. No real science or understanding save "that's just how it is".

Apparently I hit the wrong button on that last one... Spamming my own thread. Note to self for next time... Anyway, my more educated friends and I have parted ways temporarily to go prepare our respective contributions to the holiday fare, and I figured I'd recap the conversation while it was fresh for posterity: The question was asked about the pH of the leather, I said it was generally between 3 and 5, and we agreed to that having an antioxidative effect on the rancidification process that wouldn't stop it by any means, but would certainly slow it down. It was agreed that the theory regarding the mechanical properties of the leather would likely slow the propagation of free fatty acids to the surface of the leather, and it was suggested that a vapor pressure test should confirm this pretty readily (mayhap the physicist of the pair has the appropriate apparatus handy in his lab, but it wasn't pursued further). These things in combination would undoubtedly slow, further slow, and diffuse any undesirable aromatic byproducts of rancidification, and this seems to answer at least that portion of my line of questioning satifactorily. There was talk of what specific aldehydes and aromatics were in play with the decomposition of the various fats in question and whether and what would be un/desirable among them. Then the phones came out and a search for scholarly papers on the subject ensued; I can't get through the pay wall since I'm just some guy, but they're both employed at a major university as researchers so they slip right on through. Unfortunately, they quickly discovered what I had already known; that, paywall or not, there doesn't seem to have been much in the way of serious scholarly pursuit on the subject in the last century, and certainly not since the pre-internet barrier came to be. We then talked briefly about the oxidation/polymerization of the shortest of the fatty acid chains in question (specifically a-linoleic a la patent leather as mentioned above) and whether or not or how flexible and permeable the membrane would be, whether or not there was any benefit to somehow using an impermeable polymer membrane to retain a secondary tri/diglyceride/fatty acid combo and/or serve as an oxygen barrier to prevent degradation... Enzymatic, photo, and time degradation were then referenced and that train of thought sort of fell off the tracks, and then the cinnamon rolls were done. The final quote (according to my wife) was, "I don't know, but there's cinnamon rolls!" (in a thick German accent). In short, we're a lot of fun at parties. I doubt there will be much more conversation on the subject, as the beer is about to come out and our favorite brewery here locally just released a pretty excellent stout that we're all excited to try with the main course!

On a side note because I found it interesting just now, Prussian Blue is listed as a catalyzer in the oxidation of linseed oil into a hard varnish... Seems like it might make a neat wood finish depending on the concentration of Prussian Blue and whether or how it's intended to be recovered after the reaction (it's mentioned in the previous sentence that the catalysts are often recovered and reused in the context of patent leather). I found a study on the subject of resistance to UV degradation of the coloring pigment, but nothing really about the combination as a wood finish... Yet another rabbit hole in an entirely different zip code (actually more of the same)... A little rabbit hole traversal because I'll find some other shiny flittering thing in a few minutes and forget, it seems Prussian Blue and linseed oil is essentially the basis for blue oil paints, so it must stay in suspension after the oil has oxidized and hardened, so the idea has legs. It happens to be a shade within my wife's favorite color palette, and a significant portion of my life is roughly Prussian Blue, so it may unlock my ability to engage in more woodworking projects as well as stimulate my own interest therein since I don't care for chemically junk in my hobbies.

My flax oil experiment seems to have been put to bed somewhat... It has a high a-linoleic acid content (short chain polyunsaturated), and when it rancidifies and the fatty acids break off, they oxidize and harden to form a varnish. Another more traditional name for this oil when it's not used for food is... Linseed oil (I'm a wood worker... I should have known that!). Traditionally used in making patent leather. I'm seeing it referred to as Japanning, which is an antiquated term for applying a hard black lacquer to something, and I normally see the term used regarding black enamel on very old cast iron tooling... I wonder if linseed oil was somehow used there too... Yet another rabbit hole, but one that takes a second (or third or fourth) seat to the current train of thought since I have an active project in progress while I figure out this next step. So here's a little bit of circle closing... The whole "neetsfoot/neatsfoot is the only option" thing, pretty sure we can all agree that was never anything more than bunk and hooey, and oils from the opposite end of the fatty acid composition spectrum not only CAN be used, but are primarily used in certain contexts. I'm not ready to commit to something extreme and weird yet for my own current project, and every time I turn the page there's some new and exciting means of imparting some new and exciting variety of lipid into tanned leather... I have the book you suggested open in another tab in the browser, and I'm right about halfway through the one I'm currently parsing through. I have cinnamon rolls proofing in the oven at the moment, I'm drinking my morning black stuff, then all the Thanksgiving cooking excitement begins, so the reading may be a bit light today. I mentioned above that my German biochemist and physicist friends are coming over to join us in the consumption of many things, and I guarantee we dig into this topic (the wife's O-chem background may be very helpful in distilling a few questions I haven't yet resolved), so hopefully some casual chemical conjecture can make up for the lack of of reading time today! I guess the next part of the question that I didn't realize had so many parts, is essentially how 'essential oils' would play. They have a heavy triglyceride component, but there's also a significant amount of water and 'other' with the molecular content of the 'other' being a lot of small aromatic compounds. It's one of those things that certain people ascribe certain 'magical powers' (my words) in a health context without anything resembling critical thought, so it's not exactly proving a simple task to weed out the BS and get to some science on the subject. Plugged into the scientific community and familiar with the social constructs therein as I am, I feel it's probably a social taboo of sorts for any serious scientists to really dabble in it since it's so weighted down with the fluffy nonsense of the aforementioned cultural elements and could be perceived as damaging to ones' reputation and career. So... Good luck finding any hard reading on the subject, me. I think the best thing here might be to go about it indirectly and see if anyone has ever heard of some ultra-amazing-super-premium-first-born-per-square-foot tannage imbued with the special magical powers of rose oil. Rose oil is probably one of the highest profile substances through history that falls in the 'essential oils' camp, it's been around forever and ever, and if it isn't completely ruinous to the leather, I'm sure someone somewhere would have done it and charged insane quantities of currency for the pleasure of its care taking or ruin, and that person would likely have been very proud of their status as caretaker/ruiner and talked about it. What's more for the historical subset this might apply to, those people could have had literate people with pens and paper/vellum/tree bark under their employ and such an asset could have made its way to some sort of inventory type document... Maybe a bit of a stretch, but maybe not. Anyone know/heard/vaguely aware of such a thing that could tell me what it's called or where to look?

Reading, yes. No new memberships though, so I can't ask. This seems to be the most active forum on the subject on the internet right now, so here I am! I'm about halfway through a book on Archive.org on the subject of oils and greases in leather from 1919. Pretty solid reading. It's late enough that the chemistry doesn't border on alchemy, but still early enough that I'm having to look up some interesting things with some frequency. The current rabbit hole involves taking it to the extreme with waxes... Full saturation, but no mobility at normal temperatures... This seems to have a hardening effect more than that of lubricating fibers though... About to abandon the trail. New elements of understanding include the role of leather from a mechanical perspective. Still trying to see what, if any, chemical interactions are in play. There's evidence (from nearly 100 years ago, so it may have been concluded one way or another by now) to suggest that oiling is a continuation of the tanning process itself, and it seems the most common oils for this are the heavier varieties... I've not found anywhere where it's suggested that using some especially pleasant oil (rose oil or something for instance that might impart an especially pleasant smell while it lubricates fibres) is especially good or bad, or that anyone has been so inclined as to justify the cost... Current state of understanding in the same manner as the last post (to elicit comments while I continue research) shows that the primary purpose of oiling leather is to lubricate fibers, with a potential secondary function being to continue tanning (I'm only concerned with vegetable tanned leathers for anything here, btw). It seems that rancidity (break down of the triglycerides into diglycerides and free fatty acids) is going to occur no matter what, and the composition of any particular oil has no bearing on it one way or another; it just is what it is. I believe what happens is that the fatty acids and diglycerides, being smaller molecules, are more mobile than the triglycerides, and sort of just hang out in the leather not really doing a whole lot until they're able to migrate to the surface. The shorter chain fatty acids will evaporate as they normally would, but due to the inhibited mobility in the leather, the piece won't smell rancid unless there is an excess of oil at the surface due to low concentrations. The heavier molecules will accumulate at the surface with those that are solid at a given (typically room) temperature crystalizing in the form of either salt or fat spues/spews (I've seen it spelled both ways often enough to justify using both spellings whenever the need arises). Spues/spews mostly occur in or just out of the tannery as a result of lower quality (partially rancid) fat liquors at the appropriate stage of tanning, but can also occur from over oiling with heavier oils (i.e. neetsfoot). Lighter oils that degrade into shorter chain fatty acids will be less susceptible to spues/spews, with the fatty acids either leaving an oily/greasy residue or evaporating (still investigating this). Most of the oils I'm seeing reference to are animal-sourced, and the vast majority of those are of marine origin (which came as a bit of a surprise). Oils from vegetable sources do come up on occasion, but it's rare, and I've not seen anything especially detailed regarding the lighter oils. I can speculate as to why this might be, but I don't think it's for lack of suitability... Meanwhile, time to go do holiday stuff... More reading and reporting will have to wait until I have some more free time.

Beautiful! I finally found a search train that is proving fruitful! What's more, if I'm not mistaken, it sort of turns a few "time honored traditions" on their heads... Over and over again I'm finding that "oils with high melting points" should be avoided... By definition, this pretty much boils down to your more saturated oils, of which neetsfoot oil is about as saturated as I've seen among the readily available oils I've considered so far. What's more, I've also seen stated repeatedly that oils with a high stearic and palmitic content should be avoided as well (18% of neetsfoot oil is comprised of these two fatty acids). It seems that free fatty acids and diglycerides (the byproducts of oil rancidification) precipitate and crystalize on the surface of leathers in the form of spew/spue (spelling varies from source to source). I'm mostly finding it referred to as the result of problems in the initial tanning, but it seems that it would also be the result of over oiling, oiling with rancid oils, or simply time and neglect. The cure for this seems to be simply washing/cleaning like you would with normal care and reapplication of oil. If this is the case, any oil will do the job beautifully (save something like motor oil, which is a completely different animal). There does seem to be some association with spew/spue and higher stearic/palmitic acid oils as well, such that I'm not sure it's not limited to those two fatty acids. It seems like there may be some disadvantage in oils with higher saturation (often this will mean higher stearic/palmitic content) and oils that rancidify more easily... Too much to one end of the spectrum is just as bad as too much to the other end, bringing Hesiod to the fore on moderation... Additionally/on the other hand (and again, assuming I'm following correctly and my assumptions are not in error), It is stated that non-drying oils with the LOWEST melting point are preferred... Counterintuitively, those would be oils with the LOWEST degree of saturation, and thus most likely to go rancid in the normal sense. The diversity in oils suggested for treatment of leather borders on staggering such that I feel pretty confident that there is no wrong answer to the question of what oil to apply for the treatment of leather; the different fatty acid composition will only affect the frequency of care required to keep the leather in tip-top shape... This jives pretty well with my assessment that neetsfoot oil's preferential treatment lies solely in tradition, and enjoys that benefit solely due to the fact that it's what was lying around in proximity to leather production back in the day that had no other real use (i.e. foodstuffs). I have more rabbit holes to traverse, but I thought I'd leave this here to see what sort of comments might surface.

¿Que pasa, Tejano? (I grew up in Texas, and Texican is my native tongue) Unfortunately, that was pretty much exclusively on the subject of chromium tanning, I'm only interested in vegetable tanned leathers (forgot to mention it in the OP), and the only real reference to vegetable tanned leather chemistry is the dreaded "the chemistry is not well understood" plea of ignorance (in my travels, this is a common feature of very old and mature arts/crafts/disciplines where the 'how' is understood well enough that most people don't concern themselves so much with the 'why'). Fortunately, it was still pretty readable and answers the question of the source of the sulfur smell exuded by tanneries and a few others. Unfortunately, I'm an engineer and not a biochemist, and while I have a pretty solid understanding of the chemistry in the oils from my biodiesel days, the beyond my ken. Fortunately for my 'Ken', my neighbor/best friend/Thanksgiving guest, we'll call her Barbie for the sake of the joke, is a biochemist and married to a physicist who also has a better chemistry understanding than I do, so between the three of us, we might be able to sort out my understanding a bit (at least as far as chromium tanned leathers are concerned). Thanks! If you know of any other similar articles (especially on the subject of vegetable tanning), I would appreciate the reference!

Thanks! I guess that depends on whether or not 'craft' has any special meaning within the leatherworking vernacular (I honestly don't know). I have used leather in the past to make things, I'm currently engaged in a leather project, and I have tools and supplies for doing so, but it's neither my primary vocation nor avocation. I used to work in the biodiesel industry (experimental feedstocks), which is where my knowledge of this sort of thing originates.

This a question for anyone here with a organic chemistry background, or at least a thorough scientific bent. Any leather that's been cared for has had oil applied to it over time, but leather never smells rancid (assuming something hasn't been actively done to cause it to do so). What is it about the application of the oil to leather that prevents it from smelling rancid? Is it a mechanical interaction wherein the aromatics are simply unable to escape? This seems unlikely since leather has a characteristic smell. Is there some chemical or family of chemicals in leathers (if so, what?) that binds preferentially to the hydrogens in the double carbon bond kinks? Is it actually going rancid and something is just binding with the free fatty acids? What's going on exactly? Any oil will go rancid if left exposed to air, light, heat, time, etc. Neatsfoot is a nice traditional oil to use, but let's be honest... It isn't the most pleasant. As an oil, it's nothing special chemically... It has tradition behind it, and more importantly myth and lore, but really nothing more. It's more saturated than most, sure, but it doesn't really have any magic to it. Olive oil seems to be a commonly used alternative, but it's fatty acid profile is nothing special either. Olive oil is even less special (pretty average fatty acid composition with very little saturated content), and it's been used forever with positive results the same way. I have some leather scraps that I intend to oil with flaxseed oil (the highest polyunsaturated fatty acid profile I'm aware of that I can get readily and still think of a use for after the fraction of an ounce has been used for the experiment), but it will take months (or years) before I can say conclusively that fatty acid profile has no effect on anything in leather (the hypothesis)... If I can understand the mechanism in play or if it's already been done, I can save myself some time and just move onto my ultimate goal in the immediate project.

Might as well make the first post a doozie... Yup. It'll go rancid just fine. Just give it time and let it breathe. Rancidity happens when the free hydrogens in the fatty acid chains of the lipids bind with oxygen, and break off to form free fatty acids. The resulting smell depends on the length and composition of the free fatty acids. Shorter chains are more aromatic (more likely to go airborne and thus smell more), so where the double carbon bond is in the chain can change the resulting smell or lack thereof, but rancid is rancid. Neatsfoot oil is about 70% monounsaturated (oleic, palmitoleic), 18% saturated (palmitic, stearic), and 2% polyunaturated (linoleic). Fully saturated oils go rancid far more slowly than mono- or polyunsaturated oils, but free fatty acids can still be formed from UV exposure, temperature, etc. They're just less susceptible to oxidation. Nearly (nearly only because I don't claim to be an expert on all fats and oils from all sources) all natural oils/fats have some mono-/polyunsatuted content, and will eventually go rancid if exposed to oxygen, light, time, etc. Animal fats tend to be more saturated than fats from vegetable sources, but if its liquid at room temperature it is mostly unsaturated. My guess is that the myth of neatsfoot oil not going rancid is due to it smelling nearly rancid from the start... It's also possible that the resulting free fatty acids have a longer chain (likely given the animal source), and are less aromatic than the free fatty acids resulting from the rancidification of some plant-derived oils. I would further surmise that it's the preferred oil for leather because traditionally people didn't have such a wealth of choice and used what was available, applying preference only when there was a choice to be had. People processing leather would have very likely had a ready supply of neatsfoot oil due to it coming from essentially the same source, and it certainly isn't something you'd want to eat... Just a guess based on a general understanding of history and whatnot.