spectre6000

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.