Oils In Leather (Chemistry)

[spectre6000]

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.

Edited November 23, 2015 by spectre6000

[TexasJack]

The answer to your question is far more complicated than you may think. Oils go "rancid" because they oxidize into aldehydes and ketones. Oils that are highly unsaturated (lots of double bonds) become rancid much faster than those that are more saturated (mostly single bonded carbons). Heat, bacteria, and exposure to air all contribute to this. Leather, however, is also a complex and reactive material. Lots of things go on when it is treated. I found the following link that has a lot of information (that may be far over the heads of many people) and it may give you a better understanding. Chemistry of the Leather Industry - New Zealand (Please note that the document is in PDF and the link will cause it to download).

Edited November 24, 2015 by TexasJack

[spectre6000]

¿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!

[TinkerTailor]

Do a search on archive.org and restrict it to pre 1930 and look for tanning and leather books. There are a ton from the heyday of leather and quite a few do get into chemistry. I have seen manuals and textbooks on there from the 40s and 50s with more modern chemistry, however i found searching later stuff isnt as fruitful. All are free to download. Lots of recipes for leather treatments as well. Some with nasty ingredients though.

Edited November 24, 2015 by TinkerTailor

[spectre6000]

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.

[simontuntelder]

Have you tried the leather chemists forum?

They should know....

[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.

Edited November 26, 2015 by spectre6000

[TwinOaks]

In your analyses, I hope that you don't overlook the pH of leather as a contributing factor. Most veg tanned leathers will have a slightly acidic pH. That acidity may have a prohibitive effect on the oxidation.

You might look into how the collagen in the leather fibers interacts with the oil, too.

[TinkerTailor]

In your analyses, I hope that you don't overlook the pH of leather as a contributing factor. Most veg tanned leathers will have a slightly acidic pH. That acidity may have a prohibitive effect on the oxidation.

You might look into how the collagen in the leather fibers interacts with the oil, too.

I got alot out of this book: The recipes in it obviously take ph into account based on application.

Get out your bucket of spermaceti, your rape oil, and that stash of caoutchouc, Its time to make some leather dressing! Did you remember the brown sugar?

I give you:

"The manufacture of lubricants, shoe polishes and leather dressings"

https://ia600404.us.archive.org/1/items/manufacturelubr00brungoog/manufacturelubr00brungoog.pdf

All page numbers i list are pdf page numbers, not the original page numbers.

Page 29 of the pdf, they talk about the ph of oils and how it changes as it rancidifys due to the presence of free fatty acids forming. It also clarifys what are called neutral oils, which are oils that do not have the free fatty acids in a fresh state which change the ph, such as rapeseed(canola) oil as well as olive oil. They imply that some oils have the free fatty acids when fresh and are unsuitable for use with metal due to corrosion issuesdue to Ph. I would imagine the same thing applys to leather. They also get into drying vs non-drying oils, linseed oil, for eg, is a drying oil and is not very suitable for a lubricant and presumably a leather dressing, while it is used in recipes for leather varnish.

Page 102 is a writeup on neetsfoot oil and a comment about how many sewing machine and clock oils are bleached neetsfoot repackaged in tiny bottles and marked up.

pg 119 is the recipes for the fine machine oils. They also talk about freezing neetsfoot and straining out the oil that is still liquid to purify it. Also bleaching it in the sun using violet glass.....Had they discovered uv treatment? They did notice purple glass bleached it better for some reason.

Page 114 is an interesting recipe for leather belt dressing to prevent slip made from ~90% castor oil and 10% tallow.

Page 142 is where the good stuff starts. The stuff relating to leather. You will notice that many of the recipes for shoe polishes etc contain sulfuric acid or soda, Presumably to make them strong enough for a man but Ph balanced for a woman.........Or is that deodorant?....rabbit holes are fun.........squirrel

Page 148 talks about ant-acid boot leather varnish .....acid free....The effects of ph on leather were DEFINITELY known at this time.

I have from my reading determined that as a leather treatment, tallows are the best treatment for lubricating the fibers and preserving the leather for a long time, however they are hardest to apply due to being mostly solid at room temp. Tannerys hot stuff tallows and waxes to make that expensive horween stuff. Temperature and exposure time are needed for the leather to take up the fats fully. These processes are out of the reach of the average user as they require special equipment and machinery and is better done in bigger batches of hides. The tallow lubricates, and the wax protects. Both are very long lasting before breakdown, if it even happens.

We as leatherworkers try to approach this on raw leather but without the prolonged heating and tumbling. Oils are the best solution to make leather treatments that are easy to apply. In order to apply the harder waxes and tallow, we often mix then with thinner oils to make easily appliable pastes and cremes.

In my opinion,Neetsfoot oil has won over many as the oil of choice for a few reasons:

1: The general availability and cheapness of neetsfoot oil as well as its suitability for leather in that it is non-drying, long lasting, and has a long shelf life

2: It is easy to apply. It can be used to make waxes and tallows thinner and possible to apply at room temp

3: The US army chose it.

4: It works in most of the situations leather is used as an adequate dressing.

5: Grampa used it, and so did his grampa......so did stohlman and some other saddler guy...

[spectre6000]

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?