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?

[spectre6000]

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.

Edited November 26, 2015 by spectre6000

[spectre6000]

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.

Another one... man... This archive.org is a freaking gold mine!!!! A Japanning recipe that sounds super easy and doable for tools... Boiled linseed oil, Prussian Blue, and vegetable black (finely ground carbonized whatever vegetation)... Nothing special anywhere in the mix, and Japanning is one of those 'lost arts' in that space. I've got a big engine coming soon for an automotive project, and I was going to use some sort of simple paint on it, but I may just Japan the big ol' bastard for the hell of it! That would be super cool!

[spectre6000]

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!

Edited November 26, 2015 by spectre6000

[spectre6000]

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

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".

Edited November 26, 2015 by spectre6000

[spectre6000]

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.

Free fatty acids are the result of the decomposition of the triglycerides into diglycerides and free fatty acids. This typically takes place as oxidation where the oxygen binds with the hydrogen atoms hanging off the molecule in the kinks at the double carbon bond. When that happens, the 'tail' breaks off, and the larger part of the molecule (the part with the glycerine 'backbone') is the diglyceride, and the tail that broke off is the free fatty acid. The number of double carbon bonds is what makes it saturated (0), monounsaturated(1), or polyunsaturated(2+). An oil (triglyceride) is, molecularly speaking, a glycerine molecule with three fatty acid molecules hanging off the side (think "E", but with really long and occasionally kinky arms), and the properties of a given oil are dictated by how many carbon atoms are present in each of the fatty acid tails. The range is typically 16 carbon bonds up to the low 20s typically in naturally occurring stuff. Longer carbon chains typically come from animal or artificial sources. Blah blah blah... All this is background to say the free fatty acid content of an oil is often an indicator of quality since the oil, by definition, is the whole triglyceride. Any fatty acid composition means the oil was extracted in a fairly destructive way (i.e. excess heat) or was allowed to oxidize. As the triglycerides are broken down, the availability of hydrogen increases and the pH drops. This acidification increases the reactivity of the oil, which then is more likely to form metal salts when given the chance; in chromium tanned leather this may be more of an issue depending on how the chromium is fixed (though I think it's already happily locked up in a salt), but I doubt the same sort of reaction is a concern unless the animal in question ate more than its fair share of beans (the only thing I could think of aside from molasses that was especially high in iron).

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.

Heavier/longer chain (synonyms in this application) oils tend to make especially good lubricants. I do not know the reason for this.

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.

I actually just read some about this very process. It seems the tallow is not the actual lubricant (unless in the context of drum stuffing or hot dipping), but a means of mechanically holding the suspended lighter oils in the paste to the leather so that it can absorb more of it over a longer period of time. Drum/hot stuffing and hot dipping are very interesting to me, but I'm not sure it will give me the specific result I'm seeking. With the heavier oils being solid at room temperature, when in the fiber matrix of the leather, they combine to make a very hard material. It's waterproof, attractively colored, and mechanically resilient, but I need flexibility.

The current plan (subject to change when I turn the page...) is that I might be looking toward some blend of oils. The idea of the varnish produced by oxidized a-linoleic fatty acids seems like it would color the leather very nicely, and also provide some protection. I need to learn more about patent leather though... I would be looking in that general direction, but without the coloration or the polishing and only if it doesn't introduce some otherwise undesirable qualities. Something heavier would be desirable as well for the suppleness afforded and staying power... And some essential oil may provide a wonderful aroma... So much still to learn... I have both pieces roughed out, and have one more template still to go before I actually start tooling... I need to have a plan by the time I finish tooling.

[TwinOaks]

It occurs to me that you've not stated what your project is. With the incredible compendium of knowledge here, perhaps your question should be "Which leather should I use for ____________________". This just seems an awful lot like learning to build a watch factory when all you need to know is the time.

[TinkerTailor]

.. All this is background to say the free fatty acid content of an oil is often an indicator of quality since the oil, by definition, is the whole triglyceride. Any fatty acid composition means the oil was extracted in a fairly destructive way (i.e. excess heat) or was allowed to oxidize. As the triglycerides are broken down, the availability of hydrogen increases and the pH drops. This acidification increases the reactivity of the oil, which then is more likely to form metal salts when given the chance; in chromium tanned leather this may be more of an issue depending on how the chromium is fixed (though I think it's already happily locked up in a salt), but I doubt the same sort of reaction is a concern unless the animal in question ate more than its fair share of beans (the only thing I could think of aside from molasses that was especially high in iron).

A bad batch of oil is a bad batch. What I was getting at was that it seems some oils are more suseptable to these processing degredation conditions, and thus it could be surmised that they would continue to be susceptible after application to the leather in more adverse conditions, while others are not and the list seems to parallel the oils that are lauded for long term leather use.

Also, many hot stuffed leathers are not stiff at all. The stiffest leathers are compressed veg tan, the kind used for boot soles, or the half tan leather they still make in scandanavia. It has a rawhide core and gets really hard after forming. It was used for armour and sword scabbards. I think they may have made it in the states for leather drive belting as well, but i have not confirmed this. It is susceptible to rot if wet, but if covered in one of those castor oil and beeswax belt dressings, would probably be fine in the rain. Long term high humidity may be an issue.

I think one of the functions of the tallows and heavier fats is not as much to lubricate as to keep the fibres separate. When leather gets wet the fibres shrink and tighten, stiffening it. If the tallow is in the spaces between the fibers it prevents water influx and minimizes the shrinkage helping the leather stay supple.

[TinkerTailor]

It occurs to me that you've not stated what your project is. With the incredible compendium of knowledge here, perhaps your question should be "Which leather should I use for ____________________". This just seems an awful lot like learning to build a watch factory when all you need to know is the time.

He is making a drink coaster. A damn fine one. You can't even comprehend how damned it will be.........or he is........lol

[spectre6000]

You mean everyone doesn't put this much thought and attention to their furniture preservation? Water damage on fine walnut furniture is a killer!

Mayhap the atelier is part of the goal... Mayhap the skill and the knowledge is the goal, and a given project is only a means to an end... I have a particular affinity for skills and disciplines that combine art, science, and technology in a practical and aesthetic combination. In my shop you'll find tools, supplies, and materials for general mechanics, metalwork, wood working, luthiery, music, watchmaking, automotive restoration, cooking, baking, brewing, fermentation, software development, graphic design, and of course leatherwork. Probably a half dozen other things if I were to go for a stroll out there and look around. My bookshelf is even more diverse. I've been told by multiple people unilaterally and independently that in the event of a zombie apocalypse, they want to be in my band of survivors.

The project boils down as follows:

-Vegetable tanned leather (I think it's the nicer end of something from Tandy in the 5-6 oz. range)

-Carved with a reasonable amount of detail (topographical map of a meaningful section of mountain range)

-Needs to be reasonably supple, soft, and flexible

-Desired color is a fairly deep walnut brown, finish (matte/gloss) isn't especially important either way

-Color can not bleed from rubbing or contact (think use case in terms of clothing item or tapestry)

-I have a strong desire to use no man-made/petrochemicals as part of an overarching theme to a collection of work

-I think it would be neat to use an oil that smells nice

Suggestions?

I was digging through the wood shop last weekend for a bag of tiny little clamps, and found a roll of miscellaneous leather that somehow ended up somewhere it shouldn't have. I unrolled it just now, and there's some vegetable tanned belly cut offs that I might use for experiments as soon as I decide what avenues I want to explore. The time required to get a knowledge base sufficient to design and carry out the appropriate experiment and see them through to completion could take months or years... I intend to have the current project done long before then.

[spectre6000]

Finished book one as justification for a late start on a paying project with a tight deadline this morning. Interesting bit about amides and lecithin from egg yolks as being a particularly premium treatment toward the end, then a final footnote on a concoction of varied things as something of a 'scholarly pursuit' (sounds right down my alley...). Last little bit of reportable interest was the role of sugar, something else, and specifically glycerin as a softener. They seem to work by attracting moisture. This is noteworthy because as the fats break down, they're essentially migrating toward being free fatty acids and glycerin, and that would further explain some of the mechanism in play. My goal with reading that particular book was to gain a better understanding of the role varying types and weights of oils play in leather, and I dare say I got more than my fair share! I've linked it below, and strongly suggest it. The science is old enough that anyone scientifically literate should find it a quick and easy read, and it was written in the US within the last century (barely) so there are no odd linguistic stumbling blocks or having to mentally convert 'f's to 's'es as you would going back a further century.

https://archive.org/stream/applicationofoil017193mbp#page/n1/mode/2up

In summary (combined with other posts in the thread on the subject), according to the author darker color (which it seems was undesirable at the time) comes from the fibers being smooth and matted and absorbing light. Makes sense. To get this to happen, the surface needs to have a fair amount of oil content, but then you have issues of oiliness and greasiness and risk oils coming off on clothing or whatever. The section on patent leather and the polymerization of a-linoleic acids makes me think lighter weight oils might be a pretty excellent means of having cake and eating it. I need to go dig out some woodworking texts on boiled linseed oil finishes (same polymer) and see about flexibility and permeability, but I think this might be a very promising route. Fortunately for my other motives, many of the pleasant smelling oils I'm after tend to have very short fatty acid chains of the a-linoleic persuasion, and I may have found the solution I'm seeking. I need to see if I can figure out which aromatic compounds, aldehydes, etc. are generally considered pleasant and which are best avoided. Diffuse as they may be off properly treated leather, it would really suck to find that whatever pleasant smell (which would also be diffuse) was overtaken with time by something stinky.

Unfortunately, the time scale required for proper investigation of the options is beyond the scope of my most immediate project, and I need to have something nailed down by this time next week or so. I have so many irons in the fire that the project is moving at a glacial pace (compared to other things). I'm going to read TinkerTailor's referenced book next, and hopefully it gives me some further useful insight. Short of that, I'll have to see if my biochemist friend or I can dig up something that describes the odor of various decomposition byproducts of some of the oils in question between now and then.

[TwinOaks]

I can and DO respect a quest for knowledge just for the sake of that knowledge. However, the depth of your research reminds me of something I've told my kids several times: " When you learn by your actions, including mistakes, it's called Experience. When you learn from the actions of others, especially mistakes, it's called Wisdom."

Suggestion for your project: Call and speak with a person when ordering your leather (my opinion favors Springfield Leather) and order something with a soft hand/temper. Carve/tool, use a "Pro Oil Dye", and condition with neatsfoot oil or Aussie conditioner. Rub in a generous amount of carnauba cream from the flesh side (helps with suppleness), then (when all the conditioner has had a day to absorb/migrate) finish with a lacquer. As far as the scent...personally, I like the smell of regular ol' leather. If you use a scented oil, then efforts to 'refresh' the scent may eventually result in over oiling the leather.

[spectre6000]

My dad has a similar saying, albeit more colorful and with an emphasis solely on the wisdom part.

I've had the leather in my stash for a while, so it is what it is (this isn't my first leather project by a long shot, just the first down this particular path). The first part is already cut, riveted, and sewn ready to layout for carving. I'm trying to avoid dies and other heavily manufactured things. As with food, the simpler and more pronounceable the ingredients the better. I've got a pretty solid understanding of oil applications by now (the theory lines up with my experience), it's the specific chemistry I'm seeking.

Tell me more about your lacquer suggestion. What sort of lacquer, specifically, do you have in mind (I assume you're not referring to lac beetle secretions in solvent)? Have you used it? How does it hold up?

As for the scent, I like the smell of regular ol' leather too... What IS the smell of regular ol' leather though? That's the rub (pun intended). Let's be honest, not all leather smells the same in quantity or quality, and the question of the day is why? Olive oil is frequently touted as an excellent leather treatment, but some say (somewhat truthfully, though not in the way they intend) that it goes rancid in the leather. This would seem on its face to be a bad thing, but I've never personally seen anything like that occur in real life... Why? I read that cod liver oil is the secret to that beautiful old English leather aroma. Why? Similarly that Neatsfoot oil is the only option for leather treatment? Why? Also, any time anyone makes broad absolute statements, alarms go off in my head that that person has little to no clue what they're talking about, and the subject bears investigation; when MANY people within a 'tribe' (referring to the 'tribal knowledge' mentioned earlier in the thread) say the same thing, it implies there is some serious and widespread lack of understanding or misinformation afoot and then the game is on!

At this point, I can say with some authority that the "smell of regular ol' leather" is a direct result of the applied oils breaking down (rancidification) and the resulting aromatic compounds that are released from the leather. The variety and proportion of these aromatic compounds will be directly affected (and can thus be directly controlled for) by the fatty acid composition of the oils from which they originate. It would seem at least some of the aroma (the word we use for smells/odors/stenches/etc. when they are subjectively considered pleasant) comes from the byproducts of heavier oils and thus longer chain fatty acids, but what part and how much?

One of my many avocations is that of luthier; I've made several instruments, taught clinics, have written on the subject, and have even been invited to guest lecture in a university setting. Something the more adept luthiers are good at is being able to break down a fleeting and somewhat subjective sensory experience into component parts and manipulate them to be subtly more pleasing; bringing out harmonics in different ranges to affect the voice of the instrument. Timbre, as a mixture of disparate mathematically related frequencies in proportions relative to each other, is not dissimilar from smell. A similarly subjective, fleeting, and barely communicable sensation. How something looks is only part of the total experience. How it feels and smells (and sounds and tastes, but we're considerably less concerned with these from a practical standpoint) are just as important. We dislike cheap Chinese-made junk (radical overgeneralization for the sake of illustration) because it may LOOK appealing, but by the time you get up close to it and investigate it more thoroughly with your other senses, it breaks down entirely (again, pun intended). I have specifically in mind a piece of chocolate obtained in Beijing that looked especially attractive, but tasted like wax and nothingness; nothing can be more off putting than having expectations unmet (just ask Seneca). It's like the uncanny valley in robotics (I've been working on a contract to write a thin AI engine for a local robotics startup these past few days, and it seems like a pertinent analogy), the more it appears to be what you expect without achieving Turing levels of aesthetic indistinguishability, the more off putting it is. If I were to create this piece of leatherwork that looks absolutely stunning in every aspect, but felt like plastic and smelled like garbage (or nearly as damningly, nothing at all), it would be wholly off putting (to everyone that wasn't merely looking at a photo on the internet).

Updating the current plan of action as it relates to the immediate phase of the larger project: I'm going to investigate and probably make a chart/list/spreadsheet of various oils that I can readily obtain in sufficient quantities and at reasonable prices, and detail their fatty acid composition, percentages, saturation levels. If I can find the information, I'll see what I can do about nailing down the decomposition elements and see if I can't come up with some correlative understanding as to what smells pleasing on paper. Essentially, if neet/neatsfoot is the norm, olive oil is nothing out of the realm of expectation, and cod liver is somehow ideal, then compare them and figure out what they have in common.

One of the more pertinent takeaways from the 1919 book on oils and greases in leather was that of how to affect color. For starters, many things stated in the book were directly counter to my goals as I am hoping to achieve a fairly dark color, and it seems the fashion of the time dictated that lighter colors were a highly sought after norm (there is quite a bit in there about how to lighten leathers, etc.). The way this is achieved is by smoothing and slicking the fibers on the surface of the leather; essentially wetting them down. A non-wet way of doing this would be some sort of lacquer or a level-able hardened surface coating. One pretty excellent way of doing this, taken from the production of patent leather, is to use flax/linseed oil. The a-linoleic acid oxidizes at the surface of the leather and forms a polymer coating. This coating, when heavily applied, polished and mixed with colorants, is what gives patent leather its sheen. Less liberally applied, I think it would be a pretty excellent way within my aesthetic parameters to achieve the darker color I'm after (assuming there are no negative effects from hardness or impermeability of this polymer coating). Unfortunately, the concentration of shorter fatty acid chains and subsequent dearth of longer fatty acid chains could have a net negative affect on the aroma and suppleness of the leather, and thus it may not be suitable as a standalone treatment. It may be required that it be mixed with a heavier oil (TBD). Further, if the only part of the flax oil I'm interested in is the a-linoleic fatty acids, there are myriad other sources, some of which come bound up with other aromatic compounds (and water, which helps with the oil penetration). These other aromatic compounds may be too tightly bound in the leather or too fleeting to be of any real value, but just as the mechanical properties of the leather seem to slow and diffuse the negative byproducts of oil rancidification, they may similarly slow and diffuse the release of these aromatic compounds such that the leather smells additionally and subtly pleasing for a long long time...

So long term, I have some bellies that will make excellent oil treatment test subjects. Short term, I need to get something figured out for the immediate project between now and when it comes time in the current phase of the current project to oil the leather. I may see if my biochemist friend would indulge me, and see if we can't come up with a way to intentionally create a concentration of some of these aromatic compounds in a controlled setting either in her lab or here in one of our kitchens or garages or something. Getting an idea for the smell of certain things in isolation will make it considerably easier to build a desired aromatic timbre. Even more short term, I need to eat a (very late) breakfast and finish this AI engine code while it's too cold and snowy outside to work on some of the other projects on my 'to do' list...

Edited November 29, 2015 by spectre6000

[spectre6000]

Well... I hit a snag on the software contract, and can't really do a whole lot more until I hear back from the client on some hardware specifications. I know they do occasionally work on Sundays, so I might hear back, but until I do I'm a bit stuck... So.... While I wait, it's time to dig into the second book! I'm 30 pages in, and already, if nothing else, there's a very distinct difference in the overall voicing... The words 'peculiar' and 'evil-smelling' are abundant to the point that one doubts the extent of the author's vocabulary... It's very interesting in its addressing of mechanical oils toward the end of the Victorian era as I have a particular interest in clockworks of that era. Neat read. The way it addresses rancidity and fatty acids, etc. is markedly different from the other such that I wonder what the state of the science was during the intervening decade and change... It's not quite back to transitional alchemy, but certainly not written in a modern scientific voicing like the other. Very cool book pairing for anyone so inclined. That is all for now.

Finally got to the part you referenced about neet/neatsfoot oil, violet glass, precipitates, etc. Very interesting, but also very much in period. Science clearly came a long way in the 13 years between the authoring of this book and the former, and it's come far longer since. The statement about the oil not going rancid for years is directly countered by exposing it to sunlight (intentional decomposition via UV light) and the filtering of distillate (crystalized fatty acids, again from decomposition). "Rancid" may be subject to vernacular here. To me, it means the oil has decomposed from a triglyceride to diglycerides, fatty acids, and beyond. It seems to the author, "rancid" refers to a specific "peculiar"smell that likely refers to a very specific class of aromatics and aldehydes that may be present in greater or lesser quantities in neet/neatsfoot oil. Personally, I always thought neet/neatsfoot oil smelled rancid when new, so I'm not sure it would be easy to tell in any case. I don't know the state of organic chemistry in 1906, but I think it would be a pretty safe bet that the technology required to discern the chemical composition of a complex organic substance like naturally derived oils (which all the oils discussed in this book would be classified as to a greater or lesser degree) either did not yet exist or were in such a state of infancy that the author did not have knowledge of access to them.

"Green vitriol" (pre-modern science's cousin Alchemy's word for iron sulfate)... Love it!

Edited November 29, 2015 by spectre6000

[spectre6000]

Alright, I'm done waiting and I'm calling it a day. I'm going to go out and get some work done on some fun projects. For starters, I'm going to see what I can do in terms of designing an experiment to nail some of this stuff down. I have some veg tanned bellies that I don't remember buying. From the look of what's not there, I'm going to assume I used them for tool wear/friction linings. Since I don't remember buying them, it's possible they were cut offs from whole hides... I really have no idea, but they're here and I don't have any real plans for them where they're so special that I couldn't just use some other scrap cutoffs or buy some cheap stuff from Tandy or other.

So here's what I have in mind:

Acquire a collection of varied and assorted oils according to research findings regarding relative fatty acid composition in very small quantities (hopefully I can find some place that samples such things).

Cut the bellies up into smallish pieces.

Find a means of hanging them such that there is some air flow and minimal risk of cross contamination.

Find a means of labeling the samples (probably nothing more complicated than a sharpie)

Oil them up,

hang them up,

check in every so often and take notes.

I figure reapply once a quarter maybe?

Total elapsed time... 1 year?

Thoughts?

[TwinOaks]

Enjoy your experiment