Oils In Leather (Chemistry)

[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.