**The Official Shoe Care Thread: Tutorials, Photos, etc.** - Page 339
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I will try to keep the shit storm to a minimum, and the tone as civil as possible.
The chapter (chapter 1) you reference is titled “Collagen and Skin Structure” and the next chapter is titled “Skin and its Components”, “Tanning” starts at chapter 10, and while prior chapters relate to the overall processes used in a tannery, I probably would not use a chapter on Collagen and Skin Structure to support an argument on the chemical makeup of leather, as skin and leather are really two different things.
Well, you cannot "replace oxygen atoms with amino acids". Chemistry does not work that way. Amino acids contain oxygen atoms. Oxygen atoms are much smaller than amino acids, and amino acids would not fit into the locations where oxygen atoms are found in the amino acid chains of collagen- or for that matter, any other protein. To the extent that hydrogen bonds are formed then they would have to be with atoms like oxygen. Since the proteins are made of amino acids, the hydrogen bonds are between hydrogen atoms and oxygen or nitrogen. So, if you "replaced" an oxygen atom with an amino acid, you would have replaced it with a molecule that contained oxygen and nitrogen, and hydrogen bonds would still form. But you could not do this because the protein structure would not permit it.
You are simply making an argument of granular semantics. I actually do know the difference between an atom and a molecule. Clearly, you are aware that amino acids contain oxygen atoms (as you just stated such), so you understood my point of replacing the existing oxygen atom (part of the water molecules existing in the skin) with the oxygen atom of the amino acids (in chromium, in the case of chrome tanning), you just chose to argue the semantic granularity to no real benefit of anyone. The point is the amino acids are used to replace one oxygen atom for another.
Yes they are as part of modern leather fat liquoring. This is part of the reason that leather loses little fat during its lifetime. The methods used to fat liquor involve carefully controlled temperature, ionic strength, pH, emulsification, and agitation- chemistry again. The fats then bind to the leather and stay there. You might see "Theory and Practice of Fat Liquoring" and "Fat Liquoring pt1" and "Fat Liquoring pt2" ,all published in World Leather. In times past leather was treated with oils and waxes that did not bind to the fibers, would migrate out, and had to be replaced. This was the work of curriers. That business appears to be about gone, since fat liquoring took over.
Conditioning is done primarily to adjust the moisture content of the leather. In studies of leather properties, the samples are "conditioned" at standardized relative humidity to adjust the water content. Adding oil to replace lost oil may be necessary if the piece has been treated with something that can remove the fat (like the VOC's discussed above). But if you do not do this, then modern leather will lose little of its fat in normal use.
In a related discussion on AAAC, the member calfnkip, who is in the tannery business, had the following comments
“...when the leather arrives at the shoe factory, it is fully fatliquored / conditioned and ready to be made into shoes that are meant to give years if not decades of service.
Under normal use, small amounts of these fatliquors may leach out of the shoe’s leather due to the flexing it’s subjected to when you walk. The quantities that migrate out of the leather this way are pretty small and shouldn’t compromise the integrity of the collagen fibers for some time - - often quite a few years if the tanner has done his job properly. And you are correct that it is not really possible to restore all the fatliquors, greases, waxes and other compounds that are applied to footwear leathers during the tanning process.”
He does not elaborate on why is it not possible to restore the fat liquoring, but the problems are: the fat liquoring is performed with the leather in a drum, heated, with a series of chemicals that are not available to the consumer, under pH and ionic strength conditions that are adjusted for the particular liquor and leather. All sides of the leather are exposed. At the end of this process, some of these conditions are carefully altered to complete the binding, extraction of emulsifiers, or in other ways. These cause permanent changes in the leather, which may not permit more fat liquor to be added.
I do agree that the chemicals and processes used in fatliquoring are done to help bond the oils to the leather, but unfortunately all oils oxidize over time (other than volatile oils [like turpentine] that evaporate before they can oxidize). This propensity to oxidize is measured by the IV (Iodine Value) of a given oil/fat. It’s not a matter of if but when (months to years depending on the IV value of the oil, and other factors like heat).
Bonding and permanently bonding are two different things. I bonded with my first wife, but it wasn’t permanent (go figure), perhaps there just wasn’t enough liquor. Oils/fats are bonded to the leather during fat liquoring, they are not permanently bonded.
As the molecular structure of the lubricating oil changes through oxidation it becomes less bonded to the leather as the oxidation affects the hydrogen/carbon bonding in the oil.
Many of you who own shell cordovan shoes have seen oxidized oil on areas of the surface of your shoes as a white film; this is referred to as “fatty spew”
Water will, and does, flush oils from leather (even when they are bonded). As mentioned in another post, water is the ultimate solvent. Rain water, puddles, cleaning with saddle soap and water, etc... all contribute to flushing oils from leather.
In response to a question on this topic, Steve Gilbert of the American Leather Chemists Association posted this comment
“...under normal conditions there is not need or in reality no real possibility to re fatliquor a finished leather article. It is part of a chemical process during tannage.”
First, you would not want to re fatliquor a finished leather article, because it has a finish; secondly because it has been cut to and for a specific reason (shoe, jacket, whatever) and would lose its shape, and third because using conditioning oils should be sufficient.
I’m not sure what you mean by maintaining the water content of finished leather. If you mean maintaining the water content of leather as it came from the tannery, then that is unrealistic, as leather will equalize its moisture level to that of the surrounding relative humidity. I think we have seen a few charts on that in the forum recently.
In regard to water and/or humectants in leather conditioners I can tell you that one of the more popular leather conditioners for leather boots is Obenauf’s Leather Oil. They state “Dust, dry air, water and cleaning remove oils from leather.” “Regular use of our natural oils repels water, restores and protects your leather fast and easily and will extend its service life.”
Lexol leather conditioner (one of the more commonly used leather conditioners on the market) is “emulsified into microscopic droplets”. I’m not sure what the emulsifier is (it could be water). It goes on to state that “these oil droplets bond to the leather fibers, nourishing the leather and leaving no greasy residue.” They do not state that the oil droplets bond permanently to the leather, just that they bond. And, there is no mention of humectants or of moisturizing the leather. To me it would just make sense to mention it if that were the main reason for using a leather conditioner.
Dubbin is basically wax and oil, no water. Then, of course, there is mink oil and neatsfoot oil, both used for conditioning leather, neither containing water.
FWIW I think this is the perfect place for a debate between knowledgeable people, even if I don't understand the first thing about the chemistry involved. I mean, if not here, where else would such a debate take place? I would like to think that we are all getting a little more knowledge about shoe leather, and that is one of the reasons why I come here. This is SF at its best IMO. If you don't like to read long posts, just scrolll past them, or better yet, post pics of some well shined shoes! This thread can have both.
All of the above is spot on. It frustrates me to death when people act like they can't use the scroll button on their mouse to get past discussions they aren't interested in. If it is in accordance with the theme/title of the thread, people should be able to discuss anything as long as they desire. We aren't running out of pages that can be added to the thread as far as I can tell.