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A C&P- I thought this might be intresting to some

Discussion in 'Tanning' started by oldshaver, Mar 12, 2007.

  1. oldshaver

    oldshaver Guest


    The purpose of soaking is to bring the hide to the same condition in which it was immediately after separation from the carcass. Recovered softness makes it easier to introduce small-molecule substances into the hide. During soaking the mechanical impurities: scud, blood, salt of other preservations used, are removed, a part of nonstructural proteins and remains of fat and meat. The hide becomes swollen in the process.

    The collagen of glycosaminoglycons remain through the tanning process probably intact.

    Mature crosslinked collagen is water insoluble but it swells. Extent of swelling is , in such a system, inversely dependent on the crosslinks number. In a fibre network the solvent may occupy the inter or intra fibrillar spaces, the general regularity however remains. Swelling of collagen depends on two factors. Osmotic and Lyotropic ones. Osmotic swelling ( Donnan swelling) occurs due to a high concentration of bound, nondiffusing ions located inside the structure. It takes place when pH of the solution is off the isoelectric point and the ionic strength of the solution is small. Greatest swelling effects may be observed at pH 2 and 12. It is reversible by straining of the fibres, changed pH or increase of ionic strength of the solution by increase of salt concentration.

    Lytotropic swelling which is due to neutral salts at considerable ionic strength, decreases the cohesion of the fibres and is not completely reversible.

    In heating the hide one observes the shrinkage of over 50% of the sample length. This is best observed if the sample is immersed in water. The temperature of shrinkage (Ts) depends on degree of crosslinking; it is lower for the raw hide, higher for the leather.

    The non-swollen collagen is, a highky ordered polymer, which is synonymous with its crystallinity.

    Osmotic swelling is due to pH change, when the ionic strength is small and temperature low. Changes of pH in the range 4 to 8 do not affect markedly the length and diameter of fibrils. Outside these pH values almost 10 fold increase of fibre volume may be observed. If pH drops below 2, when the volume decreases. The increase of ionic strength suppresses collagen swelling. The Donnan effect comes from the increase of charge bound at protein surface, as the pH is drifting away from the isoelectric point. According to Donnan’s theory occurrence of localized charges causes formation of excess ions having opposite charges inside the gel, which in turn initiates action of osmotic forces. Donnan effect does not elucidate satisfactorily the mechanism of attachment of solvent molecules to the biopolymer, although from the thermodynamic point of view it describes very well the influence of pH on the degree of swelling.

    Lyotropic swelling may be observed in solutions of the salts, in which the forces, causing Donnan phenomenon are insignificant. One may be observe it at every pH if only salt concentration is high enough (over 0.5 molar) or in solutions having lower salt conc., and a pH neutral. Increasing salt concentration causes at first swelling increase (salting in) and then decrease (salting out) of swelling. Gelatin behaves like collagen. Comparing swelling effect of various salts have been ordered in a Lyotropic or Hofmeister series.
    So to make a long story short, acid swell, aint always acid swell.
  2. oldshaver

    oldshaver Guest

    Interesting how I mis-spelled intresting.

  3. cyclone

    cyclone Posts: 400001

    Very interesting reading OS. How about a thought on pressure pickling/tanning. I've never used the method, nor seen the results..Would this be a method that a pro tanner might employ? How about proper pH and tannin reactions?
  4. oldshaver

    oldshaver Guest

    The containers being pressurised, would be too large, and would be dangerous, in my opinion.
  5. cyclone

    cyclone Posts: 400001

    How much pressure is the norm for pressure tanners?
  6. oldshaver

    oldshaver Guest

    Maybe someone with an Auto-Tanner could help here?
  7. Matt Richards

    Matt Richards Member

    Re: Commercial Tanneries

    Good stuff OS. One sentence is either unclear or incorrect:

    "The collagen of glycosaminoglycons remain through the tanning process probably intact"

    Collagen and glycosaminoglycans are different substances. Collagen remains intact (though altered), glycosaminoglycans is typically removed from the skin, but not always depending the tanning method. Its deliberately very thoroughly removed for softer leathers (gloving for example), its not for firmer leathers (sole leather for example).
  8. oldshaver

    oldshaver Guest

    Both are important, to the structral stregnth of a skin. Pickeling with sulphiric acid, will probablly remove all the glycosaminoglycans, but long term skin stregnth, is doughtful in my opinion, because the collegen is damaged also. A chrome tanned skin, would probablly be fine, but is useless for Taxidermy. Your c&p refering to glove and sole leather was good, but is comparing apples to oranges. Think about skin memory, and how it relates to Taxidermy. Does a Taxidermist want a skin with alot of memory? This post was mainly to point out, that, osmotic swell, is what we experience most in Taxidermy Tanning. What we normally call acid swell, on here, really isnt. The after effects, of the lack of salt, in a pickle, is acid rot, where acid is left, ionically unchecked, to do its thing.
  9. oldshaver

    oldshaver Guest

    Could someone please name my avatar, or ya ll too young to remember who he is?
  10. miker

    miker Member

  11. oldshaver

    oldshaver Guest

    Thanks Mike. I was worried some of the young bucks, had forgotten da man!
  12. cyclone

    cyclone Posts: 400001

    Jerome Lester Horwitz, aka Curly. Moe and Shemp's little bro.

    Hell, I thought it was you...nyuk,nyuk,nyuk... ;)
  13. "The after effects, of the lack of salt, in a pickle, is acid rot, where acid is left, ionically unchecked, to do its thing."

    True. Cyclone mentioned the opposite, osmotic swelling. I thought it was original thinking, but someone else has thunk it!

    The ratio of "too much acid" to "not enough salt" is not reversible, as you have been saying all along OS. In fact, the hides will crack and tear as they dry, thereby rendering them to many pieces not even fit for patching or craftwork. The posts about leaving hides/capes to rehydrate for 12 hours by accident and coming back to find a rubbery mount (osmotic swelling) have been confused with acid rot. Osmotic swelling is reversible if you equalize the ionic gradient as Cyclone pointed out earlier. (Layman's terms: Use salt water to rehydrate instead of fresh tap water)

    It's real simple if you think about it, the hides have been salted from day 1. Drying, the pickle, neutralizing, tanning..... all with salts. Use salt water to rehydrate. Not much, just about 4 ounces to the gallon. It is a universal method for taxidermy types of leather

    Maybe now this post can be referenced to compare the two conditions and tie it all together.

    This was a great way to bring the confusion to an end OS, if people will search the archives for the topic.
  14. Matt Richards

    Matt Richards Member


    Yeah I appreciate the explanations, because there really are some key differences to the leather tanning I'm trained in.
  15. Glen Conley

    Glen Conley KARMA GOOSE R.I.P. 2006-2006

    HELL YEAH! A taxidermist is also going to want the elastic in his jockey shorts to have good memory!

    I remember following a link on the old forums that was supposed to be the scientific answer to leather making. That one was a whole lot like this c&p, but at least with that one the girl did present a bibliography of about twenty different writers that she thought supported her interpretation of her home work assignment.

    I just checked the old Archives, and I have pointed out protein structures repeatedly. I couldn't help but notice someone had been doggin' my ass each time I had done so. That was then, this is now. That was what gave me this great idea to use this technology now available to all and POST PICTURES!

    When I posted these three photos the other day, I did not post text with them. Reason being, it had been indicated in the past that I didn't know what I was talking about, so I figured to leave the explaining to someone else. That didn't happen.

    By the way, I don't think it is quite fair for me to be the only one to post actual photographs, how about some of you other guys posting up some pictures?

    What you are seeing is ACID SWELL.
  16. oldshaver

    oldshaver Guest

    Glen, if your talking about me dogging you, I only remember once. (the rocket science comment) Since then, I have got alot of helpful info from your posts, and enjoy them alot. If I remember right, I have asked you to post more. Im not ready, to jump off yet, but, Im not sure there is such thing as acid swell, anymore. Convince me.
  17. Matt Richards

    Matt Richards Member

    Leather chemistry

    Here is a quote from a leather chemist using terms that lay people can reasonably understand. Mucopolysaccharides are now typically referred to as glycosaminoglycans (Reed's work was published in 1979), though its probably more complicated than that (the relationship between the two terms).

    <blockquote><i>"...powerful control (is) exerted by the ground substance over the passage of ions through skin. The mucopolysaccharides in ground substance ...bind water so firmly that few other types of ions can normally reach the fibres. ...Tannage of pelt with the ground substance still present, e.g. the tanning of raw skin, tends to be slow, uneven and uncertain."

    R. Reed, Ancient Skins, Parchments, and Leathers</i></blockquote>
    Anyways, this is the concept in leather tanning that I was talking about in the other thread. Typically high alkalinity (12.5 and higher) is used to remove the ground substance as its called by Reed in leather tanning. My understanding is that a similar concept exists in hair-on tanning by using acids (I wish I could remember where I read about it, though it was in a lay 'how-to' book and mistakenly referred to them as 'glues'). Anyway, on the alkaline side you really have to get it alkaline enough for it to work. A ph of 11 does not successfully break up the mucous like structure of the ground substance, and it takes considerably longer to get larger molecules, such as oils and tannins, to penetrate the skin, if at all. You cross up to 12.5 and higher and it does the job very predictably. Lime clocks in in the low 12's, so typically it is 'sharpened' with something more caustic (such as sodium hydroxide) to bump the ph over that hump.

    My understanding/assumption was that there would be a similar threshold on the acid side, and that it would be common knowledge amongst y'all.
  18. cyclone

    cyclone Posts: 400001

    Glycosaminoglycans are polysaccharides. They contain numerous carboxylic acid ligands. This is why the alkaline soak solubolizes them so well. The hydroxide ions offered by bases extract the acidic protons from the carboxylic groups rendering the molecules quite soluble.
  19. oldshaver

    oldshaver Guest

    I hate it, when I have to go look up word meanings! ??? My point here, is, why is a skin, that has been in a pickle, without enough salt, and turns rubbbery, being called an acid swelled skin? It wasnt the acids fault was it? Wouldnt this condition be refered to as osmotic swelling, across the board? Osmotic swelling in a tan, wouldnt be refered to as tannage swelling, would it? Osmotic swell, can happen in any part of the wet end process, but only in a pickle, or a rehydration bath, is it dangerous to the skin.
  20. cyclone

    cyclone Posts: 400001

    Yes, that is true, but since the swelling has occured in the pickle it is very possible and most likely that you have a combination of acid swell with the osmotic. With acid swell the collagen strands become denatured and fold upon themselves. The process is irreversible. It's a balance, too much salt in a pickle gives a plastic hide, too little salt gives rubber...

    Matt, I did some searching on the subject and cannot find anything on the acid removal of glycosaminoglycans. The search is not over; howevr. One solution is the addition of an acid bate, but then again, the pH has to be just so and the activity of the bate must be reliable. There are standard tests using standard substrates to test the activities of enzymes which would probably prove too expensive for our purposes.

    This is why I mentioned the rehydration step being of great importance when tanning capes. It is here that most of the solubles are removed.