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Chrome tanning vs Alum tanning? Differences?

Discussion in 'Tanning' started by douglassutho, Jun 27, 2008.

  1. douglassutho

    douglassutho New Member

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    Hi Guys,

    First post, great site BTW. What is the major difference between chrome and alum tanning? I only ask because I initially read an article in a magazine outlining tanning skins. In a nutshell it was: salting, fleshing, tanning in alum sulfate, drying/oiling. Then I bought a tanning kit (after my skin was in the tan outlined in the mag), which has alum sulfate as well as chromium sulfate, the kit uses BOTH chemicals.

    What are the reasons for using chromium sulfate after the alum sulfate mix? I have a second hide that is salted, I was planning on finishing this first hide with just the alum sulfate method from the mag, then trying my kit on the second skin completely including chromium sulfate. Any info about the two processes would be greatly appreciated.
     
  2. redwolf

    redwolf Active Member

    chrome tans will not wash out of a skin, but alum tans will. chrome tanning is a metal based tan and it's toxic. so be careful not to inhale the dust, and try not to handle it too much. NASTY stuff
     

  3. cyclone

    cyclone Posts: 400001


    Alum contains a metal also..

    MSDS's seem fairly similar...

    Chromium sulfate....Chormium(III) sulfate...
    http://www.jtbaker.com/msds/englishhtml/c4389.htm


    Aluminum sulfate
    http://www.jtbaker.com/msds/englishhtml/a2914.htm


    To answer the op's question..
    Combination tannage..tanning with two or more tanning agents to achieve desired results. Alum tan final results are usually very soft and white. Chrome tans are noted for stability and longevity. By using both one would hope to end up with a final product that has properties of both..
     
  4. redwolf

    redwolf Active Member

    sorry bout that. i was under the assumption alum was a salt.
     
  5. cyclone

    cyclone Posts: 400001


    It is a salt...


    Just as chromium sulfate is a salt...

    Aluminum sulfate is a salt.
     
  6. redwolf

    redwolf Active Member

    if they are both metal based why would one wash out and the other wouldn't?
     
  7. douglassutho

    douglassutho New Member

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    Hey lads, thanks for the responses. I am a chemist by trade so the chemistry is easy. It makes sense when you look at the combination of the two methods giving you the benefits of each method. The smaller hide (which is the one I have started) will be fine as a floor covering so I think I'll leave it at just an alum tan.

    For the other guys posting who are unsure, aluminium sulfate and chromium sulfate are both salts, which when in solution will ionise to form positively charged and negatively charged ions, Cr 3+ and SO3 2-. They are a salt because they are electrically neutral (I'm sure there is more than just that to it but lets keep it basic).

    Thanks so much for the replies guys, it's great to see a forum that actually has people posting and helping out.

    Another question: I have also read its a good idea to rinse the hide in petroleum as a final step to wash off any oils and also dry it out completely. Is this wise? It sounds a bit extreme. I do have access to petroleum spirit, acetone, ethanol, diethyl ether and most other solvents through my work if they would be better?
     
  8. douglassutho

    douglassutho New Member

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    I'm interested to know this too...
     
  9. redwolf

    redwolf Active Member


    LOL i was feeling kinda stupid for a min. there, but if a chemist can't figure it out either, maybe I'm not so bad off ;D
     
  10. cyclone

    cyclone Posts: 400001

    Not all metals will react the same...

    Would you expect iron to undergo the same reactions as chrome or aluminum or gold?

    My hypothesis would be that the aluminum is forming ionic bonds with the collagen structures whereas the chromium forms covalent bonds.
     
  11. redwolf

    redwolf Active Member

    if metal is water resistant you would think it wouldn't "wash out" wouldn't you?
     
  12. douglassutho

    douglassutho New Member

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    Interesting...I would have thought aluminium and chromium would be forming similar bonds, having the same number of valence electrons...university was too long ago! Maybe the extra electron shell on the chromium allows it to react differently? Now this would have been an interesting lesson at uni!
     
  13. douglassutho

    douglassutho New Member

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    It's more about the kind of bond they are forming with the protein. Different bonds react in different ways, and in differing degrees of strength. If as cyclone is saying they form different kinds of bonds (ionic is transferring, covalent is "sharing), then they would have different properties and react differently to situations. I'd be very interested to see the pathways of these reactions.
     
  14. cyclone

    cyclone Posts: 400001

    Physical Chemistry of Leather Making Bienkiewicz
     
  15. redwolf

    redwolf Active Member

    at this point you could tell me black is white and i would believe you :p this simple little thing we call tanning can be confusing. all i know is it works :D i have ABSOLUTELY no idea how but it does.
     
  16. cyclone

    cyclone Posts: 400001

    Black is, in fact, white if you look at it negatively....but we try to keep things positive here... ;)
     
  17. redwolf

    redwolf Active Member

    sounds good, i get confused easy :p ;)
     
  18. George

    George The older I get, the better I was.

    GUYS! THis is not rocket surgery or brain science. GET A GRIP. The two chemists haven't explained to any of you yet whe should be obvious to everyone. So try this on for size.

    A "SALT" does not always mean sodium chloride. I doubt you sprinkle Epson salts on your food, at least not intentionall, or how about smelling salts which aren't crystaline? In chemistry a salt refers to the compound that replaces all or part of the positive ions in an acid with a positive charge of its own.

    METAL does not always mean a static solid. Iron is in all hard water along with other mineral METALS such as copper. Mercury is a metal that will pour. Chrome may be hard and shiny but chromium certainly isn't.

    So MY TAKE on the question of why one "washes out" and the other doesn't has more to do with the ionic process involved once water has been added to the mix. That's always been the contention with an alum tan simply because the ions that replace the hydrogen in the leather are unstable and when water is reintroduced, those ions combine with the hydrogen in the water again, diluting them and simply washing them away. What doesn't get washed way converts itself back into an acid, in this case sulfuric acid whice can, after time destroy the hide from within.

    Sure, that's only conjecture from a laymen but it's the thought pattern of why the old "pickle tan" was canned. The old guys presumed that alum tanned leather had a finite lifespan and would eventually disintegrate the hide. It's the same process libraries are faced with today in dealing with the paper in books and documents. Those items hundreds of years old are simply crumbling to dirt because they were not printed on "acid free" paper.

    Do they even require school children to complete chemistry any longer? If they do, it's hard to believe such simple problems as this are so hard to understand.
     
  19. redwolf

    redwolf Active Member

    sorry we aren't as smart as you wish us to be.
     
  20. George

    George The older I get, the better I was.

    You're never smarter than you've learned and never dumber than you've allowed yourself to be.