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A little C&P from a Leather Chemist Article

Discussion in 'Tanning' started by Rhino, Dec 11, 2016.

  1. Rhino

    Rhino Too many irons in the fire will put the fire out!

    (table pg 107 Sharphouse)

    Vegetable Tans: extracted from plant leaves, barks etc. , consist of large polyphenol molecules with some acidic groups and numerous secondary functions(dipole or hydrogen bond). The acidic groups may combine with the basic groups of the protein displacing the water of hydration. Vegatable tannage could be considered as replacing water molecules by vegetable tan molecules. Generally acid conditions favor vegetable tan fixation in increasing the ionization of the protein basic groups.

    Synthetic Tans: may be of various chemical structure. It is common for them to be made water soluble by the sulphonic acid group. This group is highly ionized and has strong attraction for the protein basic group with a consequent dehydrating effect.

    Syntans with high secondary functions will have more pronounced effect and give fuller leather(replacement syntans), those with greater proportion of sulphonic groups give a thinner less flexible leather(auxiliary syntans).

    Minneral Tannages: the basic salts of chromium, zirconium and aluminum behave in a rather different way. Their initial fixation is on the acid groups of the protein where they displace some of the bound water, but they may form cross links between adjacent acid groups, which will stabilize the wet hydrated skin structure.

    The dehydration effect of theses tannages and the quantity fixed is less than with vegetable tannages and therefore the shrinkage and hardening on drying is more pronounced. Invariably some type of oil is applied to the wet fibres before drying. Its effect on softening the dried leather should be more pronounced on chrome leather than on vegetable tanned leather.

    Aldehyde Tannages: formaldehyde, gluteraldehyde or the aldehydes produced in chamois tannage, combine with basic groups of the protein and form cross links with basic groups on adjacent molecules in the wet protein. Quite small amounts of aldehyde are sufficient to produce a significant effect.

    Does this tell you anything??
     
  2. 3bears

    3bears Well-Known Member

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    It explains quite a bit to me, thank you. What message are you trying to convey? Is this in regards to a post about adding alum to truebond to set hair?
     

  3. Rhino

    Rhino Too many irons in the fire will put the fire out!

    Naw. What does this say to yall about when to neutralize. Mineral tan - syntan Hint-sidegroups
     
  4. 3bears

    3bears Well-Known Member

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    Am I off base thinking it refers to the tan being attracted to the acid from the pickle?
     
  5. Rhino

    Rhino Too many irons in the fire will put the fire out!

    EXACTLY! Mineral and vegetable tans. Lutan=mineral tan, so you don't neutralize before the tan. After. Attracted to acidic side groups.

    SYNTANS- are attracted to BASIC side groups, or being neutralized BEFORE tanning.

    All tans are not neutralized the same way!

    I can produce a dozen articles by guys with masters and doctorates that say the same thing.
     
  6. 3bears

    3bears Well-Known Member

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    Good to know. What happens if you neutralize before on mineral and vegetable tans?
     
  7. Rhino

    Rhino Too many irons in the fire will put the fire out!

    You mounted nothing but a pickled skin.
     
  8. Tanglewood Taxidermy

    Tanglewood Taxidermy Well-Known Member

    Even though this info exists in the archives, I believe it needs to be brought up periodically because there are new people on here that need to hear it. There still is much confusion on this subject. Thanks for posting this.
     
  9. cyclone

    cyclone Posts: 400001

    Dang it, where is the "LIKE" button. :eek:

    Bill, we need LIKE buttons... ;)
     
  10. Rhino

    Rhino Too many irons in the fire will put the fire out!

    Vegatable tannage could be considered as replacing water molecules by vegetable tan molecules. Generally acid conditions favor vegetable tan fixation in increasing the ionization of the protein basic groups.

    Synthetic Tans: may be of various chemical structure. It is common for them to be made water soluble by the sulphonic acid group. This group is highly ionized and has strong attraction for the protein basic group with a consequent dehydrating effect.

    This part of this C&P also helps to explain why I don't like plastic bags for paint-on tans. After applying the tan to the skin, the tannage will cause the skin to continue to "weep"water long after the application of the tan. Most if not all of the water within the collagen will be "pushed out". I just have a problem with skins setting in water of that pH, with no way for the water to escape.(remember you neutralized that skin) Call me anal, but in tanning its always better to stand on the side of caution.
     
  11. 3bears

    3bears Well-Known Member

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    oldshaver do you recommend folding and freezing prior to placing in a bag?
     
  12. Rhino

    Rhino Too many irons in the fire will put the fire out!

    No, just let the skin hang and drip overnight, then wash, bag, and freeze.(or mount)
     
  13. 3bears

    3bears Well-Known Member

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    Good answer that is just what I did. Thank you for answering my questions.