Tannins in leather - quantity and quality related?

[SUP]

To explain my question,

 A while ago I dyed a piece of leather with Vinegaroon. It only turned grey. Research suggested that the Vinegaroon was depleted. So I made a fresh batch and the leather turned black as expected.

Fast forward a couple of months and again, another piece of leather turned grey. Fresh Vinegaroon and it still remained grey. Further research said that this happened because there were not sufficient tannins in the leather. Dipped the leather in strong tea, dried it and then used Vinegaroon, it turned black. However, because I used the tea, color transfer did happen. Now, I know to use a sealant if I need to use tea when dyeing with Vinegaroon.

The question is, evidently the leather I used in the 2nd incident had lower tannins. Does that in any way reflect the quality of the leather?

Do better quality leathers have more tannins or less or is it unrelated to the quality of leather?

I have been wondering about it and thought I would put it up here.

Edited October 3, 2023 by SUP

[ArkieNewbie]

32 minutes ago, SUP said:

To explain my question,

 A while ago I dyed a piece of leather with Vinegaroon. It only turned grey. Research suggested that the Vinegaroon was depleted. So I made a fresh batch and the leather turned black as expected.

Fast forward a couple of months and again, another piece of leather turned grey. Fresh Vinegaroon and it still remained grey. Further research said that this happened because there were not sufficient tannins in the leather. Dipped the leather in strong tea, dried it and then used Vinegaroon, it turned black. However, because I used the tea, color transfer did happen. Now, I know to use a sealant if I need to use tea when dyeing with Vinegaroon.

The question is, evidently the leather I used in the 2nd incident had lower tannins. Does that in any way reflect the quality of the leather?

Do better quality leathers have more tannins or less or is it unrelated to the quality of leather?

I have been wondering about it and thought I would put it up here.

I have not read this whole thesis but this may give you a scientific perspective. Apparently a student in Kenya going for a Master of Science in Kenya asked the same question.

http://erepository.uonbi.ac.ke/bitstream/handle/11295/93419/Kuria%2C Alex Nduru_Evaluation of tanning strength and quality of leathers produced by selected vegetable tanning materials from Laikipia county%2C Kenya.pdf?sequence=3

[Littlef]

I don't have any scientific data to back this up, but no, I do not believe the level of tannins in the leather has a correlation to leather quality.   Tannins are present in the plant material that animals eat.  The levels regularly fluctuate based on diet, and a multitude of other factors.  I think the largest problem was because you sealed the leather before applying the vinegaroon.  That physically made a barrier that it couldn't pass through.  Adding the tea did 2 things. Tea is fairly high in tannins, so that strengthened the chemical reaction; but in addition, you added more water, and the saturation of the leather also helped the 2nd application of vinegaroon better distribute through the leather.

Using vinegaroon isn't an exact science.  If you run it through once, and its not dark enough, you can always apply it again.  a word of caution, the acid in the vinegar does degrade the natural oils in the leather, so it normally will need a good dose of neatsfoot oil afterwards.  I've had leather that got hard and brittle after using vinegaroon.  I found that the leather needs to be oiled before its fully dry after the vinegaroon.  If the leather is allowed to get brittle, oiling it might not be able to restore the flexibility in the leather.

 

[chuck123wapati]

Buy a book of find an eBook on tanning leather its not rocket science veg tanning is thousands of years old. Tannins are used for vegetable tanning that's why they are in the hides and is the main ingredient in the process they literally soak the hides for months in the solution( made from woods high in tannins ,Oak for one,. how much is left in the leather depends on the strength they used and how much they rinsed out after the hides were done, if I remember right its the ph they worry most about. But all veg tanned hides have tannins. if you sealed the leather then used tea the black probably will rub off or bleed. The iron in the vinegaroon reacts to tannin you can put tea on wood and it too will turn black in fact it is used as a stain for wood as well as leather. The roon can be put directly on oak for example as it has mucho tannins.

 

[SUP]

@Littlef Actually, I did not seal the leather before dyeing it with vinegaroon. I cleaned it with saddle soap like I usually do and no, repeated treatment with vinegaroon did not help. I tried that as well.

I do apply neatsfoot oil oil after the dye, always. It does darken of course, but I cannot depend on it. These were always trials on  scraps of leather, which I always do before I actually dye the piece I am working on, so I could adjust accordingly.

@chuck123wapati I absolutely agree about it not being rocket science. I can read the process up but my question is about whether the quantity of tannins differs  in good quality and average quality lather.

4 hours ago, ArkieNewbie said:

have not read this whole thesis but this may give you a scientific perspective. Apparently a student in Kenya going for a Master of Science in Kenya asked the same question.

http://erepository.uonbi.ac.ke/bitstream/handle/11295/93419/Kuria%2C Alex Nduru_Evaluation of tanning strength and quality of leathers produced by selected vegetable tanning materials from Laikipia county%2C Kenya.pdf?sequence=3

@ArkieNewbie thank you for that. I will read the article. 

When working with leather, there is so much time to think that all sorts of questions come up!

 

 

[TomE]

I have read that the stepwise increase in tannin concentration during the tanning process produces a more uniform matrix of cross linked collagen fibers. If the concentration is too high at the beginning there’s a lot of cross linking near the surface, creating a barrier to penetration deeper in the leather. It is my understanding that tannin solutions are used for multiple runs of tanning, starting the process with used solutions that are depleted in tannin concentration and working towards newer solutions with a higher tannin concentration. In the end some tannin molecules are tightly bound to collagen (hydrogen bonds and salt bridges) and a portion of the tannins can be extracted through back soaking. If you’re interested I can post some articles describing these aspects of the chemistry. 

[SUP]

@TomE Could you please post them? I would love to read about it.

[chuck123wapati]

Tom is right while there are consistencies in the process that can be determined most of the professional tanners have their own methods, solutions and variations closely guarded so your question wont be as easy to find as you may think it should be. In the end all tanners will say theirs is the highest quality and their process the best therefore the amount of tannins in their leather is correct. The various steps from start to finish all effect the final quality of the leather the amount of tannins is not the only variable in the process.

 Think of it as cooking, two different chefs given the same ingredients and recipe may both make the same dish but each will have its own flavor and texture differences due to their own  personal choices and knowledge base. 

 

 

[SUP]

@chuck123wapati yes of course that is true. But then, that is why some chefs' food is much preferred over that of others.

And of course tannins are not the only thing that affect the quality of leather!  Not rocket science either to know that!

It could be one of the things that affect quality though, along with other things.

My question is just whether the quantity of tannins affects the quality of the leather or not. What @TomEstated above suggests that depth and extent of bonding may differ as well. So this might be a more complex answer than I expected!

TomE's articles and the earlier one sent by @ArkieNewbie could provide that answer. 

 

[chuck123wapati]

14 minutes ago, SUP said:

@chuck123wapati yes of course that is true. But then, that is why some chefs' food is much preferred over that of others.

And of course tannins are not the only thing that affect the quality of leather!  Not rocket science either to know that!

It could be one of the things that affect quality though, along with other things.

My question is just whether the quantity of tannins affects the quality of the leather or not. What @TomEstated above suggests that depth and extent of bonding may differ as well. So this might be a more complex answer than I expected!

TomE's articles and the earlier one sent by @ArkieNewbie could provide that answer. 

 

My point was there may be a recipe but "quality" is still somewhat subjective. good luck on your endeavor I'll be interested to hear what others say as well. 

[TomE]

3 hours ago, SUP said:

@TomE Could you please post them? I would love to read about it.

A couple of articles on tanning chemistry and several on currying - fats and oils.  The files are too large to upload here, so here is a link to my Dropbox folder containing the files. https://www.dropbox.com/scl/fo/1unoj2zvcdb9xjo65u993/h?rlkey=4rx8dv5fz2obchca3n6k27qyh&dl=0

[SUP]

@TomE Thank you! That is a lot of information. 

[Ddat]

I've always been curious about the chemistry of tanning, and was never able to find information sufficient to scratch the itch. Thanks for sharing!