Grain Or Flesh, Which Is Stronger?

[TomE]

I've had the impression that the grain layer of veg tanned leather contributes more to overall strength than an equal thickness of the flesh layer.  This has been suggested as a reason to not use a stitch groover (removes the strongest part of the leather), and why splits are stretchy and weak compared to full grain leather.  Well this paper from 1926 says the opposite.  On per thickness or per cross sectional area basis the flesh side is stronger than the grain.  For example, the grain side of a split that retains 70% of original thickness has half the strength of the unsplit leather.  The flesh side split to 40% of original thickness retains half of the strength.  Maybe not a big deal but it indicates that the grain doesn't have an outsized contribution to strength, at least not for these authors' cow hides.  The sum of the strengths of grain and flesh splits is always less than the strength of the unsplit leather- that much I can understand. 

Does this comparison of grain and flesh agree with your thinking?  I am particularly interested in the strength/stretchiness of straps.  Seems it would be stronger to line a thick strap with a thin liner instead of using 2 straps of equal thickness that add up to the same overall thickness.  

Wilson and Kern - 1926 - Effect of Splitting on the Tensile Strength of Lea.pdf    

[bruce johnson]

Yes I think the grain side generally has more tensile strength. If it didn't we would be using splits to make good lace. That said, some tannages of splits carry some good strength like pearl apron splits for aprons and horn wraps. 

My opinion on grooving  is just this - my opinion. I am a groover, I have stickers on my sewing machines to remind people who use them to groove. First and foremost it gives you a visual target to stitch in. Edge guides on machines are nice but not foolproof. You can have the nicest machine  - dialed in with the prefect thread and needle combination and tensions just right. Make one wiggle in the line and it is like a zit on the prom queen's nose. You see it first and last. 

  People who say grooving weakens the leather  - here's your test. Groove off a section and pick up that little leather thread that you just grooved off. Pinch it between your fingers on each hand and pull. Pretty flimsy and about zero tensile strength. It is not taking away anything from the strength of your leather and likewise leaving isn't adding anything.  Edger trimmings have more strength and we edge our leather religiously. 

[chuck123wapati]

5 hours ago, bruce johnson said:

Yes I think the grain side generally has more tensile strength. If it didn't we would be using splits to make good lace. That said, some tannages of splits carry some good strength like pearl apron splits for aprons and horn wraps. 

My opinion on grooving  is just this - my opinion. I am a groover, I have stickers on my sewing machines to remind people who use them to groove. First and foremost it gives you a visual target to stitch in. Edge guides on machines are nice but not foolproof. You can have the nicest machine  - dialed in with the prefect thread and needle combination and tensions just right. Make one wiggle in the line and it is like a zit on the prom queen's nose. You see it first and last. 

  People who say grooving weakens the leather  - here's your test. Groove off a section and pick up that little leather thread that you just grooved off. Pinch it between your fingers on each hand and pull. Pretty flimsy and about zero tensile strength. It is not taking away anything from the strength of your leather and likewise leaving isn't adding anything.  Edger trimmings have more strength and we edge our leather religiously. 

well said.

[AlZilla]

I've always assumed that when we say the strength is in the flesh side, it means abrasion/cut resistance, too.

It looks like they were stretching the test straps to 1.25 times the original length. Too bad they didn't go until they broke.

[chuck123wapati]

My two cents is this doesn't mean much in terms of helpful info for the average leather craftsman and here's why IMO.   We all know that when leather is split it becomes weaker, how much is what this study tries to determine accurately. We also know however that the hide on the back differs from the hide in other areas of the animals body, that's why we use different cuts for specific applications. Its a good study don't get me wrong it does show repeatable results I'm sure. 

1. This study while determining the amount of weakness in one specific part of the animals hide does not put any numbers on the other 70/ 80 percent so IMO it cant give good data as not all the hide is tested and compared. So how do we know the results would be the same for the butt as compared to the neck or any other area.

2. do you plan on using calf leather for straps or chrome tan? does leather from a mature animal have similar results? The study doesn't say that either.

So with that in mind basically it tells you what you already knew leather gets weaker if it is thinned, but what you don't know is how strong it is to begin with or even why separating the two layers has such an effect. So if you are still asking if two equal layers are the same strength as a thick and a thin layer it doesn't really answer that imo. That would be a whole different experiment.

 

[TomE]

Thank you @bruce johnson, @chuck123wapati, and @AlZilla for your thoughtful replies.  You all raise interesting points that will influence my experiments with leather this winter.  Hope to spend more time in the leather shop once I finish the fall/warm weather chores - spreading manure, replacing broken fence posts, and weaning the foals.  Thought I might turn this hobby into a business someday but my wife's business uses up most of my time.  Maybe there will be a brief span when I'm too old to mess with horses but young enough to mess with leather.