HOW TO PREVENT GRAIN CRACKING ON BARKTANS
First some definitions.
The grain layer is the most external layer of skin on a hide. You may have heard of the epidermis, which takes up real estate in thick hides like cattle. The epidermis is negligible on deer, and even most elk. It exists as a thin film that can be swabbed away with a light touch of the finger. So for hide tanners, the outermost layer of hide is the grain.
Barktan refers to any and all hides in the bark-tanned method: leathers that have been tanned with the tannic acid from tree bark, roots, or aerial plants.
If the grain is left on the hide during bark-tanning, we call it full-grain leather. Hide tanners use this term a little differently than the chrome leather industry, which thins hides out with planer blades and leaves only the grain; for us, we keep the dermis + grain.
And some anatomy:
The grain layer sits on top of the dermis, which sits on top of the hypodermis. The grain + dermis are both made of collagen, but the collagen is structured differently in each. In the dermis, it is like a plush woven knitting. In the grain, it is like a rigid mat. This makes the dermis flexible + stretchy, while the grain is brittle.
Take note of that.
When we make full-grain leather, we are dealing with two distinctly-structured layers, stuck to each other for eternity. One wants to be squishy and stretchy, the other wants to be firm and rigid.
So sometimes, when leather is drying, the grain (the brittle one) will crack.
Here's how to prevent that
In order of importance…
(1) Tan the hide thoroughly
Simply put, grain isn't likely to crack if the hide has been thoroughly tanned into leather
Some signs that the leather is not ready to come out of the bark liquor include: uneven colouration (dark and light spots), light or white colour on the inside of the leather when you cut a cross section, and a hardening of the leather if you cut a piece off and let it dry. Do all these tests before considering taking leather out of its bark liquor.
Pro Tip: if the hide is allllmost fully leather but taking a very long time to finish, put it in a strong solution of first extraction bark liquor.
(2) Dry the leather slowly
I'm going to tell you something: I've never had grain crack on any of my leathers. This is partly because I follow the advice I'm giving you, but it's partly because I live in a rainforest + the air is saturated with moisture. Nothing dries fast. And in hide tanning, that can be a good thing.
Avoid blasting heat directly to your barktan as it dries. Ambient heat is great, a space heater right underneath your hide frame, not so great.
Let it take 2-3 days for a full sized leather to dry
(3) Roll the leather a bit as it dries
When leather is 90% dry, I take it out of the frame and roll it back + forth on a table.
This gets the grain to start stretching, and it locks in that stretch
This also conveniently turns the hide horizontal for our next step:
(4) Oil the grain side before it is dry
Make sure to oil the grain side before it is dry. Waiting until the grain is dry is too late.
I use a 1 litre fat emulsion of lecithin + liquid oil mixed with water for this. I will then finish with a leather pomade when I'm ready to craft with the leather.