Leather can be hardened or softened depending on the treatment it is given. Leather will become harder and stiffer if it is allowed to dry out over time. Often, this can lead to cracking and damage, especially in weaker leather. However, those needing harder leather, perhaps for leather armor or bags, can harden leather by speeding up the loss of moisture and heat-treating the leather. The more fragile the leather, the more care is required to prevent damage.
Boiling Vegetable-Tanned Leather for Greater Hardness
Fill a pot with tap water. Make sure the pot is large enough to hold the leather. Turn heat to high until water boils.
Be sure to boil vegetable-tanned leather only, as other kinds of leather may leach harmful chemicals or disintegrate. Do not boil very delicate leather, like lamb. This procedure is for thicker, stronger leather, not for jacket leather.
Place the piece of leather in the boiling water. For the first two minutes, the leather will darken and stretch. The leather can be taken out and stretched or manipulated into different shapes. After stretching it, you can leave it to dry and it will be harder than it was prior to boiling.
Leave the leather in the pot for over 2 minutes if you do not intend to stretch or change the shape of the leather. The longer the leather is left in the boiling water the harder it will be once it dries. Leather that is hardened in this way will become more brittle and will also shrink to around 7/8 of its original size. Leaving the leather in boiling water for more than 30 minutes will result in very brittle leather.
Remove the leather and place it flat on some towels to air dry. After a few days the leather should be dry and much harder than it was before.
Water Hardening for More Delicate Leather
Put the leather in a bucket of water that is around room temperature until the leather is completely soaked. This should not take more than a minute or two.
Remove the leather and pat it dry with a towel.
Lay the leather flat on a folded dry towel to air dry. As the leather dries it will become harder. Drying leather in this way removes the moisture and can lead it to crack. You can apply leather conditioner oil to the outside of the leather if it feels too dry or cracked, but this may also soften the leather.
Check the label to see if it is vegetable tanned or tanned using metal or other chemicals. You might also be able to ask the staff where the leather was purchased if the leather was vegetable tanned or not.