Metal Free tanning: a more Sustainable way to make leather

What is Metal Free Leather?  Can Leather be sustainable?   In most of the case Metal Free leather is a more sustainable option, especially for Be Green that went through a crowdfunding round (the first tannery ever) to push the sustainability to the next level.   But not all the leather can be done in this tanning. In this cases we talk about ovine leather that are usually tanned without chrome