This is something that came up recently in a conversation about a custom order, so I thought I would write it all out for easy reference.
So first things first. Tooling leather is sized in ounces (oz). The bigger the number in oz, the thicker the leather is.
These are the two sizes of leather that Eve and I work with. The left is 4oz leather, the right is 8oz leather.
8oz is roughly 1/8th of an inch thick. As such, 4oz is about 1/16th of an inch.
So why choose one over the other?
beadmask is a beautiful example of what can be done with thin leather! This is a 4-5oz mask.
Sometimes masks with such dramatic shaping look better without tooling because the tooling can get a little warped and distorted.
As the thickness increases, it becomes more difficult to achieve such sharp folds in the leather, but tooling becomes less of a pain because there is actually depth to hammer into.
Autumn Forest Mask, which is 4oz leather. As you can see, each leaf is hand tooled, but there is still a significant amount of what I call "tight flexibility." The vines can be twisted into tiny little tight curls. 4oz is a great middle ground if you want something that can be tooled as well as intricately shaped. There's also a lot of layering that can be done without making the mask too heavy or unstable.
That's not to say that you can't do some awesome things with 8oz leather in terms of shaping! The dragon mask is a great example of shape paired with detail. This is what the full mask looks like, in all of its horned and finned glory. The key difference here is that while the horns do spiral up, those spirals are wide and not at all tight like the curls in the Autumn Forest Mask.