Here is a quick look at the difference applying some tallow to dyed leather can make.
This is a pair of early 16th century style low shoes I’ve been working on, they’re my first ever right-side-out lasted and welted shoes so don’t look at them too closely. Neither shoe has fastenings yet.
The shoes are both cut from the same hide, and dyed with the same batch of iron-based black dye. The one on the right is finished and has had warm tallow applied to the leather. The one on the left is still waiting for it’s outsole and is the color of the dye before application of any oils.
As you can see the tallowed shoe has a much shinier, darker and more consistent finish than the un-tallowed shoe. I was expecting some change but I was quite surprised at the degree of difference between the two.