Fans of sartoria will be well aware of the benefits of oak bark-tanned leather. Its denseness of fibre and yet ability to be moulded make it perfect for the soles of men’s shoes. But because it is hidden beneath the shoe, and because it is often painted, few appreciate how the leather changes colour over time.


This quality of tanned leather was highlighted by a collection of bags from Bill Amberg, and it’s one I discovered recently. Serendipitously, as a new collection is slowly arriving in store, that oak bark line currently has 30% off.


In its natural state, the oak-tanned leather is pale and creamy. Over time, it turns a rich, deep brown. This is not as a result of contact with your hands or other contaminants, just photosensitivity. It’s a lovely process to witness in a natural product – the neck leather, selected for its texture and stretch lines, slowly acquiring character and depth over three months or so.


It can, however, be a hard thing to explain to customers. You’re not buying a bag in that colour, but in a very different one. Bill admits it’s not easy to communicate, “which is why we’re polishing up and preparing a bag that can sit in the shop, to show people the final colour,” he says.


Bill also has a strip of the leather around his wrist, in case you happen to bump into him. (Among its many characteristics, the leather is one of the most benign against the skin and is hypoallergenic, which means it is often employed in prosthetics.)


Each tanning process produces a different colour – chestnut is more yellow in its natural state and turns orange, mimosa is almost white and goes very yellow. Oak, for Bill, produces the most natural, wood-like tone.


But little of any tanned leather is used for bags. This is because its hardiness makes it difficult to work with. Fortunately, Bill has a lot of experience here. Most of his products are made with bridle leather: skins that are treated with oil to give them strength and water resistance. They are also harder to work with, but Bill prefers them because they retain the structure of a bag.


“It means you can make a bag with no internal structure, no metal frame or cardboard – just leather on the outside and a coloured suede on the inside. It’s beautiful and simple and strong,” he says. That also makes the bags surprisingly light – particularly briefcases or similar designs, which conventionally have a wooden or metal frame.


The size of leather required, particularly in the neck, is not usually produced by tanneries. So it takes a fair amount of planning to produce a collection like this – up to a year to commission the skins and plan production, then nine months for the (pit) tanning before anyone can start making bags.


So probably worth checking out the remaining ones in this collection.