The top layer.

The thickness of a hide

A bag is drawn in millimeters before it is drawn in shapes. Leather is bought by the tenth of a millimeter, and never as a single figure: a hide is not the same thickness on the back and on the flank, so it is bought as a range, 1.2 to 1.4 for instance.

3 millimeters separate a lining from a belt. Those millimeters decide whether a piece falls or stands up on its own.

None of those figures is the figure at the edge. A folded edge is skived from the back, on a slope, until it folds without a ridge: 2 millimeters come down to 0.6 over the last 10 millimeters, and at the very end the leather is as thin as a sheet of paper. It does not show. That is where you see the workshop.

The ranges are in millimeters and come from the trade; no standard fixes a thickness for a given object.

What is taken off

A hide is two very unequal thicknesses. On top, the full grain, a film a millimeter deep. It is the part that grew with the animal, the part that carries the pores and the marks.

The second thickness lies under the grain and is called the corium: 5 millimeters of thick, tangled fiber, with no pores and no pattern. It is not a surface, it is matter. It is what holds, and it was never meant to be seen.

Cross-section of a bovine hide, to scale. The full grain is only a sixth of the thickness; all the rest is corium.

You read everywhere that the grain is the strongest layer. That is not accurate, and it is measured by a tear: it holds 5 times less than the corium. The grain makes the surface, the corium makes the strength. A full-grain leather is the two of them together.

A hide is split through its thickness, along the dotted line above. The upper side keeps the grain. The lower side has none: it is bare corium, also called a split.

It can be made to look like a hide. A grain is printed into a sheet of plastic, the sheet is glued onto the split, and out comes a perfectly regular surface, with pores that never let a hair through.

That surface does not wear, it peels. What comes away is not leather but a sheet of plastic in one piece, and underneath there is only the fluffy fiber that was never made to be touched. The phenomenon is predictable enough that a standardized test counts the rubs before the plastic separates.

French law is not fooled. A split cannot be sold under the single word leather: the label has to say split leather. In 2016 the French fraud-control authority inspected 539 businesses in the sector and had 47 samples analyzed; 60% of them were non-compliant.

Corrected grain does the same thing, more quietly. The full grain is sanded down to erase the marks, a new grain is printed on with a roller, a pigmented finish covers it all. It is legally leather. It is simply no longer the skin you are touching.

The number of pores

The grain of a leather is the trace of the hair. An animal is born with all its follicles and never makes another one. But the skin grows with it: the same number of points spreads over a larger and larger surface, and they move apart.

Calf. Less time to take on wrinkles, marks and scars, and a tight grain because the skin has not stretched yet. It has another quality, a rarer one: it keeps its body when it is split thin. That is what makes it irreplaceable in small leather goods, where we go down to 0.8 millimeters without the piece collapsing.

Lamb. The softest of the three: it falls, it follows the shape instead of holding it. Its fragility at the edge has a precise cause: lamb carries more hair than any other skin, and every hair taken out leaves a minute hole; beside each hole, a small pocket of fat. End to end, that makes a line of weakness inside the skin itself, like a perforation. On an edge, where the leather is skived, folded and sewn, that line is the first to give. We protect those edges for that precise reason.

Cowhide. Its fibers are thicker and tighter than a calf’s, and they form a dense network. It keeps its shape without anything added inside. We put it where a piece has to stand up.

Click a skin to enlarge

Tanning

A fresh hide rots. Dried as it is, it hardens like cardboard. Tanning settles both: a substance is driven into the skin, it grips the fibers, stops them welding back together as they dry, and makes them impervious. Without it, a skin dries, hardens, and stays a dead skin.

Two families do this work. Chrome salts, which are a mineral. And tannins, drawn from bark and wood. 85% of the leather produced in the world goes through chrome.

The difference starts with time. A chrome tannage takes 15 hours. With tannins, in the pits, it takes a year at the few houses that still work that way: 3 months in the bath, then 9 months stacked with bark between the hides. Today’s industrial version runs to 10 days.

It carries on with heat, and there it can be measured. Chrome-tanned leather only shrinks above 212 °F (100 °C), tannin-tanned leather at around 176 °F (80 °C). Those 20 °C change everything you can do with a leather: chrome takes ironing and hot glue, and it can be split very thin without collapsing. Tannin-tanned leather molds, embosses, and keeps the impression you give it.

The temperature at which leather begins to shrink. Those 20 °C decide what can be ironed, hot-glued and split thin.

It ends with color. Tannins are a list: chestnut, quebracho, mimosa. Quebracho, for instance, pulls the leather toward a warm red.

That leaves patina, which everyone talks about and nobody defines. It is a reaction, and it has a name: under light the tannins oxidize and the leather darkens. Pushed further, the same reaction lightens it, because the fiber comes apart. Patina and wear are the same phenomenon at two moments.