Leather and How It's Tanned

Before you cut a single strap, it helps to know what leather actually is, because the kind you buy decides what you can do with it.

What tanning does

A raw hide rots. Tanning turns it into leather by chemically reacting the hide’s protein fibers, its collagen, with an agent such as plant tannins, chromium salts, or alum. Tanned leather resists decay and wetting and stays supple and durable ¹. Most leather comes from cattle, sheep, and pigs ¹.

Every piece of leather has two faces. The grain side is the outer surface of the animal, where the hair grew: smooth and dense. The flesh side is the underside: thicker, softer, and fibrous ^{1 2}.

Two tannages

Vegetable tanning is the oldest process still in use. After the hides are soaked, fleshed, limed, and dehaired, they go through a series of tanning baths made from tannins extracted from tree bark, starting weak and growing stronger. It usually takes about three weeks for the tannin to reach the center of a hide ¹. Vegetable tanning makes the firm, heavy leathers used for soles and saddles ¹, and the “veg-tan” that leatherworkers prize for carving and dyeing ². It’s the leather in this course: it takes stamping and carving, and when it’s wet it can be molded to a shape it will hold once dry.

Chrome tanning uses salts of trivalent chromium. It’s faster and makes soft, flexible leather for bags and clothing ², and it accounts for about 90 percent of U.S. leather production ¹. It won’t take carving or molding the way veg-tan does.

Ounces are thickness

Leather thickness in the United States is given in ounces, which confusingly has nothing to do with weight. One ounce is about 1/64 inch, or 0.4 millimeter, so 4-ounce leather is about 1/16 inch thick ³. Tandy Leather’s chart gives rough uses ³:

• 1–3 oz: linings, wallet interiors, watch straps.
• 3–5 oz: wallet exteriors, small bags, notebook covers.
• 5–7 oz: medium and sturdy belts, tool pouches.
• 7–9 oz: heavy belts, holsters, sheaths, saddlery.
• 9–10 oz and up: harness, heavy straps, armor.

Too thin, and stitches pull through; too thick, and it won’t fold or sew well ³. For layered items, the total thickness is what matters ³.

Milwaukee leather

Wisconsin is part of the story. The EPA notes that Pennsylvania, Massachusetts, New York, and Wisconsin together hold almost half the country’s tanneries ¹. In 1847 two German immigrants, Guido Pfister and Frederick Vogel, set up shop in Milwaukee, Vogel with a tannery on the Menomonee River. Within a year their firm made half the city’s leather. By the 1870s it was called the largest tannery west of the Alleghenies, and during World War I Pfister & Vogel employed more than 3,500 people and was one of the largest tanners in the world. Its last Milwaukee plant closed in 2000 ⁴.

Try it

Next time you hold a belt or a wallet, flex it. Firm and holding its shape, with a smooth, tan grain? Likely vegetable-tanned. Soft and floppy? Likely chrome-tanned. Buy a small piece of 6–8 oz veg-tan for your first projects in this course.