There are four generally recognized types of tannage:

The first is vegetable tannage. In this process, tannin, a substance present in many plants, is infused into the prepared wet skins. The oldest, and many say the best method, is. pit tannage. Hides are simply placed in a pit which is filled with a mixture of ground bark and water. This process can take a year or more for heavy bovine hides, starting with very weak infusions, and moving through stronger ones. Too strong a tanning solution at first tans the outside of the hide and seals in the middle which remains untanned. -

Even when natural bark is used, the pit system has generally been replaced with paddle vats which speed up the process considerably. But instead of ground bark, tannin extracts are often used. Different tree barks produce different leather color. Pure oak makes a mellow, pale leather, strong and flexible. Hemlock makes a firmer, better wearing leather that is reddish in color. Extracts are made from various trees and plants: que-bracho and mimosa trees, valonia and myrobalans fruits and leaves.

The second type of tannage is mineral tannage. This modern form of tanning was invented over the period 1860 - 1885, and revolutionized the leather industry. Leather could be tanned much more cheaply and quickly than in the past. Basically, hides are immersed in a bath of water and chromium sulfate, agitated in a drum, and are tanned in a few hours. Although chrome tanned leather lacks the body and moldability of vegetable leather, it is at least as abrasion resistant, more flexible, stronger, and better able to stand heat. Although it is universally called "chrome tanning," other materials are used: zirconlure sulfate, for example, produces a white leather; also used are salts of aluminum, potassium and iron.

Also classed as mineral tannages are specialized processes which tan using either formaldehyde or glut-araldehyde.

The third type of tannage is a combination of vegetable and mineral tannages. This more expensive process is used to create special purpose leathers with the desirable characteristics of both tannages. The skin or hide is first tanned by one method then retanned by the other. Looking at a cross section of such leather, the center will clearly show how it was first tanned, and the grain and flesh sides will show the features of the second method. "Semi-chrome" leathers have been vegetable tanned first, then chrome tanned; "chrome retans" are done in the opposite order.

The fourth and last method of tannage is oil tanning. This is actually tanning with oil, not treating leather with it. The leather commonly referred to a "oil tanned" is not oil tanned: it is normally chrome tanned and then oiled.

After a run of hides is tanned and finished, they are then sorted into grades. This is all relative since there are no absolute standards, and one tannery's grade "A" might be equal to another's grade "B".

What factors does the tannery consider when grading leather? First, size. Often, this is just a square foot measurement, but sometimes skins are graded as S - M - L - XL. Secondly, weight (thickness) is measured: although hides are usually split to a uniform thickness, full substance hides may be rated as light, medium, heavy, etc. Thirdly, defects are considered. Often, this is the only factor affecting grading: brands, holes, and scars detract from the grade of the hide in proportion to the size and quantity of defects.

Two slightly less noticeable characteristics are sometimes considered by graders: temper, the general "feel" of the leather, and uniformity, how great the variations in color and temper are within a single hide or skin.