Once the hide is shaved, they enter into the tanning drum. This is where the base color of the hide is applied. The hides along with the dye and other chemicals are added to the drum. The drums spin at a slow steady rate for sometimes hours. Tanning drums can hold any amount of hides from 2 to 100, the ones in this photo were some of the larger at this facility.
The use of tanning drums goes back hundreds of years. Even as metal and plastic manufacturing has improved over the years, wooden drums are still the most widely used. This part of the process uses a fair amount of chemicals and the wood can withstand strong alkaline solutions and chemicals. They are also cheaper when modifications or repairs are needed as opposed to their stainless steel or plastic counterparts.