Is the act or deliberate action of the tanner required for the purification of a hide?

Primary text

The process of tanning does not require a specific deliberate action (fi'l) because it constitutes the removal of impurity, similar to washing the earth. Therefore, if a hide from a dead animal falls into a tanning area and is purified by the tanning agents without any direct action, it becomes pure, just as rainwater purifies an impure patch of ground.