before froth flotation can work, the ore to be treated is reduced to fine particles by crushing and grinding (a process known as comminution) so that the various minerals exist as physically separate grains. this process is known as "liberation". the particle sizes are typically less than 0.1 mm (100 µm), but sometimes sizes smaller than 7–10 µm are required. there is a tendency for the liberation size of the minerals to decrease over time as the ore bodies with coarse mineral grains that can be separated at larger sizes are depleted and replaced by ore bodies that were formerly considered too difficult.