Shading, pooling and pile reversal all refer to various kinds of pile distortion. These terms describe similar conditions and so are often used interchangeably.
Shading is the most general term, this occurs when light reflects off the carpets fibres differently from nearby fibres. This will cause the appearance of the carpet to have light and dark areas.
Shading can happen when fibres become scratched and abraded due to heavy traffic or lack of proper maintenance.
Tufts leaning in a different direction from adjacent tufts will also cause shading. The light reflecting from the sides of the yarn appears brighter in colour than light striking the ends of the yarn.
Pile reversal describes a localized change in the orientation of pile yarns. Pile reversal along a seam may be the result of an installation error, two lengths of carpet being installed in opposite directions.
Pile reversal may also occur when the bottom of a roll is crushed, for example by the weight of other rolls stacked on top prior to fitting. This type of pile reversal can sometimes be corrected by the use of wet steam and grooming.
Another form of pile reversal may appear as a wavy line along the edge of a traffic area such as down a hallway or at a pivot point.
Pooling or water marking is seen as an irregular roughly circular area with light/dark contrasts that make the carpet look like it is wet, or a pool of water. This is frequently seen on cut-pile wool products as well as less resilient fibres and the cause remains undetermined.
Unfortunately most pile distortion can’t be corrected or the correction will only be short lived, however, maintaining your carpets correctly will reduce the chances of this occurring.