Particle settling times for gravity sedimentation and centrifugation: a practical guide for palynologists.

Palynological analyses are becoming increasingly integrated into archaeological studies. Central to the preparation of samples is the use of centrifugation or still settling to remove sediment components and/or separate the palynomorphs. However, although there are many standard recipes describing palynological preparation, there appears to be no discussion of optimal centrifugation times and speeds or settling times. Since centrifugation may take up as much as 20–40% of preparation time, it is reasonable to consider potential reduction of this time; this may be achieved by reducing centrifuge time. This note discusses the theoretical bases for selecting settling times and centrifugation times and speeds, and provides indication of optimal values for the removal of particles of different sizes, especially those in the clay-sized range, in media of various viscosities. By returning to first principles, it appears that typical centrifugation times provided in recipes are conservative, and that considerable savings in time may be made by more judicious use of the centrifuge. Tables of values are provided as example, indicating order of magnitude of timing required.