The extraction procedure is an essential step to retrieve and concentrate phytoliths. It consists in the separation of the biogenic silica fraction (not only phytoliths but also quartz, diatoms, sponge spicules, etc.) from the soil matrix, so that the residue obtained can be mounted on a slide for observation under the microscope. There is no single method of phytolith extraction, and different researchers use different approaches. However, extensive research has been carried out comparing different extraction protocols (Powers and Gilbertson 1987; Buckler et al. 1994; Lentfer and Boyd 1998; Zhao and Pearsall 1998; Zucol and Osterrieth 2002; Piperno 2006) as well as approaches recovering multiple microfossil (e.g. phytoliths, pollen, starch, etc.) data sets (Coil et al 2003; Lentfer and Boyd 2000).
Some protocols have as a first step the fractionation of the sediment using a fine mesh to remove the bigger particles (Rosen & Weiner 1994) or to separate silt, clay and sand fractions (Piperno 2006). In some protocols this first step has been removed to reduce mechanical damage to the more delicate, ornamented morphotypes such as the fine spiny rods and dendritic forms, or the silica skeletons, as well as to speed up the processing (e.g. Madella et al. 1998).
Although there are differences between the various existing methods, all protocols consists of the following basic steps:
Also, most methods can be divided into two main groups: the ones that separate the phytoliths using a heavy liquid to float them and those that do not use this approach. The most commonly used heavy liquids are zinc bromide (ZnBr2) and sodium polytungstate [Na6(H2W12O40)H2O]. The latter is a non-toxic (Munsterman and Kerstholt 1996), high density (1·0–3·1 g/cm3) separating compound that generates an almost neutral solution (pH 6) of relatively low viscosity. Although sodium polytungstate is expensive, it can be recycled several times, using a membrane filter of 0.5 microns and the help of a vacuum pump. The filtered liquid is then evaporated at low temperature in a fume cupboard until it reaches the desired density.
Also, most methods can be divided into two main groups: the ones that separate the phytoliths using a heavy liquid to float them and those that do not use this approach. The most commonly used heavy liquids are zinc bromide (ZnBr2) and sodium polytungstate [Na6(H2W12O40)H2O]. The latter is a non-toxic (Munsterman and Kerstholt 1996), high density (1·0–3·1 g/cm3) separating compound that generates an almost neutral solution (pH 6) of relatively low viscosity. Although sodium polytungstate is expensive, it can be recycled several times, using a membrane filter of 0.5 microns and the help of a vacuum pump. The filtered liquid is then evaporated at low temperature in a fume cupboard until it reaches the desired density.
To speed up the drying process it is possible to add the following steps after the supernatant has been poured out: