Thursday, 18 April 2013

Micrograph of the Month: Pseudomorphic Voids

Plant voids in different materials in thin section
I mentioned these features briefly in a previous post on wall plasters - these micrographs all show pseudomorphic voids in different contexts. It would actually be technically correct to call these pseudomorphic plant voids, as there can be other 'pseudomorphs' found in geological thin sections, which occur where a void/space originally formed by one type of material is replaced by another type. This leads to the crystal shape being that of the original mineral rather than the mineral that is currently present. Pseudomorphic plant voids in an archaeological context occur in materials where plant remains were once present but have now decayed. This leaves an 'impression' of the plant remains, which can give important clues on the use of plant materials in archaeological contexts.  It gives us a different insight to that of the charred macrobotanical record. Materials such as those shown above may not contain plant remains in charred form, and looking at these types of voids may be the only way of determining the type of volume of plant remains that were used for tempering clay materials, or that were being consumed. In some cases we can combine this with phytolith analysis, though in the materials I have examined the presence of phytoliths within the voids is suprisingly rare. In fact I have only seen one example, shown above (upper right), where you can see a grass leaf/stem phytolith within a void in mudbrick. In the upper left you can see seed shaped voids in calcitic plaster (maybe an archaeobotanist could hazard a guess at the type of plant?). In  the lower left is a great example of how the angle of the cut impacts what we seen in thin section, in this case showing both transverse and longitudinal voids in grass tempered clay aggregate. In the lower right we can see an omnivore coprolite with plant voids.

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