Tuesday, 8 January 2013

Fun with phytoliths, part 2

Deflocculating the clay particles. Don't forget to label the beakers.
After carbonate removal and washing away any remaining acid, stage 2 is to get rid of any clays. Again it depends on the type of sample how long this step will take - whilst ashy samples will have more carbonates, they are likely to have minimal clay content. Conversely, these deposits from Elbląg have much more organic and clay components. Again, this is a very straightforward process, but very time consuming. In fact, for samples with a significant clay component, it can take all day! The samples are transferred from their tubes into tall-form 400mL glass beakers. The height and volume of the beaker is important, as we are using a bit of fluid mechanics to seperate out all the clay and non-clay sized particles.

Rinsing beakers into crucibles
Distilled water is added  along with a few mL of sodium hexametaphosphate (otherwise known as Calgon), up to a height of 8cm (it's easiest to mark this out on the side of the beaker). Calgon is a deflocculating agent - a lovely word which is the opposite to coagulation i.e. it encourages the little clay particles to seperate rather than stick together. Then you give it a good stir and let it stand for 1hr 10 mins. During this time, all the particles which are larger than clay-size will settle into the bottom of the beaker, whilst all the clay particles stay in suspension. This is determined by Stokes' Law, which involves a few equations with greek letters. In brief it is based on the radius of a spherical particle, it's velocity in a liquid of particular viscosity, and gravity. Luckily this has all been previously calculated, so all we need to remember is the height of water and the time (I should mention this method is based on Rosen 2005 in the Catalhoyuk monographs).

When the time is up, carefully pour the water + clays down the sink - you sohuld be able to pour most of it away before the silt and sand sized particles start moving from the bottom. As soon as this happens, stop pouring or you'll lose some sample. Then, add the water back up to the mark and repeat, until the water is clear. As you can see in the photo above, some of the samples are very dark. This tends to mean a lot more clay particles, and can take up to 8 hours to remove. When the water is clear, it's time to transfer the samples into crucibles for Stage 3 - removal of organic materials!

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