If a slide prep contains a great deal of debris, the microfossils can be difficult to view. Nine methods that can be used alone or in concert with one another are presented on this page.
My Slide is Complete and it is “Dirty”
The first thing to try is (1) engagement of the Lambda waveplate on your scope. This simple step frequently makes debris that “shines” fall to the background so that you can see each particle more easily and pick out the microfossils.
Another option is (2) to divide the material from one sample into a large number of slides so that there is ample space for the particles to lie separately instead of piled up on one another.
If there is too much debris for these options to work, and microfossils are being obscured, materials can always (3) be washed off slides and re-floated. The best solution, however, is to prevent the problem from happening during the sample prep before the slides are made.
Equipment Choice and Methods
Simple preventive measures can help insure clean slide preps. (4) Consider using a centrifuge with buckets rather than one with a fixed-angle rotor. The horizontal separation created in a swing-out bucket leaves less residue on the side of the tube to re-enter the floated matrix and add debris.
If you decant your floats, as I do, keep a box of starch-free fiber wipes close by. If you see residues of sediments on the rims of the tubes, (5) use a new wipe, and gently clean it off, being careful not to let the debris fall into the tube. This sediment will not be decanted with the floated materials, and the prep will be much cleaner.
If your sediments are highly organic, (6) Cesium Chloride will give you a clean float in cases when SPT produces a very dirty float. I encountered this problem with Andean sediments in which a thick layer of black material floated with the SPT, while starch grains floated cleanly with the CsCl. I have no idea why it works, but the difference can be dramatic. The issue is documented and discussed in Coil et al., 2003 (Journal of Archaeological Science, 30: 8, 998-1008.)
One of the most common sources of debris on a prepared starch mount is clay in the sediment. Tiny particles of clays will fill a slide, and large numbers can obscure your view of microfossils. Small numbers of clay particles can be less noticeable if you engage the Lambda plate (1, above). There are, however, methods to remove clays from samples, the first of which is common practice in prepping phytolith samples: settling.
Using Stokes’ Law, calculate how long it will take for the microfossils of interest to settle out of the water/sediment suspension. Once the starches and/or phytoliths hit the bottom of the vessel, (7) you can decant or pipet off the cloudy, clay-filled supernatant. Complete this settling process several times for sediments that are clay-heavy. You can speed up this process (8) using the centrifuge, but you will need to calculate the centrifugal forces using Stokes’ Law, and they will be unique for each model of centrifuge. *If you do not know how to make calculations using Stokes’ Law, you should not risk pouring off your data with the supernatant.*
Sometimes (9) a deflocculation step can help prevent clays from floating during the heavy liquid separation. Sediment samples are usually deflocculated, however, if there is enough soil extracted from an artifact that you are fairly certain clay could be an issue, a bit of baking soda or Sodium hexametaphosphate may help make a float cleaner. I let samples deflocculate in water and the salt for 72 hours.
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