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UCL Home  /  Geography  /  Resources  /  Laboratory  /  Laboratory Methods  /  Lake Sediment Analysis  /  Diatom Slide Preparation

Diatom Slide Preparation

Diatom slides are usually made up by allowing the diatom suspension to settle out on a cover slip overnight, as described below. This produces an even spread of diatoms over the cover slip but it can take up to two days. It is possible to speed up the procedure (resulting in lower quality slides) by gently heating the coverslips after the diatoms have been allowed to settle for 30 minutes. This may result in some clumping of the diatoms but the slides can usually still be counted.


Hotplate, round glass cover slips 19mm diameter, glass slides, 1 ml pipettes for each sample, Naphrax diatom mountant , rigid metal tray, distilled water.


  1. Dilute the cleaned diatom suspension to a suitable concentration. It takes practice to get the concentration right. The suspension should look neither totally clear or milky. Fine particles in suspension should be just visible when the suspension is held up to the light.
  2. Place metal settling out trays with cover slips in a position where they will not be disturbed, away from dust sources and air currents.
  3. Using the 1 ml pipette, place up to 0.5ml of well mixed diatom suspension on each cover slip, cover the tray to keep off dust and leave to dry. This may take up to two days.
  4. Heat a hotplate in a fume cupboard to 130oC.
  5. Place 1 drop of Naphrax on a glass slide and invert the cover slip with the dried diatoms over the drop.
  6. Heat the slide on the hotplate for 15 minutes to drive off the toluene in the Naphrax.
  7. Allow the slide to cool and then check that the cover slip does not move when pushed with a fingernail. If it does move then the slide will need to be heated a little longer.