Before starting to prepare a permanent diatom slide, organic matter must be removed from the sample. The following supplies and process are for the preparation of simple (strewn) diatom mounts.

Be safe. This method should only be attempted in a laboratory and with appropriate eye protection, lab coats, closed toe shoes, and laboratory gloves. The process must be carried out in a fume hood because of the toxic fumes (toluene) of the mounting media.

Supply list:

fume hood
#1 thickness glass coverslips
glass microscope slides
pipettes (disposable glass works best)
pipette bulbs
aluminum plate for drying slides
hot plate
high refractive index (1.7) mounting medium (e.g., Naphrax, Zrax, Pleurax, Hyrax)
single edge razor blades
slide storage boxes


Prepare diatom slides in an area free from disturbance and with minimal vibration. Place clean glass coverslips on the aluminum plate.

Using a pipette, carefully place up to 1 ml of clean diatom slurry on each coverslip. Take care not to disturb the coverslips and allow the diatoms to settle and the water to evaporate, at room temperature. Coverslips usually dry overnight, but the amount of time depends on the ambient humidity. When the coverslips are dry, place the aluminum plate on a hot plate (~ 130 C) to drive off remaining moisture.

Place a drop of mounting medium on a glass slide. Using the forceps, invert the coverslip with the diatoms (turn it diatom-side-down), and place it on the glass slide. Then place the microslide on the hotplate. The mounting medium will begin to bubble and the toluene (solvent in mounting medium) will become more volatile. Allow the mounting medium to bubble for 1-5 minutes. Remove the slide from the hot plate and allow the slide to cool. Check that the mounting medium is neither brittle and brown (too much heat), or clear and soft (not enough heat).

Once the slides are cool, excess mounting media can be removed with a single edge razor blade. Be careful not to dislodge the coverslip from the slide. Store the microslides in a slide storage box.

Zrax is not currently available. Naphrax is available through

Brunel Microscopes Ltd.

Unit 2, Vincients Rd., Bumpers Farm Industrial Estate
Chippenham, Wiltshire, SN14 6NQ, United Kingdom

, 620 Broad Street, Suite 100, St. Joseph, MI 49085

Yellowstone 0 1Cm 40X
Image Credit: Sarah Spaulding
Aulacoseira, Yellowstone Lake surface sediments.
Yellowstone 0 1Cm 04 60X
Image Credit: Sarah Spaulding
Aulacoseira and Stephanodiscus, Yellowstone Lake surface sediments.
Iwb 0001 Heart Lake 02
Image Credit: Ian Bishop
Diatoms from Heart Lake, Washington.