How to prevent large diatoms from getting crushed?

[Wes]

I made a couple of diatom mounts and in both cases once the slides were cured I found crushed frustules.

Is there a technique to prevent that? Maybe use more mounting media or spacers?

[MicroBob]

Hi Wes,
which mountant did you use? Pleurax is heated to remove the solvent, it solidifies when cooling and there should result no pressure.
Can you write you preparation steps?

Bob

[Wes]

Hi Wes,
which mountant did you use? Pleurax is heated to remove the solvent, it solidifies when cooling and there should result no pressure.
Can you write you preparation steps?

Bob

Hi Bob,
Yes I use Pleurax. I arrange the diatoms on the coverslip, then add a small drop of Pleurax and after that the coverslip with diatoms goes on a slide with the Pleurax being sandwiched.

What you're saying is that I should let the coverslip with Pleurax stay on the hot plate to evaporate the solvent and only then add it to the slide.

Is there a particular temperature program I should follow here (e.g. 70˚ C for 30 min or something similar)?

[MicroBob]

Hi Wes,
I use Pleurax like this:
Cover slip with ditoms
Drop of isopropanol
Drop of Pleurax
Heat to 180°C
Pick cover slip up with warmed slide
Cover slip sucks itself to bottom of slide
Let cool cover slip down supported at the ends
Hard when cooled
Apply paint ring

This way the solvent is gone before the diatoms are between the glass planes. There is no pressure on the cover slip before the Pleurax solidifies. It is good to have an elevated surface to put the cover slip on for the heating. I turned stepped aluminium discs for this.

Bob

[Wes]

Thanks Bob. I will try it out like you say.

Do you apply adhesive to the coverslip or thats only for arranged diatoms, not strews?

And while we are an on the topic may I also ask if you ring your slides? And if so what ringing cement are you using?

[MicroBob]

With diatom arranging there three main substances:
- glue layer for ridgid fixation (in my case thin shellac, melts at about 60°C)
- sticky layer for temporary fixation (in my case skin fat)
- mountant (in my case Aroclor, don't worry about PCB content, it is pure PCB )

These three have to work together. The mountant has to solve the sticky layer but must not solve the glue layer. Other combinations are possible.

For strew slides just
cover slip
diatoms
Iprop (Pleurax-solvent, faster penetration)
Pleurax

The diatoms can be applied as a suspension in water. For absolutely clean material (acid cleaned without mercy) it can just be shoveled on and spreads nicely when the mountant in heated.
Here a beginners coures in diatom mounting: http://www.mikrohamburg.de/Programm/Pro ... 170218.pdf

Ringing cement: I have used black ALDI rust protection paint with good success. It should be a paint that resists the lighter fluid or brake cleaner that I use to remove immersion oil. From one of our cemists I got two ringing cements after an original microscopy recipe but so far have not had good success with it. It is not sure that Pleurax has to be ringed but for long term storage it is safer. I have other slides about 20 years old mounted probably with Naphrax that start to deteriorate. .

Bob

[Wes]

Many thanks Bob! I have taken your advice on board and finally I stopped having crushed diatoms in the mount.