In that case, do you think incineration helped cleaning at all? In other words, if you clean the same sample without the first incineration step, would cleaning be less complete?
No, I think that incineration did not contribute significantly to the cleaning. But, your rational was that incineration will save most of the expensive chemical reagents used for further cleaning, and I think that this point may still be valid.
In my opinion this goes as follows. The H2O2 you add is a volume of liquid and you add an excess, like 2:1 or 5:1 (or any other ratio) to the VOLUME of crude unclean diatoms. The excess is important, since you start with a strong reagent (30-35% as marketed) and you want to preserve the strength, not dilute it.
But that crude volume of diatoms and debris, although to the eye it is an opaque "concentrated" suspension, may actually contain just a small amount of solids. So, a relatively large volume of unwanted water.
Hence, to save on reagents, we should reduce our initial crude mass of diatoms and debris into as smaller volume, by removing the water. To do this, I see three options, and each means labor:
1. Rapid centrifugation
. I think we object to it. Combined together, literature data and forum member opinions about it are quite inconclusive. So forget it.
2. Filtration through highly dense paper or membrane filter, under vacuum even
. This is too time consuming and/or nuisance.
3. Heating to evaporate all the liquid
. The idea just jumped into my mind, and is relatively simple to do.
A primary step of incineration of the mass, in fact, covers option 3 and beyond
- get rid of redundant water as well as organic residues, so reduce the initial crude material to the absolute minimum volume before chemical treatment. Again, in my opinion, I think it is still worth testing, if one avoids the softening and deformation of the diatoms. So I hope to try it yet.
And the "incineration" we are doing here is different than that with a torch. Maybe torch would incinerate better and more evenly? I don't know if it will blow away diatoms though, if not controlled well. Maybe slightly higher temperature than 400C for a shorter time would actually work better than lower temp and longer time? I am only guessing.
Yes, certainly different from what I did, like you say, a higher temperature and shorter duration. A torch may prove useful. However, I do not know how they applied the torch to the diatoms. My own experience (years ago) with directly heating coverslips in a gas burner or an alcohol flame was negative.The slips cracked, or spattered, or melted. The steel plate that I used above (the dimensions are borrowed from forum member MicroBob's protocol
- thanks Bob!) prevents such problems, and at least, yields undamaged coverslips, and is fairly reproducible.
Using a torch on diatoms in a deep container, such as crucible or Pyrex bowl or beaker might work, but for me at least, it is too much trial and error (duration, distance from flame, container type etc).
BTW, a longer time (say double) and higher temperature (say by 10C) often yield the same result; not always. To melt a solid, one needs to heat to the melting point, and heating below melting point will not cause melting even if done through ages. But to burn an organic residue of a plant, time and temperature might be interchangeable.
Black powder does not sound as good as white powder.
Perhaps this is not important, as long as it is a powder and not big lumps of tarry stuff.
Your incineration + chemical cleaning looks about as good as it can get with your mild reagents. I would add HCL abd 30% H2O2 after incineration, but I know/understand you want to avoid them.
The tiny mineral particles you have there may be very difficult to remove, without micromanipulation. They are probably only slightly denser than diatom frustules.
So, I plan to collect some fresh algae (without silt) and start again, at a lower temperature. I will do the same as above, only lower the stove flame to medium power. We shall see... and perhaps other people will find it interesting...
P.S. The temperature on the coverslip is lower than the temperature on the metal plate, since glass, thin as it is, is an insulator. can be a difference of 30C and more. It does not change the conclusions though. In between the 1:0, 1:1 and 2:1 scores from Moscow, I managed to incinerate the black residue, created on the heated coverslip, some more on the bare plate. It remained mostly black, supporting the guess that it is mainly iron oxides.