I think definately the packing is too fine. Recommended packed column support mesh size is 80-100, and my alumina has the (very scientific) consistency of potato starch - feels much finer than 100 mesh!
Sand is a possibility. some inorganic salts might do it too (moisture sensitive!)
but they're both straying further from usable and closer towards classroom demonstration. I'd like to be able to make decent columns.
Diatomaceous earth is definitely ideal, used commercially too. It's just a bit harder to find for sale for laypeople, but I have seen health shops selling it online.
Either way I'm gonna buy some 80 and 100 mesh test sieves and make a little shaker so I get good particle size control. It's a fairly easy variable to take out of the picture.
The first support coating i just shook the support with a bit of mineral oil. Now i've adapted a method outlined in ASTM E260:
https://archive.org/details/gov.law.astm.e260.1996/page/n1it's a pretty good reference for packed GC, actually. Basically my method is:
1. 1g mineral oil dissolved in 50 ml heptane(s)
2. addition of 9g support (Al2O3)
3. swirl gently and pull a vacuum on the mixture to draw adsorbed air out.
4. filter suspension on a coarse glass frit funnel under vacuum
5. gently transfer dry-ish phase to a dish and let dry in air at slightly elevated temp.
This should give a max 10% loading, probably much less. It's adviced to extract a known amount of the finished support's phase to determine actual loading but I don't have the capacity for that.
Thank you all for your good advice! I'll look out for Supina's book!
--Chris