Just gunna get me a dri- AAAH!

[Doc Ezra]

As soon as somebody figures out what was in Greek fire, I imagine we'll have an answer (it's really all that alchemist's fire is, only without the real-world historical name). Current theory is that it was a mixture of petroleum, naptha, and quicklime, with another "secret ingredient" that nobody really knows.

And they carried it on ships in sealed casks, ejecting it from large bronze tubes onto other boats. It burst into flames when exposed to air at the end of the tube. Basically a primitive flamethrower that self-ignites. Very cool stuff.

 

[Steverooo]

Secret Ingredients

Sodium, dissolved in kerosine?

 

[isoChron]

Take Potassium for better effect

 

[dinsdale]

As suggested above, under water or oil is probably the most probable and historical answer. For example, float a layer of oil over one ingredient (or the solvent, etc) then carefully drop the other ingredients through the oil layer to the bottom.

Under an inert atmosphere is another option, as it is likely not truly "air" that causes the problem but oxygen. There are all sorts of reactions you can use to produce an inert atmosphere. The easiest would probably be to just burn candles in a box. That will pretty much convert the O2 to CO2 which would (hopefully) not react with the alchemists fire.

(is my chemistry background showing yet?)