Sand is a real pain to get out (I know, I've tried it). You end up with a fine layer of sand in the bottom of your glass for ages afterwards and as a maker, it's not what you want.
You can also use rice but the starch that leaches out onto the damp leather makes it difficult to dislodge. Trial and error has definitely led me to using barley. You can buy barley quite cheaply in Sainsburys or Tesco in small packets next to the soup mixes. Barley is also historically accurate and is great if you ever end up doing a period demonstration of your craft work. From a flask making point of view there's no real difference in using barley and wheat grains.
Barley last a lot longer though as it isn't effected by moisture. I found wheat softens and sometimes sprouts after the third or fourth flask. The barley I'm using has been stored in a canvas bag for two years now and is just as good as the day I bought it. It'll still be good for making soup as well. If you want any info on my waxing techniques just ask.
I've found (again through trial and error) that it's necessary to get the leather completely impregnated with beeswax for archival permanency. Just coating the inside is not enough. It is total impregnation that makes the leather totally liquid proof and hard as nails too.
Eric
