Ash is an interesting idea. It should be readily available and renewable. I would replace it every now and then. I expect it would act less powdery once it had taken up some moisture.
Do be aware that silica gel must be re-dried every now and then because its capacity for taking up moisture is rather easily overloaded. Heating it in the oven for several hours should drive off the water (same for rice). I use anhydrous calcium sulfate. There is a small amount of indicator calcium sulfate in there as well. It turns from blue to pink when it has picked up water, so I know when the material is no longer picking up moisture. I find that even using Mason jars with lids and rings is not
that tight to moisture.
With any of the drying materials, you do want to refresh them (drive off the water they've adsorbed) on a regular basis, especially if you live in a humid area.
I've read about using the hammer method for testing for bean dryness. If the bean shatters when hit with a hammer, it's dry enough.
