I have tried three desiccants: silica gel, anhydrous calcium chloride (DampRid), and anhydrous calcium sulfate (Drierite). Here's what I found:
One can find silica gel in some larger craft stores for flower drying. It costs between $10-$20 for 5 pounds of the flower-drying-type silica gel. Silica gel will bring down the relative humidity of a closed container to about 40% while it is doing its job, and is a fairly good desiccant.
The white crystals are simple silica beads so are no more toxic than beach
sand. One notable exception is the dust of broken beads, which is harmful to
the lungs (like asbestos fibers). It should be noted, however, that a very small
percentage of the crystals are of the indicating type and contain up to 1% cobalt
here (PDF) for a material safety data sheet for indicator silica gel
crystals). This bad stuff is a heavy metal salt and is toxic and carcinogenic.
It is there to turn these crystals blue when they are active (i.e. dry) and
light pink when they need to be recharged.
The beads are not soluble in water, so they will not dissolve in your mushrooms like, for example, DampRid (calcium chloride). Never the less, don't let your mushrooms touch the tiny crystals because they tend to get stuck in the mushrooms, and the blue ones in particular are not good to eat (heavy metals are bad).
When the indicating crystals turn from blue to light pink, the crystals need to be recharged. To recharge them, place the beads in a shallow pan in an oven at 250°F for about five hours. Any higher heat than 250°F will cause excessive beads to fracture and turn to silica dust which is, as I mentioned above, bad for the respiratory system.
Anhydrous calcium chloride (the active ingredient in DampRid)
is a slightly stronger desiccant. It is also cheaper. Locally, I can find this
stuff at ACE hardware in the economy refill size of 48oz (3lb) for $3. I find
that it will often bring down the relative humidity of the inside of a closed
container with mushrooms to 25% or even less while it does its job.
On the down side, this desiccant is very hard, if not impossible, to re-use. It tends to dissolve in the water it absorbs from the air, so when you try to dry it out, it often dries to useless, large chunks rather than to its original form of small beads. Also, it is highly soluble in water so that even if you place your mushrooms on paper covering it, it will still find a way to get in your mushrooms. On the other hand, it is not toxic like the silica gel indicator crystals. Still, if you have the unfortunate experience of eating some DampRid, it tastes like shit and will make you as thirsty as hell (will set the stage for a really bad trip).
The last type of desiccant I tried is anhydrous calcium sulfate (Drierite). Drierite is quite expensive ($10 a pound for indicating and about $7 for non-indicating). It comes in indicating which, like the indicating silica gel beads, turn blue when dry and pink when it needs a recharge. It also comes in a number of rock sizes, but I have no idea which size is best for drying mushrooms. The grade I used was about like aquarium gravel.
I have not looked over the toxicity of Drierite (Click
here for a material safety data sheet for Drierite), but if I had to
guess, I'd guess the indicator is the toxic cobalt chloride salt. I definitely
keep it away from the mushrooms. I don't have any humidity readings on this
desiccant either, but I found it to be about average strength. Again, once the
indicating rocks (and you should buy at least some of the indicating type) turn
from blue to pink, it is time to recharge. To recharge this desiccant, you place
it in a 400°F oven for two hours.
I liked the fact that Drierite didn't turn to goo like DampRid and can be recharged, but it was just too damned expensive for my tastes. I used it a few times, but wasn't too impressed considering the price.
In general, the key to using any desiccant is to first fan
dry the mushrooms. Buy a cheap fan, and place the mushrooms on the
grill of the fan so air blows through the pile for 24 to 48 hours. If you live
in a dry climate (40% relative humidity or lower) your mushrooms will dry completely
out using this technique and you won't even need a desiccant. If you aren't
that lucky, you should continue the drying process using a desiccant. Just place
the mushrooms on a screen right above the desiccant of choice in an airtight
container. If you want to push your desiccant to warp speed (several times faster),
use a tiny fan on low speed inside the airtight drying container to gently
circulate the air