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[klaus]

I frequently receive mics where a small silica gel or silica crystal bag is tucked into the mic box.
The idea: to let the hygroscopic (moisture seeking) silica desiccate excess humidity from the mic, especially from the capsule, whose performance is greatly affected by humidity.

But simple logic dictates that there are only so many moisture-starved silica crystals in these tiny pouches, and, unless you severely restrict the amount of humid air getting in contact with the silica, the effect will be essentially over, once the crystals have sucked enough humidity to neutralize their ability to suck more - about 2-4 hours.

Only two types of active management of the process can make the idea work:

Severe restriction of the amount of air the silica bags are exposed to

Suppose you have a fully charged (i.e. moisture-starved) silica pouch of big enough size to be effective (at least 3-4 inches square), place the pouch against the mic’s head basket, then wrap a sealable plastic bag tightly around the mic (a rubber band can assist in sealing).
This limits the air available to the silica crystals.

How do we know when the silica pouches have neutralized? Silica crystals usually have a chemical added that will change the crystals’ color once the chemical comes in contract with humidity and the crystals are moisture-saturated.
Usually, dry crystals are dark blue, or another strong color, and neutralized crystals (those which can no longer absorb additional moisture) turn towards neutral or slight pink or brownish.

Reconstituting neutralized bags

Even with most careful restriction of air to the crystals, they will neutralize eventually, requiring reconstitution:
Place the pouches in a 350º heat oven for several hours*. I use a toaster oven, to limit my electrical bill. Once all the humidity has been baked out of the crystals, they are ready to work again. If you don’t have an immediate use for them, place them in small canning jars, barely big enough to fit (again, the idea is to limit exposure to air), until ready to deploy.

* Make sure that the silica pouches are of the type that can be reconstituted. Some small ones are one-time only (the tiny ones you find in pill boxes and camera cases). The larger, rechargeable ones usually have instructions on the pouch how to re-dry them.

[Kai]

There are cheap household zipper plastic bags available, made for frosting food.
They serve very good for airtight seal storage.

[Jim Williams]

Turkey basting bags seal very well. Those don't let any moisture or any smell out.

[ratite]

My concern has always been can silica dust get out of the pores in the bags and is it electrostatic?Mind you it's not stopped me using them,though I wont wrap  them right over the basket.

[klaus]

Good point.
What I should do is rub or shake one of the bags hard over a black piece of paper and see what comes out. (Or maybe someone else can do this, so I can work on microphones today?)

[Noah Scot Snyder]

I use the metal cased rechargeable desiccants with success. The style I use starts orange and turns clear as it absorbs water. Once it is clear you toss it in a 300 degree oven for 3 hours and it dries out ready to be used again. One lives in each of my well sealed mic cases (the style often used for camera equipment). 

[DarinK]

What is the best humidity range for microphones?

There are de-humidifying cabinets, made for camera equipment and selling for a couple hundred dollars, that could be used for microphone storage. I don't live in a humid enough area to ever really need something like that, but they could probably come in handy in studios in humid environments, like in Florida or Southeast Asia.
With an air-tight dehumidified cabinet, you wouldn't need to bother with plastic bags and desiccants every time you used a mic. For those with very expensive microphone collections (not me, sadly), a couple/few hundred dollars for such a cabinet may not seem that big of an expense.

[klaus]

Darin,
All high-end studio condenser mics are resistant to moisture back-down in high humidity environments, as long as the capsule surfaces are clean.

A contaminated capsule in high humidity behaves differently: when the the level of contaminants (often specks of dust or a film of dried saliva) is dense enough it will form a conductive path between diaphragms and backplate, as soon as enough moisture is present.

The conductive path between the two capacitor plates discharges the plates, and the mic becomes noisy.

[DarinK]

My question is, if desiccant packs are useful for mic storage, why not instead use a dehumidifying cabinet for storage? And if such a cabinet were to be used, for what humidity range should it be set?

[klaus]

The opening sentence in my reply suggests that a dehumidifying cabinet or room for microphones is overkill. We have enough data from more than 50 years of condenser mic use all over the world to confirm this.

As a matter of fact, the original AKG C12 was specified to be impervious to even high levels of humidity, as its primary market initially was South America.