Silica gel is mis-named, as it is NOT a gel. Silica gel is simply
silicic anhydride, SiO2. Most of us are familiar with those little packets that come with cameras and high end electronics. How much can it absorb? Twenty (20) percent of its own weight. One pound of silica gel can absorb 3.2 ounces of
water from the atmosphere.
Those familiar packets we know in the US, that turn from blue to pink once they have absorbed their maximum water, have been outlawed in the European Union.
Here is a cut/paste from an article on long term seed saving @
http://www.seedcontainers.net/a_guide_to_long-term_seed_preservation.html QUOTE:
The colour usually shown by silica gel is due to an indicator,
added to see directly when it is dehydrated and when it has absorbed moisture.
For many years, cobalt chloride (Cl2Co) has been used.
This substance gives the dehydrated gel a strong blue colour and a pale pink colour
to the gel having absorbed moisture. Recently, the European Union banned its use
because of considering it carcinogen through inhalation. A search for
new alternatives
led to some iron salts, where the change in colour can be poorly
distinguished. At present, the most advisable alternative is methyl violet, which
gives the dehydrated gel an orange colour and a green colour to the hydrated gel.
END QUOTE
As a side note, silica gel will also absorb some toxins from the atmosphere. Given the reputed carcinogetic nature of the readily available form in the US, I consider it a "trade-off". I have NOT researched price/availability of the methyl violet version in the US, but would suggest that you do so if you want to go this route for home use.
For reference, the "green" color they refer to looks more like olive drab to me.
Here are views of (L) untreated, (next 2) the type common in US, and (right2) the euro version.
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