thanks for the replies. Humidity in the workshop is lowish. The paper envelopes are kept there, strings are in a sealed bag with silica bags, and the bag kept in a drawer. I never thought the strings would corrode in an envelope?? Some strings I prepared earlier in january are quite corroded...
In the near future I will prepare lots of spares for an institute First Aid Kit, and wonder how to arrange it to keep the strings fresh. I will pass the strings ,once looped, with a bit of greasy sheepwool for better protection. But I need an alternative to the paper envelopes, obviously.

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