I did say " think."
And thank you all, for your thoughts.
I last flew gliders in the 1950s when the most advanced available to me were the Olympia / N2000 / Weihe. ( All three had been made by different makers to much the same design IIRC. Some would creak a little to remind one that the LIFT had altered. Would this now be called " a feature " in the Sales literature, I wonder!)
I saw one other at that time, which had an early jettisonable water ballast for flying at higher speeds between one strong thermal to get to the next one, perhaps earlier in the day. It was something like an enclosed water tank, I think.
The thought of actually PUTTING moisture inside a wooden wing, perhaps melting some of the glued structure... Would have been avoided, then.