Hello! I have some frost essences in my inventory and was wondering what happens if the festival ends before a sculpture is finished? I don't want them to disappear and lose 5k gold but it says they'll expire on the 30th so should I use them or not?

there's still a little over a week before fest fully ends, which is enough time to get sculpts finished.

Once fest closes, Krin will look at the stats of the remaining unfinished sculpts and determine the parameters for which ones will be autoharvested and delivered to player's hatcheries

All fest-activity items such as left-over frost essences, ice waters, snow wars tokens, etc. will expire once fest ends (I beleive they might be auto-deleted after a month or two? but I don't keep the stuff around after fest so that's second hand info)