Hi! I know this is an old post but this may end up helping someone new to this issue (like me!). Humidity where I live (Los Angeles area but on the coast) has been around 80% or more lately, and my bath bombs are are reacting and turning into goopy or ugly messes (they still work great, though). I don't have AC and don't intend to get, and I've only been making small batches, but this is what I've found works:
-MINIMAL liquid--mixture shouldn't even really feel damp at all, and NO water--use witch hazel from a spray bottle
-Reducing by half or eliminating epsom salt
-Use smaller molds
-Unmold almost immediately and let dry in a large paper bag (like a grocery store bag) with the top folded over and clipped. This seems to absorb the humidity and they come out great and are hard enough to handle after 1 hour and fully hard overnight.
This is my recipe, I have not tried to make larger batches, and I am using small molds--I get 3-5 mini bombs depending on the mold:
1/2 C. Baking Soda
1/4 C. Corn Starch
1/4 C. Citric Acid
1/8 C. Epsom Salt
2 t. Dried Flowers (optional)
3 t. Carrier Oil
15 Drops Essential Oil
4-5 Spritzes Witch Hazel with 14% Alcohol
I add the carrier oil mixed with essential oil slowly with a whisk and then add the witch hazel a spritz at a time, whisking in between. It feels pretty light and dry when I put in the mold.
Edited: The total amount of witch hazel from the spritzes measures ~1 t.