I know i'm new on here but have been making bath bombs for a couple of years now and just wanted to add this.
So many times i have read that people can't dry their bath bombs because of humidty and the fact they have oil in them.
i have a no fail recipe that i use and i have never had a problem in making bath bombs, if the humidity is high i set the oven on low and put them on a baking sheet in the oven for a couple of hours.
my recipe is
2 parts sodium bicarbonate
1 part citric acid
1/2 part of cornflower
1/2 part epsom salts (although sometimes i leave them out)
seive the above together and mix well stir and stir again, add a few drops of color to get the desired color you want, then add your fo or eo,
blend and blend and blend
then get a spritzer bottle full of witch hazel spray the mix while stirring with the other hand, depending on the size of your batch you will need to spray until it clumps together and molds in your hands without falling apart,
put loosely into molds and leave a slight mound on each one, push them together and leave in molds until you have finished molding the whole batch, then gently tap them out. lay then on baking sheet to dry and spritz the outside with witch hazel, after an hour turn them over and spritz the other side, hey presto one batch of hard bath bombs that fizz for around 2-3 minutes in the bath and don't even break when you drop one from table height onto a hard floor as i did the other day.
if humid then before you begin simply turn the heat on your oven on the lowest setting possible and let it warm up, make you bath bombs, when ready turn the oven off and pop them in the oven for an hour to two to dry, then spritz gently with witch hazel and again leave to dry.
i have to say everytime i've tried to use an oil or butter in the mix it always fails to harden.