In a large bowl, combine the Epsom salt, baking soda, cornstarch and citric acid.
Mix well with a whisk, making sure to remove any clumps.
Pour the olive oil, water, fragrance oil and soap colorant into a jar. Secure the lid and shake well to form the colour.
Very slowly add the liquid mixture to the dry mixture. Add just a few drops at a time, while continuously whisking. If you’re not pouring slowly enough and whisking while pouring, the mixture will start to fizz and the bath bombs won’t form correctly.
Once all of the liquid has been poured into the dry mixture, mix very well until the colour is consistent.
Add the rose petals and give the mixture a quick stir.
Immediately fill the molds with the mixture, firmly stuffing each side full.
Secure both parts of the mold together and let set. I always give it about a day, just to be on the safe side.
Carefully release bath bombs from molds and put into an airtight container, or individual cello bags.