It is possible to have a bath bomb within a bath bomb. Called an embed. That is one way to get a bath bomb to spin. However the embed needs to be on one side of the bath bomb to offset the weight. Using cornstarch in the recipe tends to harden the bath bombs and they may last longer. Packing the mold very very tightly means a longer lasting bath bomb. Using water that is not too hot helps extend the in water life of the bath bomb. And I suspect but cannot prove that the type of salt used can affect the length of fizzing (Epsom vs. Himalayan vs other salt types.) Have noticed that Lush does not add salt to all their bath bombs and suspect that is due to the hygrooscopic nature of salt. No salt means no warts on bath bombs while they are drying down in a humid environment. Another thought is that fizzing time length could be the difference of using liquid oil vs an oil that has to be melted (coconut oil.). Have been making bath bombs since 2018. Switched to a new recipe that incorporates the liquid differently and there is no alcohol added. This recipe seems to make harder bombs. It adds the wet ingredients to the baking soda before adding the acidic ingredients. Much much easier way to make bath bombs. What to do with all the alcohol I have on hand? Am using it to decorate the bombs (mix isopropyl alcohol with mica.) Here is a link to the new recipe:
Winter Solstice Bath Bombs - Humblebee & Me