Anyhoo - on to fizzing bath cubes. The recipe I ended up using was pretty simple (no cornstarch, epsom salts, and only four ingrediants) but I took a bath with a couple cubes the other day and it was heaven. The magical ratio is 2:1 (baking soda to citric acid).
To make 12 cubes I used 1 c citric acid, 2 c baking soda, 1 tsp sweet almond oil, and fragrance oil to my own preference (approx 6 ml). I mixed the dry ingrediants, added the oils, and then spritzed the mixture with witch hazel (in a 3 oz spray bottle) while mixing with the other. You want the dry ingrediants to only get damp enough to barely stick together when squished in your hand (the less witch hazel you use the better, otherwise you could end up with a fizzing blobby mess).
Once your bath fizzy mixture is ready, pour it into a mold (or in my case, a silicone ice cube tray) and compact, compact, compact with your fingers - then try to compact even more of the mixture on top. The more you compact, the longer your cube will fizz in the bath.
Let your bath fizzies sit in the mold for a few minutes, then lightly tap out. Let dry for 24hrs.
Note: if you don't have fragrance oil, use a scented bath oil (3tsp) instead of the sweet almond oil + fragrance oil. Also, just the combo of 1tsp of oil + the fragrance oil made for a pretty slippery bath, but if you want more moisturizing - you can always use shea butter or up the oil content. Also - you can either buy citric acid online, or in a store that sells canning supplies.
Grass scented fizzing bath cubes