I've just started making bath bombs and I've decided to keep a record of my experimenting, so I can look back and see how I've developed my 'recipe'. It's very trial and error at the moment, with scents and colours, but also with the basic ingredients and how to put them together.
So for my first batch I put the basic dry ingredients together, and divided this into two, so I could try two different scents and colours. So I started with 600g bicarbonate of soda and 300g citric acid, sieved them into a bowl, and then sieved them back and forth between two bowls to make sure they were thoroughly mixed.
To half the mix I then added 1/2tsp of scent made up of 35 drops bergamot, 20 drops jasmine, 30 drops palmarosa and 5 drops ylang ylang. This makes a beautiful scent! I chose this combination for myself, as they are supposed to have theraputic qualities for people with stress and depression. I then added 1/4tsp of red liquid colour. The colour clumped terribly, so I sieved it through and kept rubbing the clumps, like you would with a crumble mix. If you look in the picture you will see the overall effect was very pale, but with dark flecks. I am not happy with this and will experiment.
This mix made 2 large hearts and 4 mini hearts (ice cube tray). I put rose petals in the base of the moulds before pressing the mixture in. The mixture has to be pressed firmly into the mould, and slightly overfilled if you are going to press to halves together. Then when you press the two halves together you need to use a slight grinding motion and a fair amount of pressure to get them to bind together effectively.
I think next time I would make the large hearts in only half the mould, instead of putting two halves together. These hearts seem excessively big, and this doesn't seem to match up with the delicate romance and prettiness of the scent and colour I was going for.
With the second half of the mix I again added a 1/2tsp of scent, this time made up of 25 drops vetiver, 40 drops bergamot and 5 drops ylang ylang. It's not my taste at all, but it's not for me! This blend was for my OH who gets muscle aches. This time I added 1/4 tsp of blue liquid colour, and again had the same problem with clumping, as you can see in the final product. These ones have some cone flower petals mixed in before putting in the moulds. This amount made 3 medium balls and 1 small ball, plus I made 2 or 3 hearts in the same ice cube tray mould.
They were just right in terms of consistency. I didn't measure the amount of water I used, just did it by feel, and this seemed to work well. They stuck together well, came out of the moulds easily, and retained their shape 48 hours later.