Since I started making bath bombs 18 years ago I've been using pretty much the same recipe, its very basic and I guess its kinda old school. You can easily use this recipe as a jumping off point if you want to make different variations but I would say all bath bomb recipes start out with these ingredients.
Bath Bomb Recipe
2 cups of baking soda
1 tsp of sweet almond oil
1 tsp of Fragrance or Essential oil
Witch Hazel in a fine mist sprayer
Mix the fist 4 ingredients really well then start spritzing with the Witch Hazel. Mix as you spritz, you are going for a slightly damp sand kind of feel. When you can squeeze some of the mixture in your hand and not have it fall apart you can start packing it in to whatever you are using for a mold. You can also shape it by hand into mini snowball shapes but most people like to use a mold.
You should be able to pop the bath bomb out of the mold right after you pack it. I have used one single mold for a batch of 18 bath bombs ( that was a double batch), they are pictured below and made with a sand toy.
And below are some bath bombs made in silicone ice cube trays. The fishy ones are actually toilet fresheners which are a type of bath bomb.
The recipe is here.
Round bath bombs are usually made in two part plastic Xmas ornaments and you can also buy metal ones specifically for bath bombs too.
I also like to use Milky Way soap molds, below are bath bombs made in a Milky Way guest mold, I have some of these for sale and I really like the size for baskets and bath packages.
Since these are smaller I get 12 to 15 out of a batch. For average sized bombs you well get around 6.
Now for a few notes. The sweet almond oil adds a bit of moisture to the bath bomb and a bit of oil to the bath. If you absolutely hate oil in your bath, leave it out. I've used a little grated coco butter in place of the almond oil and that's really nice too. Some fragrance & essential oils are stronger than others and some people like more or less scent so the tsp of fragrance or essential oil is just a guide.
I will say if you are making smaller sized bath bombs, like ice cube sized, and you are only using one per bath you will probably want to add a little extra fragrance.
When I colour my bath bombs I tend to use a powdered colourant and mix it well with the dry ingredients. The little trees above were sprinkled with some cosmetic glitter after they were all unmolded and the glitter was set with a spritz of rubbing alcohol. The star fish bath bombs below were coloured gold for Christmas. I put each bath bomb in a shoe box and gave it a light spray with some rubbing alcohol that had a pinch of gold mica added to it. You have to shake the bottle to mix the mica with the alcohol.
I have used alcohol in place of witch hazel when making my bath bombs, the scent of the alcohol will evaporate but I prefer witch hazel which has almost no scent at all.
Here's a link to an old post with some recipes for bath bombs with powdered milk. Sorry there's no pictures, its a repost from a really old newsletter:)
Anyway, hope this helps anyone who is new to making bath bombs!