Complete disclosure: I've never used a premade bath bomb. I see them everywhere and they look really nice but they always seemed too expensive for a one-time use item and too big. I mean, that whole thing is supposed to dissolve in one bath? That didn't seem right.
So I thought it might be worth my while to try making some myself as a nice Valentine's gift to... myself. Obviously, I have no idea how proper bath bombs are supposed to work, but these fizz delightfully, add a very subtle tint and scent to the water, and soften my poor winter-dry legs amazingly well. So, for me, I think that's a successful bath bomb!
Probably the hardest part was finding citric acid. I eventually found mine at the pharmacy counter at Superstore. It was a special order but (thankfully) I only had to wait one day for it to arrive. You can buy citric acid in varying amounts but since I didn't know how much I'd use, I started with the smallest which was a 100 g bottle. This only cost me $3 and I made about 14 bath bombs with it (this includes my testers which bombed prior to making it to a bath). Not too shabby. If you can't find citric acid, you can use cream of tartar but it doesn't have the same reaction citric acid does (it still makes the bath water nice, though). Just use half of whatever amount the citric acid was (so if you need 1 tablespoon of citric acid, you'd use 1/2 tablespoon of cream of tartar).
Initially, I used an old cupcake container I had as a mould just to do some testing, but I realized I really liked the rustic/cute look it gave the bombs so I continued using it. If you were gifting these to someone special for Valentine's day, I did spot a ton of different heart-shaped containers at the dollar store that you could easily use as a mould for bath bombs.
What you'll need to make one bath bomb (or two like mine):
2 tablespoons baking soda
1 tablespoon of citric acid (or 1/2 tablespoon cream of tartar)
1 tablespoon of cornstarch
1 tablespoon epsom salts (you can find this in any store these days)
1/4 teaspoon oil (you can use any light oil you like. I actually used plain ol' canola since it's fragrance free and wouldn't compete with the liquid)
3/4 teaspoon liquid*
a drop or two of food colouring
Dump the dry ingredients (baking soda, acid, cornstarch, and salts) into a bowl and whisk together to remove lumps.
In a small jar, shake together the wet ingredients (oil, liquid, and colouring).
*About the liquid -- most recipes I tried had a bit of water and a bit of essential oil but I stopped bothering with water altogether. For the green tea and cinnamon bombs, I brewed a very strong, very concentrated cup of both green tea and cinnamon tea and I used that instead of water or essential oil. For the coconut and vanilla bombs, I used straight up vanilla and coconut extract as the only liquid (my coconut extract was older and had mysteriously changed colour so it was a good way to get rid of it). The tea and the extracts aren't as strong as essential oils would be, but the smell is kind of a small factor for me. I can smell them in their dry state, but I'm really just liking the way the bath bombs soften bath water. I don't necessarily need my bath water to smell all fancy, too.*
While whisking, dump the wet ingredients into the dry mixture. You'll see a slight reaction (if you're using citric acid), but keep on whisking until the mixture has started to clump together and is completely dyed the colour you added. It should still be a little crumbly which is what you want. If you add any more liquid, the citric acid will start to foam and you'll end up with a pre-bombed bomb (I got a few of those and your little bath bomb will keep foaming and growing right out of your mould until it completely dries).
Spoon the mixture into your mould and pack it in as tight as you can. Allow to dry for at least 5 hours before trying to get them out of the mould. Let dry for another 4 hours before plopping one in the bath or let them dry another 1-2 days before wrapping them as gifts or putting them into a storage jar, like so:
This is a jam jar that I fancied up by painting the lid with some gold paint. I liked this storage idea the most because it has the rustic charm that matches that of the bath bombs plus it'll keep them nice and dry in the bathroom (and it was just as easy as making the bath bombs themselves!).