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Bring together all of your dry ingredients in a mixing bowl, (baking soda, corn starch, Epsom salts) excluding your citric acid. We want to leave the citric acid until later, as adding it now can complicate things during the liquid pour step, just trust me.
In a seperate container, you can go ahead and mix your wet ingredients as well, including any food color or soap dye you’re planning to use.
Working slowly so as not to disturb the baking soda too much, add your wet ingredients to the dry ingredients a bit at a time and mixing thoroughly with your hands. (Adding your dye to the Epsom salts rather than your liquids is an interesting alternative worth trying at some point. Just mix thoroughly before adding the salts to the rest of your dry ingredients).
As you work the bath bomb ingredients, you’re looking for a texture that is similar to that of damp, but no saturated, sand. You’ll notice that the mixture will begin to stick together at this stage, a bit like wet sand does in your sandcastle bucket.
Once all of your liquid has been added and you’ve mixed thoroughly, add your citric acid and mix once more. It’s okay to use your hands but as citric acid can cause irritation and burns, make sure you’re using gloves.
Now to get that orb-like shape we associate with a bath bomb. Take both halves of your mold and pack them with your mixture. You want it ever so slightly heaped so that your halves stick together well.
When you’re ready, press the two halves together tightly so that the mixture bonds into an orb. After a few seconds, you should be able to remove one half of the mold.
Lastly, gently set the exposed half onto a silicon cupcake case. You don’t have to use a cupcake case, some people use muffin tins for this, but I find it’s the best thing to rest these on without them falling apart. Hopefully, it should just pop out of your bath bomb mold, but if it doesn’t gently tap it to try and loosen it up. If this becomes a regular issue, take a look at how much liquid you’ve used. Also try working a bit quicker and don’t leave the mixture in the mold for more than a few seconds.
You can use plastic bath bomb molds but prefer to use metal molds, and haven’t had a problem with them. If you’ve had problems with a mold made from a particular material (say plastic) let me know how it turned out. Otherwise, stick to metal. You'll then want to leave your bath bombs to dry for at least 24 hours, but usualy a couple of days. Pop them somewhere they won't be disturbed easily, just reduce the chance of dents and imperfections.
Ingredients in these quantities should get you anywhere from 8-10 DIY bath bombs depending on the size of your molds. Just keep going until you've used up your ingredients, you'll find uses for them I'm sure. You can use whatever essential oils, fragrance oils, food color, or soap dye you like, although I do think almond works the best for bath bombs.