Bring together all of your dry ingredients in a mixing bowl, 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.
You can go ahead and mix your liquid ingredients as well, including any coloring agent you’re planning to use.
Working slowly so as not to disturb the bicarbonate of soda too much, add your liquid to the dry mixture 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 mixture, 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 bath bombs. 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 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 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 them to harden for 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 bath treats 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 oil, fragrance oil or color dye you like, although I do think almond oil works the best for this recipe.