Welcome to our first DIY (and theme!) for the 2025 Humblebee & Me holiday making season! We’re kicking things off with some bath bombs—a must-make for me every Christmas because my mom loves them (hi, Mom!).

All the way back in 2016 I shared a couple “Lumps of Coal” themed DIYs (a soap and lip balm), and I thought it was high time I revisited this fun theme with some more formulations and nearly an extra decade of DIY know-how.

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Everything about the idea of “lumps of coal” makes me think of free-standing DIYs, so bath bombs were a must. My 2016 Christmas bath bombs were part of the other theme I did that year (Winter Wonderland), and I’ve got a decent guess as to why: 2016 me didn’t have any water soluble dyes, so the idea of making black bath bombs was… bad.

Those Winter Wonderland bath bombs used powdered indigo root or blue ultramarine to colour them, and other bath bombs I shared around that time were coloured with micas. While easy to get, there’s a big reason micas and ultramarines aren’t the best way to colour bath bombs and other bath things: because they’re insoluble, they have a tendency to leave a messy ring in the tub. Ahh, there’s nothing like taking a lovely, relaxing bath and then scrubbing a dingy ring out of your tub right afterwards. Bath bombs seem like such a no-brainer for this theme idea that I’m guessing 2016 Marie decided to spare herself, her friends, and her family that mess. (Activated charcoal is insoluble, too, so also not a great choice.)

Now, though, I’ve got water soluble dyes! Water soluble dyes are awesome for bath bombs and all sorts of bath treats. If you like making colourful bath goodies and don’t have a selection of water soluble dyes, I can’t recommend them highly enough. You only need the tiniest amount to colour both your bath bombs and your bath water—and because they fully dissolve in water, they wash down the drain easily. Hooray! They can and will stain skin, fabric, and anything else porous if given the chance, so keep usage levels low and wear gloves when working with the dry, concentrated powder or you’ll have colourful hands for a few days.

I used Acid Black 2; just 0.03% is enough to make the bath bombs charcoal and dye your bath water a soft lavender colour. Weigh it out carefully and resist the temptation to use more unless you’re willing to chance emerging from your bath with a plummy tint to your skin!

You’ve got room to play on the scent front. What do you think a lump of coal should smell like? Surely not like actual coal, ha. Something else that’s black—like anise/liquorice? Vanilla and anise? Something warm and deep? As I was planning out this year’s holiday DIYs the folks over at Bulk Apothecary reached out, so I told them what I was working on and asked what they thought would be a good fit. They sent me a couple gorgeous fragrance oil options for both of this year’s themes (stay tuned for the second!), and they knocked it out of the park. The coal-y selections were all rich, warm, and intoxicating—the sort of swoon-worthy scents that make me think of a low, crackling fire in a cozy library lined with mahogany book shelves and plush button-back chairs.

For this formulation, I chose Bibliothèque; it’s deep, rich, warm, and the name is also French for “library”, which probably helped sway me in that direction. This fragrance is Bulk Apothecary’s version of Byredo’s Bibliothèque scent; I’m not familiar with the original, and since 50mL (1.69fl oz) of the perfume is an eye-watering $310CAD, I doubt I ever will be! Please choose a scent that strikes your fancy—just be sure whatever you use is safe for use at 0.75% in IFRA Category 9 formulations (products with body and hand exposure, primarily rinse-off). If you aren’t sure how to do that, please read this blog post.

I used a 1.5″ spherical mould to shape these bath bombs by hand, but you could also use a press or a cavity mould. In my experience, spherical moulds are the trickiest way to shape bath bombs, but once you’ve got it down it’s like riding a bike. I typically only make bath bombs once a year these days, but once I’ve got those two half-domes in my hands, it comes right back to me. A good formulation is also essential; getting a spherical mould is what got Cream of Tartar a permanent home in my bath bomb formulations as it makes them way easier to mould (and much harder when they’re dry). So, if you’ve struggled with making round bath bombs in the past, try this formulation and watch the full video tutorial for a more in-depth how-to!

After bath bombs are shaped they need to dry so they fuse together. I used a single wire mesh drawer from a wheely drawer unit I’ve got in my studio, but any flat-bottomed, shallow box will do (I’ve used leftover containers in the past). If you’re making round bath bombs, I definitely recommend something with walls/sides to reduce the chances of any roll-y mishaps!

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Relevant links & further reading

• Baking Soda in the Humblebee & Me Encyclopedia
• Witch Hazel in the Humblebee & Me Encyclopedia
• Polysorbate 80 in the Humblebee & Me Encyclopedia
• Apricot Kernel Oil in the Humblebee & Me Encyclopedia
• Citric Acid in the Humblebee & Me Encyclopedia
• More bath bombs:
  • Frosted Cranberry Bath Bombs
  • Wedding Cake Bath Bombs
  • Ice Palace Bath Bombs
  • Argan Rose Valentine Bath Bombs
  • Christmas Tree Bath Bombs
  • Vinolia Lemon Rose Bath Bombs
  • Lavender Spruce Bath Bombs
  • Milky Tea Bath Bombs

Lumps of Coal Christmas Bath Bombs

Basic phase
530g | 53% baking soda (USA / Canada)
96g | 9.6% Epsom salt (USA / Canada)

Wet phase
5.2g | 0.52% witch hazel distillate (USA / Canada)
0.3g | 0.03% black water soluble dye
16g | 1.6% Polysorbate 80 (USA / Canada)
25g | 2.5% apricot kernel oil (USA / Canada)
7.5g | 0.75% fragrance or essential oil

Acidic phase
270g | 27% citric acid (USA / Canada)
50g | 5% Cream of Tartar (USA / Canada)

To mould: 70% isopropyl alcohol or witch hazel in a mister bottle

Put on a pair of nitrile gloves.

Weigh the dye and witch hazel into a small beaker and stir to dissolve the dye. Then, weigh in the Polysorbate 80, apricot kernel oil, and fragrance/essential oil into a small dish or beaker and stir to combine.

Weigh the baking soda and Epsom salts into a large bowl and stir to combine. Add the witch hazel mixture and use your gloved hands to blend the mixture thoroughly.

Last up, weigh in the citric acid and Cream of Tartar, and blend that into the mixture as well. When you’re done you should have a clumpy mixture that holds together well when squeezed firmly.

Now you’re ready to mould! You can press these bath bombs with a press (I love my Bath Bomb Press!) or hand-mould them. I opted to hand-mould this batch using a spherical mould; please watch the video for full instructions! If the bath bombs start to become finicky as you work, that’s because the mix is starting to dry out—mist in just enough 70% isopropyl alcohol or witch hazel that they become workable again, and carry on.

Once the bath bombs have been moulded, leave them to dry overnight .

That’s it!

To use the bath bombs, just drop one or two into a hot bath and enjoy.

Shelf Life & Storage

Because these bath bombs don’t contain any water once they dry, they do not require a broad-spectrum preservative (broad spectrum preservatives ward off microbial growth, and microbes require water to live—no water, no microbes!). Be sure to keep them dry to ensure they last as long as possible—don’t let any water get into the container/bag you store them in and they should easily last a year.

Substitutions

As always, be aware that making substitutions will change the final product. While these swaps won’t break the recipe, you will get a different final product than I did.

• As I’ve provided this recipe in percentages as well as grams you can easily calculate it to any size using a simple spreadsheet as I’ve explained in this post. As written in grams this recipe will make 1000g (1kg!) of bath bombs.
• To learn more about the ingredients used in this formulation, including why they’re included and what you can substitute them with, please visit the Humblebee & Me Encyclopedia. It doesn’t have everything in it yet, but there’s lots of good information there! If I have not given a specific substitution suggestion in this list please look up the ingredient in the encyclopedia before asking.
• Don’t substitute the baking soda or citric acid.
• You could try a different salt in place of Epsom salts.
• You can try replacing the Cream of Tartar with more citric acid, but the Cream of Tartar makes the bath bombs much easier to mould and harder once they’re dry.
• You can try Poly Suga®Mulse D9 as a natural alternative to Polysorbate 80.
• You can substitute another lightweight oil like sweet almond, grapeseed, or sunflower seed oil instead of apricot kernel oil.
• The fragrance is up to you—what makes sense for a “coal” theme to you? You can use a bath-safe essential oil or any fragrance oil you like (always within IFRA guidelines, of course). If you aren’t sure how to check that, please read this blog post.
• To colour the bath bombs: stick with water-soluble dyes as they’re super potent at low concentrations and will dye your bath water without leaving a ring in the tub.