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How to Make Bath Bombs
How to Make Bath Bombs That Hold Together and Fizz
A bath bomb is a pressed mix of baking soda and citric acid, about two parts baking soda to one part citric acid by weight, bound with a little oil and a light mist of liquid. When it hits bath water, the two powders react and release carbon dioxide, which is the fizz. The secret to a hard bomb that does not crumble or fizz in the bowl is weighing every ingredient, adding very little moisture, and pressing the mold firmly.
The master recipe below is written in grams and percentages, followed by color, scent, and shape variations and a troubleshooting guide.
What Each Ingredient Does
| Ingredient | Typical % | Job |
|---|---|---|
| Baking soda (sodium bicarbonate) | 45 to 55% | Base of the fizz reaction |
| Citric acid | 22 to 28% | Reacts with baking soda in water |
| Cornstarch or kaolin clay | 5 to 15% | Slows the fizz, gives a smooth surface |
| Epsom salt | 0 to 10% | Texture and a mineral-salt bath feel |
| Carrier oil or melted butter | 3 to 5% | Binds the powders and softens the water feel |
| Polysorbate 80 | 1% | Disperses oil, scent, and color into the water so they do not float or ring the tub |
| Essential oil | Up to the oil's rate | Scent |
Equipment and Sanitation
- Digital scale that reads to 0.1 g
- Large glass or stainless steel bowl, whisk, and a fine sieve
- Two-part stainless steel or plastic bath bomb molds
- Fine-mist spray bottle filled with 99% isopropyl alcohol or witch hazel
- Gloves and, for anyone sensitive to dust, a simple mask
Make sure every tool, bowl, and mold is completely dry. Even a few drops of water can start the reaction early. Wear gloves, and keep citric acid powder away from your eyes.
Master Recipe: Classic Bath Bombs (500 g, about 4 large bombs)
| Ingredient | Grams | % |
|---|---|---|
| Baking soda | 250.0 | 50% |
| Citric acid | 125.0 | 25% |
| Cornstarch | 50.0 | 10% |
| Epsom salt (fine) | 40.0 | 8% |
| Sweet almond oil (or melted cocoa butter) | 20.0 | 4% |
| Polysorbate 80 | 5.0 | 1% |
| Anti-Age Essential Oil Blend | 10.0 | 2% |
The Anti-Age blend's product page directs 2%, the highest rate listed, so 10 g goes into a 500 g batch. Measure scent by weight for a batch this size.
- Sift the dry ingredients. Sift the baking soda, cornstarch, and Epsom salt into the bowl and whisk out any lumps. Keep the citric acid separate for now.
- Mix the wet ingredients. In a small cup, stir the almond oil, polysorbate 80, and essential oil together. If you use cocoa butter, melt it gently first and let it cool until just warm.
- Combine. Drizzle the oil mixture slowly into the dry ingredients while whisking, then rub it in with gloved fingers until there are no wet clumps.
- Add the citric acid. Whisk in the citric acid last so it spends as little time as possible near any moisture.
- Mist to bind. Spray 2 or 3 spritzes of isopropyl alcohol or witch hazel and mix. Repeat only until the mixture holds together like damp sand when squeezed. If it starts to fizz, stop and mix faster.
- Pack the molds. Overfill both halves, press them together firmly with a slight twist, and scrape off the excess. Tap the mold and lift one half off, then the other.
- Dry. Let the bombs dry on a tray in a cool, dry room for 24 to 48 hours before wrapping.
Variations
- Two-color bombs: divide the finished mix into two bowls and color each with a cosmetic-grade mica or bath bomb colorant. Press one color into the edges of the mold like a small pinch pot and fill the center with the second.
- Botanical tops: sprinkle a pinch of dried rose petals or cornflowers into the mold before packing. Keep the amount small, since petals float and can clog a drain.
- Calming lavender bomb: replace the Anti-Age blend with lavender essential oil. Use the same 10 g (2%) and patch test it first.
- Butter-rich bomb: use 20 g melted cocoa butter in place of the almond oil for a harder bomb and a creamier water feel.
- Sparkle: use cosmetic-grade mica instead of craft glitter, which is plastic and does not rinse cleanly from skin or the tub.
We leave lemongrass and other citral-rich oils out of bath bombs. In a bath, the scent spreads over a large area of skin, so we save those oils for small, measured leave-on products.
Shelf Life and Storage
A finished bath bomb is dry, so it does not need a preservative. The goal is to keep water and humid air out. Wrap each bomb tightly in plastic wrap or shrink wrap and store in a sealed container away from the bathroom. Used within about six months, they keep their fizz and scent best. That is a formulator estimate; essential oil scent and fizz both fade over time, faster in humid climates.
Patch Test and Safety
- Patch test the essential oil you plan to use, diluted, on your inner arm for 24 hours. If your skin is sensitive, make the bombs unscented or start at the lowest rate (about 1%, or 5 g per 500 g batch).
- Oils make the tub slippery. Rinse and wipe it after your bath.
- Strong colorants can tint grout or light tubs. The polysorbate 80 helps, but rinse right away.
- For young children, choose an unscented, uncolored bomb and ask your pediatrician first.
Troubleshooting
- Crumbles out of the mold: too dry or not packed firmly. Add one more spritz and press harder.
- Grows warts or expands: too much moisture or a humid room. Use alcohol instead of witch hazel and mist more lightly.
- Cracks while drying: too much liquid. Cut back on the spritzes next batch.
- Fizzes in the bowl: water got in. Work faster, use dry tools, and add citric acid last.
- Weak fizz: old citric acid or a bomb that absorbed humidity. Store ingredients sealed and wrap finished bombs quickly.
- Oil ring on the tub: add the full 1% polysorbate 80.
Frequently Asked Questions
What is the best ratio for bath bombs?
Most recipes use about two parts baking soda to one part citric acid by weight. Cornstarch or kaolin at 5 to 15% softens the fizz, and 3 to 5% oil binds the mix. Weigh everything rather than using cups for consistent results.
Why do my bath bombs crumble?
Crumbling usually means the mix was too dry or was not packed tightly enough. Add moisture one spritz at a time until it holds together like damp sand, overfill the mold, and press firmly.
What can I use instead of citric acid?
Cream of tartar can replace part of the citric acid, but the fizz is weaker. For a strong, long fizz, citric acid is the standard choice.
Can I use water instead of witch hazel?
Water starts the fizz reaction quickly, so it is easy to overdo. Isopropyl alcohol evaporates fast and gives the most control, and witch hazel is a gentle second choice. Use only a few light spritzes of any of them.
How long do homemade bath bombs need to dry?
Let them dry for 24 to 48 hours in a cool, dry room before wrapping. In a humid climate, allow extra time, or dry them in a room with a dehumidifier.
How long do homemade bath bombs last?
Wrapped airtight and stored away from humidity, they are best used within about six months. The scent and fizz fade over time.
Are bath bombs safe for sensitive skin?
Choose unscented, uncolored bombs, or keep essential oil at the lowest rate, and patch test first. Skip glitter and strong dyes, and rinse off after your bath if your skin feels tight.
Shop and Learn More
Scent your bath bombs with the Anti-Age Essential Oil Blend or browse our essential oils collection. For more bath recipes, see our bath salt recipes and our guide to making your own bath salts and bubble bath.
Sources
- Skin Perfection product page: Anti-Age Essential Oil Blend (2%).
- Ingredient percentages, drying times, and shelf life are Skin Perfection formulating practice, not tested figures.
These statements have not been evaluated by the Food and Drug Administration. These products are not intended to diagnose, treat, cure, or prevent any disease.
About the Author
Written by Kari Thomas, cosmetic formulator and founder of Skin Perfection.