Bath Bombs: A Reaction That Starts in Water
A bath bomb is a compressed powder that fizzes and breaks apart the moment it touches water, releasing its fragrance, oil and colour into the bath. The chemistry behind it is simple: when sodium bicarbonate meets citric acid in water, carbon dioxide is released.
The hard part is not the chemistry but moisture. Because water is what starts the reaction, you have to keep the mixture dry throughout production, yet moisten it enough to hold together in the mould. Making bath bombs is walking that fine line.
Ingredient Table and Ratios
The formula below is for a 100-gram batch; with the calculator under the table you can scale it to any quantity.
| Ingredient | CAS No | Ratio | For 100 g |
|---|---|---|---|
| Sodium bicarbonate | 144-55-8 | 55% | 55 g |
| Citric acid (anhydrous) | 77-92-9 | 27% | 27 g |
| Corn starch | 9005-25-8 | 8% | 8 g |
| Epsom salt (magnesium sulphate) | 10034-99-8 | 3% | 3 g |
| Carrier oil (sweet almond, MCT) | — | 4% | 4 g |
| Fragrance oil | — | 2% | 2 g |
| Polysorbate 80 | 9005-65-6 | 1% | 1 g |
Bicarbonate and citric acid work at a ratio of roughly two to one; this is the body of the fizz. Starch slows the reaction and leaves a silky feel in the water. Carrier oil is both a binder and a skin softener. Polysorbate 80 disperses the oil and fragrance into the water; without it, oil leaves a ring on the water surface and around the tub. If you want to add colour, use a cosmetic-grade water-soluble dye or mica.
Preparation Steps
Work in a dry environment free of humidity; production becomes noticeably harder on a rainy day.
- Sieve and mix the dry ingredients
Pass the bicarbonate, citric acid, starch and salt through a fine sieve. Lumps come back as bumps and cracks on the surface of the bomb.
- Combine the liquids in a separate container
Mix the carrier oil, fragrance oil and polysorbate 80 in a small container. Do not pour fragrance oil straight into the dry mix; it collects in one spot.
- Work the liquid in little by little
Add the liquid mixture to the dry ingredients drop by drop, mixing constantly. If you see fizzing in one area, disperse it at once.
- Check the consistency
The mix is ready when it holds together without crumbling when squeezed in the palm; aim for the texture of damp sand. If it does not hold, add a very small amount of isopropyl alcohol or witch hazel by spraying. Do not spray water; it starts the reaction.
- Mould and compress
Overfill both half-spheres, press them together, and separate without twisting. A loosely filled bomb falls apart once dry.
- Dry
After removing from the mould, leave for 24 to 48 hours somewhere dry and cool. A bomb packed before it is fully dry swells inside its packaging.
- Pack moisture-tight
Shrink film or a sealed bag. A bomb left in the open absorbs moisture from the air and loses its fizzing power.
Safety and the IFRA Framework
A bath bomb looks like a rinse-off product, but the person stays in the water for a long time; choose your fragrance accordingly.
Keep the fragrance level in the range of 1 to 3% and check the category limit covering bath products in the IFRA certificate of the fragrance oil you use. The limit is determined not by the total fragrance level but by the substances within the oil; the permitted level differs for each fragrance oil.
Citric acid and bicarbonate dust irritate the airways and the eyes. Wear a dust mask and goggles while sieving and mixing, and ventilate the area. A bomb containing oil makes the bath slippery; add a slip warning to the packaging.
A bath bomb you offer for sale is a cosmetic product: product notification, safety assessment and labelling obligations apply. If you use a colourant, choose from the colourants permitted in cosmetics. Cosmetic fragrance oil is not edible; on products that look like confectionery, add a warning against access by children.
Troubleshooting, Tips and FAQ
Almost all problems stem from the moisture balance.
| Symptom | Likely cause | Solution |
|---|---|---|
| Fizzing or swelling in the mould | Mix too moist, or humid environment | Reduce the liquid, add the binder by spraying, work in a dry room |
| Cracking or crumbling once dry | Mix too dry or loosely compressed | Increase the binder slightly, press the mould more firmly |
| Bubbles and pimples on the surface | Unsieved lumps, local fizzing | Sieve the dry ingredients, work the liquid in more slowly |
| Weak fizz in water | Has absorbed moisture, wrong ratio or stale raw material | Pack moisture-tight, check the bicarbonate-to-acid ratio |
| Oil ring in the tub | No dispersant, or not enough | Add polysorbate 80; lower the oil level |
| Scent weak or quick to fade | Fragrance heavy in volatile top notes, left in the open | Choose a fragrance with a strong base, pack immediately |
| Blotchy colour or staining the tub | Water-insoluble dye, or too much | Use water-soluble cosmetic dye, reduce the amount |
Why does a bath bomb fizz?
How much fragrance oil goes into a bath bomb?
Can bath bombs be made with essential oils?
Why did my bath bomb swell in the mould?
Is corn starch essential?
Can I leave out polysorbate 80?
How long does a bath bomb keep?
Do I need permission to sell bath bombs?
What size mould should I use?
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