Quick answer: weigh 2 parts baking soda to 1 part citric acid, sift them together, rub the oils into the dry mix, then spritz witch hazel a little at a time until a squeezed handful holds its shape. Pack both halves of the mold, press, pop the bomb out, and let it dry for a day or two before storing it airtight. Most bath bomb problems start with moisture getting into the mix too early.
The method
- Weigh the baking soda and citric acid.
- Sift them together into a wide bowl, breaking up any lumps.
- Mix in any dry extras, like cornstarch or SLSA.
- Drizzle in the carrier oil and fragrance, and rub them through the powder with gloved hands.
- Spritz witch hazel a little at a time, mixing between sprays, until a squeezed handful holds together like damp sand.
- Overfill both halves of the mold, press them together hard, and pop the bomb out.
- Let it dry on a soft surface for 24 to 48 hours.
- Store it airtight.
For a small test batch, use 200 g baking soda and 100 g citric acid. The bath bomb calculator scales the recipe to your molds and works out the extras as a share of the base.
Use fragrance at the rate your supplier gives for bath products on the oil’s IFRA certificate. That limit is different for every oil.
What you need
| Item | Notes |
|---|---|
| Baking soda and citric acid | Fine or granular citric acid both work by weight |
| A carrier oil | A teaspoon or two per batch; sweet almond and fractionated coconut are common |
| Fragrance or essential oil | One your supplier lists for bath products |
| Witch hazel in a fine-mist spray bottle | Rubbing alcohol works too |
| A scale that reads to 1 g | The ratio only holds by weight |
| A sieve and a wide bowl | Lumps turn into holes and cracks |
| Two-part molds | Metal, rigid plastic or silicone |
| Gloves, and a mask if you use SLSA | SLSA is a very fine powder and gets into the air |
| A folded towel or a piece of foam | Gives round bombs a soft place to dry |
| Airtight containers or shrink wrap | Damp air slowly uses up the fizz |
Moisture: why batches fail
A bath bomb is two powders that only react when they get wet. Baking soda is a weak base and citric acid is a weak acid. In water they react and give off carbon dioxide, which is the fizz. As long as they stay dry, nothing happens.
Water that reaches the mix before the bath starts that reaction early, and most of it comes from the binder. Witch hazel and alcohol are used because they dampen the powder enough to hold a shape and then evaporate before much reacts. Too much of either starts the fizz in the bowl, and plain water is worse.
Damp air does it too. On a humid day the powder picks up moisture from the room, so the same recipe needs less binder than it did last week. Bramble Berry, a soap and bath supply company, recommends drying bath bombs in a room below 50% relative humidity. A cheap hygrometer will tell you where you are.
Liquid colorants and other wet additives bring water in with them.
When the reaction starts early, you get cracks, bumps, bombs that grow out of the mold, and bombs that barely fizz in the tub because they’ve already used some of it up. On a humid day, run a dehumidifier or the air conditioning if you have one, spritz less, and do the squeeze test more often. If the mix fizzes or puffs up after the first few sprays, stop adding liquid and keep mixing.
Shower steamers run on the same reaction and have the same moisture problems. The shower steamer guide covers how they’re made differently.
Packing and unmolding
Fill each half of the mold until it’s heaped a little over the rim, and press it down firmly with your fingers. Pile a bit extra on one half, then press the halves together hard with a slight twist. Scrape off whatever squeezes out at the seam.
With metal or rigid plastic molds, tap the top half lightly with a spoon and lift it off. Then turn the bomb into your palm and tap the other half free. Do this right away. A bomb left to dry in a rigid mold tends to stick.
Silicone works the other way around. Leave the bomb in until it’s firm enough that pressing the top doesn’t dent it, then peel the silicone off.
If a bomb crumbles as soon as it comes out, the mix was too dry or packed too loosely. Put it back in the bowl, add a spritz or two, and pack harder.
Drying
Bramble Berry puts drying time at 24 to 48 hours at room temperature with some airflow, and several days in humid weather. A bath bomb is dry when it’s hard all the way through and doesn’t feel cool. A damp one feels slightly cold, because the liquid inside is still evaporating.
Dry round bombs on a folded towel or a piece of foam. On a hard counter they flatten where they sit. Leave a little space between them.
Don’t dry them next to a heater. If the outside dries much faster than the inside, the surface can crack.
Why bath bombs crack, grow or get bumps
| What you see | What happened | Fix |
|---|---|---|
| Cracks as it dries | Too wet, so the reaction started and the gas split it. Sometimes drying too fast near heat | Less binder, a drier room, no heater nearby |
| Grows out of the mold or domes up | Too wet; the reaction started in the mold | Less binder, a finer mist, more mixing between sprays |
| Bumps or “warts” | Small wet spots that reacted, from heavy spraying, humidity or a liquid colorant | Finer mist, longer mixing, a less humid room |
| Splits along the seam | The two halves didn’t bond | Overfill both halves and twist as you press |
| Crumbles when unmolded | Too dry, or packed too loosely | Another spritz or two, and press harder |
| Flat on the bottom | Dried on a hard surface | Dry on a towel or foam |
Cracks, growing and warts are all the same thing at different strengths. If a batch shows any of them, use less liquid next time, or the same amount on a drier day.
Fizz, foam and color
Cornstarch slows the fizz down. It doesn’t react with anything; it just takes up room between the powders that do. In a classroom bath bomb activity from Scientific American, the recipe with less cornstarch fizzed harder and dissolved faster than the one with more. Add it if you want a longer, gentler fizz, and leave it out if you want a fast, strong one.
SLSA (sodium lauryl sulfoacetate) makes foam on top of the water. Makers say it slows the fizz a little, more so the more you add. Wear a mask when you measure it. The calculator page gives a starting amount.
Mica and other powdered colorants can leave a ring around the tub and color on skin, and the more you use, the more likely that gets. Polysorbate 80 is often added to help oils and colorants spread out in the water instead of floating to the top and sticking to the tub. Some deep reds, purples and blacks stain anyway, so try a bomb in your own tub before making a batch to sell.
Storing them
Once they’re dry, put them in an airtight container or shrink wrap each one. Damp air slowly sets off the reaction on the surface, so a bath bomb sitting out in a bathroom loses its fizz long before anyone uses it.
Suppliers usually give a shelf life of six months to a year, with the fizz and the scent fading over that time. A bomb that has gone soft or crumbly on the shelf has taken on moisture, and it won’t fizz much.
Problems in the tub
Weak fizz, or none. The bomb reacted early, from too much binder or damp storage, or it’s just old.
Chalky water or residue. Extra baking soda with no acid left to react with. Stay near 2:1. The calculator page explains why the ratio already carries some extra baking soda.
Oil floating on top. More carrier oil than the water can spread out. Use less, or add polysorbate 80.
A ring around the tub. Too much colorant, or a colorant that stains. Use less, and test before selling.
Not much scent. Fragrance fades in storage. Wrap them airtight and use them within a few months.
Frequently asked questions
How do you make bath bombs?
Weigh 2 parts baking soda to 1 part citric acid and sift them together. Rub the oils and
fragrance into the dry mix, then spritz witch hazel a little at a time until a squeezed
handful holds its shape. Overfill both halves of the mold, press them together, pop the bomb
out and let it dry for a day or two before you store it airtight.
Why are my bath bombs cracking?
Usually the mix was too wet. The extra liquid lets the baking soda and citric acid start
reacting in the mold, and the gas splits the bomb as it dries. Use less witch hazel, make
them on a drier day, and keep them away from heaters while they dry, since drying too fast
can crack them too.
How long do bath bombs take to dry?
Usually 24 to 48 hours at room temperature in a room with some airflow, and several days
when it’s humid. They’re done when they’re hard all the way through and no longer feel cool
to the touch.
Why do my bath bombs have bumps on them?
Those bumps, often called warts, are spots where the reaction started on the surface.
Something made that spot wetter than the rest: a heavy spray of witch hazel, a humid day, or
a liquid colorant that wasn’t mixed in well. Spray a finer mist, mix longer, and dry them
somewhere less humid.
Why didn’t my bath bomb fizz?
It probably reacted before it got to the tub, either from too much liquid in the mix or from
being stored in damp air, so the fizz was used up. Old bath bombs lose their fizz the same
way. A lot of cornstarch, or much more baking soda than 2 parts to 1, also makes for a weak
fizz.
How long do bath bombs last?
Suppliers usually say six months to a year. The fizz and the scent both fade over that time,
faster if the bombs are stored somewhere damp. Wrap them airtight as soon as they’re dry.