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What you need
Dry Ingredients
300 g baking soda, also called sodium bicarbonate
150 g citric acid
40 g kaolin clay
Liquid Ingredients
40 g cocamidopropyl betaine
10 g Polysorbate 80
Up to 5 g skin-safe fragrance oil
Cosmetic lake colourant or bath-safe colourant, as required
Equipment
Digital scale
Large mixing bowl
Fine sieve
Whisk
Silicone spatula
Smaller bowls for dividing colours
Gloves
Dust mask for handling fine powders
Baking tray
Wax paper, baking paper or silicone mat
Clean airtight jars
Small cosmetic scoop
You can usually find digital scales, clear cosmetic jars, small scoops and basic bath bomb supplies through Amazon. For specialist ingredients such as cocamidopropyl betaine, Polysorbate 80 and bath-safe colourants, use a reputable soapmaking or cosmetic ingredient supplier.
Instructions
Bath bombs are great fun until you reach the part where they need to come out of the mould.
One half sticks. The other half crumbles. Then the next one expands overnight until it looks less like a neat bath bomb and more like a science experiment.
Bath bomb crumble removes the most frustrating part of the process.
Instead of pressing the mixture into metal or plastic moulds, you spread it onto baking paper, leave it to expand and dry, then break it into chunky pieces. The finished crumble is scooped into the bath, where it fizzes and foams just like a traditional bath bomb.
The irregular pieces are part of the design, which makes this a wonderfully forgiving bath and body craft for beginners.
Bath bomb crumble is sometimes called bath bomb brittle, fizzy bath rocks, foaming bath crumble or scoopable bath fizz. It is different from ordinary bath dust because it dries into a brittle slab before being broken into pieces.
Instructions
Step 1: Prepare the Tray
Line a large baking tray with wax paper, baking paper or a silicone mat.
Leave plenty of space around the edges because the mixture may expand after it has been spread onto the tray.
Do not grease the paper.
Step 2: Prepare the Dry Ingredients
Put on gloves and a dust mask before measuring the fine powders.
Sift the baking soda, citric acid and kaolin clay into a large mixing bowl.
Whisk the ingredients thoroughly, making sure there are no lumps.
Uneven pockets of citric acid or colourant can create spotty colour and inconsistent fizzing, so this is one of those steps worth doing properly rather than pretending the lumps will sort themselves out.
Step 3: Divide the Mixture
For a two-colour bath crumble, divide the dry mixture evenly between two bowls.
Leave one portion white and colour the other, or use two coordinating colours.
Try:
- Pink and cream
- Aqua and white
- Lavender and lilac
- Orange and purple
- Blue and pale grey
- Peach and soft yellow
Use lighter colours than you think you need. Strong colour can stain pale bathtubs and grout.
Step 4: Combine the Liquid Ingredients
In a small jug, combine the cocamidopropyl betaine, Polysorbate 80 and fragrance oil.
Stir slowly to avoid creating lots of bubbles.
Check the fragrance supplier’s recommended usage rate before adding fragrance. Only use fragrances approved for bath and body products.
Candle fragrance oils and diffuser oils are not automatically safe for cosmetic use.
Step 5: Add the Liquid to the Dry Mixture
Pour the liquid ingredients into the dry mixture.
Mix quickly using a silicone spatula or gloved hands.
The mixture may begin to foam or puff slightly. This is expected with this type of bath crumble.
Unlike traditional bath bomb mixture, it should feel soft, damp and slightly sticky rather than dry and sandy.
Do not keep kneading once everything is evenly combined. Too much mixing can use up some of the fizzing reaction before the crumble ever reaches the bath.
Step 6: Add Colour
Add a small amount of cosmetic bath colourant to each portion.
Mix until the colour is even.
Use only colourants approved for bath products. Food colouring can stain the tub and skin, while ordinary craft pigment and glitter should not be used in bathwater.
Step 7: Spread the Mixture
Transfer the mixture onto the lined tray immediately.
Spread it into a rough layer approximately 1–2 cm thick.
Do not flatten it perfectly. Ridges, bumps and thicker areas create more interesting pieces once the slab dries.
For a marbled effect, place spoonfuls of two colours beside each other and gently swirl the edges together.
Avoid overmixing the colours or they may turn muddy.
Step 8: Allow the Mixture to Expand
Leave the tray somewhere dry and undisturbed.
The mixture may puff up, spread slightly or develop a foamy surface.
Do not press it back down unless it is in danger of escaping the tray entirely.
The expansion helps create the light, brittle texture that makes bath bomb crumble easy to break apart.
Step 9: Dry the Bath Crumble
Leave the tray to dry for 24–48 hours.
In a humid climate, it may take longer.
The slab is ready when it feels dry and firm, including through the thicker sections.
Do not place it in a hot oven. Excessive heat can affect the fragrance, colour and finished texture.
A dry room with gentle air circulation is usually best.
Step 10: Break It Into Pieces
Lift the dried slab from the paper.
Break it into irregular pieces using clean, dry hands.
Keep a mixture of:
- Large bath bomb rocks
- Medium-sized chunks
- Small crumbs
- Loose bath bomb dust
These mixed textures look attractive when layered in clear jars.
If the entire slab turns into powder, it is still usable as bath bomb dust. Nothing has to go into the bin simply because it failed to behave photogenically.
Step 11: Package the Crumble
Transfer the bath bomb crumble into clean, completely dry airtight jars.
Add a small cosmetic scoop and close the lids tightly.
Bath bomb crumble absorbs moisture from the air, so it should not be left uncovered in a steamy bathroom.
How to Use Bath Bomb Crumble
Fill the bath with warm water.
Add approximately 2–4 tablespoons of bath bomb crumble.
The mixture will begin fizzing and foaming as it dissolves.
Start with a smaller amount until you know how strongly coloured and fragranced your batch is.
Stir the water gently to help disperse the ingredients.
Take care when stepping into or out of the bath, as bath products can make the surface slippery.
Rinse the bathtub after use.
Bath Bomb Crumble Variations
Pink Lemonade Crumble
Colour half the mixture pale pink and the other half soft yellow.
Use a skin-safe pink lemonade or citrus fragrance oil.
Layer the dried pieces loosely inside a clear jar.
Do not use real lemon juice, as it will activate the citric acid before the mixture has time to dry.
Lavender Moon Crumble
Leave half the mixture white and colour the other half pale lavender.
Use a suitable lavender fragrance or safely diluted lavender essential oil.
Avoid adding large dried lavender buds. They look lovely in the jar but tend to end the evening gathered around the plughole and stuck to the bather.
Mermaid Bath Rocks
Divide the mixture into aqua, blue and pale purple portions.
Use an ocean, coconut or tropical fragrance approved for bath products.
Add a tiny amount of biodegradable cosmetic glitter only if it is specifically approved for body and bath use.
Halloween Witch’s Brew Crumble
Use purple, green and orange crumble.
Package it in a jar labelled Witch’s Bath Brew.
This variation pairs nicely with other spooky bath projects, particularly handmade soap, Halloween bath salts or cauldron-style bath bombs.
Troubleshooting
The Mixture Fizzed Too Quickly
The room may have been humid, or the liquid may have been added too slowly while the mixture was repeatedly worked.
Combine everything promptly and transfer the mixture to the tray without prolonged kneading.
Some foaming is normal.
The Mixture Overflowed the Tray
The layer was too thick or spread too close to the edges.
Use a larger tray or divide the next batch between two trays.
Once dry, any escaped pieces can still be broken up and packaged.
The Crumble Is Still Soft
It needs more drying time.
Move the tray to a drier room with gentle air circulation.
Break open one thick section to check whether the middle is still damp.
Do not package soft crumble in an airtight jar.
The Crumble Barely Fizzes
Too much of the reaction may have occurred during mixing or drying.
High humidity, too much liquid and excessive mixing can all weaken the final fizz.
Work quickly and store the finished product in an airtight container.
The Colour Sticks to the Bath
Too much colourant may have been used, or it may not be intended for bath products.
Use less colour and test a spoonful in a white basin before packaging the entire batch.
Polysorbate 80 can also help disperse oils and colour more evenly.
The Crumble Became Clumpy in the Jar
Moisture has entered the container.
Keep the lid tightly closed and use a dry scoop rather than wet fingers.
Storage
Store the finished bath bomb crumble in a cool, dry place inside an airtight container.
Avoid keeping it directly beside a shower or bath where it will be exposed to steam.
Discard the product if it develops an unusual smell, visible moisture or an unexpected change in appearance.
This recipe is suitable as a personal-use craft project. Anyone planning to sell bath products should investigate the cosmetic regulations, labelling requirements, insurance and product testing obligations that apply in their location.
Bath Bomb Crumble Gift Label
A simple front label could read:
Foaming Bath Bomb Crumble
Suggested directions:
Add 2–4 tablespoons to warm bathwater. Keep dry between uses. External use only. Take care, as the bath may become slippery.
Include the full ingredient list, fragrance name, date made and any warnings required for your chosen ingredients.
Bath bomb crumble is proof that bath products do not need to be perfectly round to feel special.
Mix it, spread it, let it puff up and then enjoy breaking the entire thing into pieces. For once, cracking your bath bombs is not a failure. It is the whole point.
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