Epsom salt and additives in bath bombs
Seven things people put in bath bombs, what each one is actually for, and the percentage past which it starts working against you.
Use fine epsom salt at around 8% of the batch weight — 12.1 g in a standard bath bomb. Coarse crystals are the mistake: they sit in the mix like grit and give the bomb lines to crumble along. Its ceiling is about 20%, past which there is not enough acid and bicarbonate left to fizz properly.
A bath bomb only genuinely needs two ingredients. Everything else on the shelf is there to change how it fizzes, how it survives the post, what it does to your skin, or what it does to your bath afterwards — and a surprising amount of it is doing nothing at all, at the percentages people use.
Epsom salt is the one most people ask about, and it is the clearest case of the ingredient being for the bath rather than the bomb. At 8% of the batch — about 12.1 g in a standard 65 mm bomb — it dissolves into the water and does whatever you believe epsom salt does. What it must be is fine. Coarse crystals sit in the mix like grit and the bomb breaks along the lines they make.
The table below gives every additive at its working share and its ceiling, weighed out for a 6-bomb batch. The percentages are of total batch weight, which is the only convention that stays true when you change the ratio — quoting an additive against the bicarbonate means it moves when you have not touched it. Full recipe context is in how to make bath bombs.
Every additive, weighed out
Shares are of total batch weight. The last column is what goes into one 65 mm bomb, which is the figure worth sanity-checking against — a "generous handful" of anything is usually several times its ceiling.
How much epsom salt
Enough to say it is in there. Sensible if you are also running a lot of cornflour and clay and do not want to crowd out the fizz.
Where most recipes land, and where the bath actually feels different. Start here.
Past this you have made a bath salt that happens to fizz a bit. That is a perfectly good product — it is just not a bath bomb, and it will not hold a sphere well either.
What each one is actually for
Magnesium sulfate. It contributes nothing to the fizz, the binding or the shape; it dissolves and does its work in the water. 8% of the batch is the usual figure and 20% is where the bomb stops being one. Buy the fine grade, and if all you have is coarse, run it through a grinder — the crystals are exactly the size of the flaw they create.
Sodium lauryl sulfoacetate. The only thing on this list that makes real bubbles rather than fizz. Past about 10% the bath foams more than it fizzes. It sits at 3% of the batch and tops out around 10%. SLSA is a very fine powder and a respiratory irritant. Weigh it wearing a dust mask, in still air, and do not lean over the tub — a lungful sets off coughing that lasts. It is harmless in the bath and unpleasant on the way there.
The honest answer to a stained tub. It ties oil and colour into the water instead of letting them ring the bath. If a bomb has left a coloured ring or an oil slick, this is the fix — around 2% of the batch, and it wants to be at least as much as the oil and mica it is emulsifying. No amount of rinsing the bath afterwards is a substitute.
These are the three that decide whether a bomb survives being handled. Kaolin clay at 3% binds and hardens; Cream of tartar at 2% firms it without the clay dust; Cornflour (cornstarch) at 10% slows the reaction so the fizz lasts more than a minute. All three have low ceilings — 8%, 5% and 20% — and past those the bomb holds together beautifully and barely fizzes.
Colour only. More does not make a stronger colour past a point — it makes a bomb that leaves a tidemark. 0.5% is plenty and 2% is the ceiling. The failure here is visible rather than functional: too much colour rings the bath, and that is what polysorbate 80 is for.
Not an additive in the sense of the others, but worth listing with them because it is the ingredient most likely to ruin a batch. 3% of the batch is generous and 6% is the ceiling — 54 g on this batch. Fat coats the powder and stops the particles locking, which is why an over-buttered bomb is still slightly greasy the next morning.
Using them well
Every percentage here is a percentage of the batch that is not fizzing. The starter mix in the recipe builder runs 25% of the batch as additives, butters and fragrance combined, which leaves 75% as acid and bicarbonate. Past about 40% you have made something that holds together well and does very little.
Clays and starches clump, and a clump is a soft spot in the finished bomb. Sieving the dry ingredients together also mixes them, which saves a step and does it better than stirring does.
SLSA, kaolin clay and mica all go airborne the moment they meet a scale. None of them is dangerous in a bath; all of them are unpleasant in a lungful. This is the one piece of protective kit the craft genuinely needs.
The rule of thumb worth remembering is that polysorbate 80 should be at least as much as the fixed oil and mica it has to carry. A bomb with a lot of butter and a token amount of emulsifier still rings the bath.
Bath bomb recipes accumulate ingredients the way old recipes accumulate steps. If you cannot say what something does, take it out for a batch and see whether you notice — most of the time nobody does, and the fizz improves.
See what your additives leave for the fizz
Every additive in the recipe builder carries its ceiling, and the tool tells you how much of the batch is still acid and bicarbonate.
Open the recipe builder →Keep the ratio to hand
The bicarb-to-citric ratio, binder rates and mould weights on one printable card. Free.
Estimates, not advice. Every figure on this page is computed from published reference values and standard measures, so your own inputs, equipment and conditions will move it. Nothing here is professional advice — talk to a qualified professional before acting on it.