Writing the Ingredient List for a Pure Castile Bar

If you want to learn how INCI naming actually works without getting lost in a twelve-oil recipe, castile soap is the easiest place to start. One oil, lye, water, and whatever's left of the lye after saponification. That simplicity makes it a great worked example, because every line on the ingredient list has to earn its place, there's nowhere to hide a naming mistake.

What goes into a true castile bar

Traditional castile soap is made with 100% olive oil, saponified with sodium hydroxide (lye) and water. No coconut oil, no palm oil, no added butters. It's a soft, slow-curing bar with a reputation for gentleness, mostly because olive oil alone produces a mild soap even though it takes months to fully harden.

The finished bar, once saponification is complete, is technically sodium olivate, the salt formed when the lye reacts with the fatty acids in olive oil. That's the compound your ingredient panel is describing, not "olive oil" sitting inertly in the bar.

Building the INCI list

Here's where the worked example gets useful. Even though your recipe has two raw materials, oil and lye, the ingredient list doesn't simply say "olive oil, sodium hydroxide." It needs to reflect the saponified product and account for excess unreacted oil if your recipe is superfatted.

Recipe component	What it becomes in the bar	INCI name to use	Notes
Olive oil (bulk of the batch)	Reacts with lye to form soap	Sodium Olivate	The saponified salt, not the raw oil
Sodium hydroxide	Fully consumed in saponification if properly calculated	Not listed separately if none remains	Excess lye is a safety problem, not a labeling one
Water	Evaporates substantially during cure	Aqua	Some remains in the final bar
Superfat (extra unreacted olive oil, often 5 to 8%)	Remains as free oil, unreacted	Olea Europaea (Olive) Fruit Oil	Listed separately from the sodium olivate because it never reacted

Notice that a superfatted castile bar actually needs two olive-related entries: the sodium olivate from the saponified portion, and the plain olive oil INCI name for the portion left deliberately unreacted to boost moisturizing feel. That distinction matters because it's chemically accurate, not just a formality, the two are different substances even though they came from the same bottle.

Where people get it wrong

The most common mistake with castile soap listings isn't a Hotlist issue, it's a naming issue. Makers sometimes list "Olive Oil" when they mean the saponified soap, or skip listing the superfat oil separately because it feels redundant with the sodium olivate line. Both of these get flagged when someone actually checks the formula against the process.

The other common gap is skipping additives that seem minor. If your castile bar has a trace amount of citric acid added to reduce lye scum, or sodium chloride added to harden the bar, those need INCI entries too, even at low percentages. A truly minimal castile bar might only have two or three lines, but check your actual recipe card against what's on the label, not what you remember adding.

If you make any claim beyond cleansing

Everything above assumes you're staying in "cleanses skin" territory. Real castile soap, sold purely as a cleansing bar, can be treated differently from a cosmetic. The moment you add language like "gentle enough for sensitive skin" or "leaves skin soft," you've likely crossed into cosmetic claim territory, and that pulls in the full set of obligations: a Cosmetic Notification Form filed within 10 days of first sale in Canada, concentrations or ranges for every ingredient, a Hotlist screen, and a bilingual label.

Castile makers often make exactly this claim, since olive oil soap really does feel mild, so it's worth deciding upfront which lane you're in and keeping your marketing copy consistent with it.

Why simple recipes are still worth double-checking

It's tempting to assume a one-oil recipe is too simple to get wrong. In practice, the simplicity is precisely why small errors go unnoticed, there are only two or three lines on the label, so a mislabeled ingredient represents a bigger share of the total panel than it would in a fourteen-ingredient lotion. If you're filing a notification for a castile bar with a cosmetic claim, Cosmetic Comply can map even a short ingredient list like this to the correct INCI names and CAS numbers, confirm nothing trips the Hotlist, and carry it through to a filed notification with a trackable number, useful even when the formula itself is about as simple as soap gets.