Natural, and the Green Guides
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Handmade soap sells substantially on being natural, and natural is the least defined word on the label.
The word the whole category leans on
Walk a market and nearly every stall says natural somewhere.
Which is an environmental and general marketing claim, and the FTC's Green Guides are the place those claims are evaluated.
The Guides do not give "natural" a safe-harbor definition, which is precisely the problem: an undefined term still has to not be deceptive.
What the Green Guides actually say
Qualify a general claim so it is clear what specific benefit is meant, because unqualified general environmental claims are difficult to substantiate.
Have substantiation for the claim you make, in the sense a reasonable consumer would understand it.
Do not overstate a benefit, and do not let a claim imply more than the fact supports.
The same logic reaches biodegradable, compostable, and recyclable on your packaging.
Claims that get made anyway
"All natural" on a bar containing a synthetic fragrance oil, which is the commonest one.
"Chemical free", which is not true of anything.
"Organic", which is a regulated term with a certification behind it and is not a synonym for natural.
"Non-toxic", which reads as a safety claim.
And note that lye is in every bar of real soap, so a "no lye" claim is either false or describes a melt and pour base somebody else saponified.
Saying it without the claim
Describe rather than characterize: name the oils, say where they came from, say how it was made.
"Made with olive, coconut and castor oil" carries the whole impression and is simply a fact.
Which is also better copy, because it is specific.
And keep the environmental claims on the packaging separate from claims about the soap, since a compostable band does not make the product anything.