Lye pockets and lye heavy soap
Updated
The only failure in this property that can injure a customer, so it is the one to understand properly.
The failure that hurts somebody
Finished, correctly made soap contains no free lye. It is not caustic and it is safe.
A batch where the lye did not fully react, or where the calculation was wrong, can be.
Which is why superfat exists and why the calculation is checked.
Lye pockets
Localized areas of unreacted lye in an otherwise sound bar.
Causes: undissolved lye in the solution, poured without noticing; an under-emulsified batch that separated; or a poorly mixed batch where the lye was not distributed.
They show as clear or crystalline patches, as a soft weeping spot, or as a bar that stings on contact.
The prevention is procedural: stir the lye solution until completely dissolved and check the bottom of the container, and blend to a genuine emulsion rather than to an appearance of one.
Lye heavy throughout
The whole batch is caustic because the recipe was wrong.
Causes: a miscalculation, a substituted oil without recalculating, a mis-weighed lye, a zero superfat with a scale that drifts, or lye that had absorbed moisture and was not the purity assumed.
It shows as a bar that feels harsh, that stings or burns, that is unusually hard and brittle, or that develops a crumbly surface.
What to do
Do not sell it, do not give it away, and do not use it yourself to be sure.
A suspect batch is a disposal, not a discount. The cost of the oils is small against the alternative.
Then find the cause before making another batch, because the cause is in your process rather than in the soap.
And note that zap testing, touching soap to the tongue to feel for a sting, is a traditional practice that puts a possibly caustic material on a mucous membrane. A pH reading on a dissolved sample is the better answer, and disposing of a genuinely doubtful batch is better than either.