Lye soap sounds old-fashioned, even a little alarming. It is neither. Lye soap is simply true soap: fat combined with an alkali until the two become something new.
What lye is
Lye is a strong alkali. For bar soap, it is sodium hydroxide. Generations ago, soapmakers drew their alkali from wood ashes. The FDA mentions this history in its soap FAQ: soap was once made from animal fats and lye extracted from wood ashes.
What lye does
Fat and lye react in a process called saponification. The fat's fatty acids bond with sodium to form soap. The rest of the fat molecule is released as glycerin.
When the fat is tallow, the soap that forms is called sodium tallowate. Issue 20 covers that chemistry in detail, and issue 3 walks through how a cold-process bar is made.
Is there lye in the finished bar?
The FDA answers this directly: when made properly, no lye remains in the finished product.
The key words are made properly. What controls remaining alkali is the formulation and the making: the right amount of lye for the fat, measured and mixed correctly. Curing time on its own is not what makes soap safe. Curing is the weeks a bar spends drying and hardening after it is made.
What about soap without lye?
You will see "no lye" on some labels. True soap cannot be made without an alkali. The reaction between fat and alkali is what soap is.
A "no lye" label can mean the person selling it didn't handle lye themselves, for example because they started from a soap base that was already made. Somewhere earlier in the chain, an alkali did its work. It can also mean no remaining free alkali in the finished product. Ask the seller which meaning they intend.
What Tallow Soap’s bar is
True soap. One fat, grass-fed beef tallow, saponified cold and cured hard. The lye's job is to turn the tallow into soap, and in a properly made bar, it is used up doing exactly that.
That is also the regulatory meaning of soap. The FDA defines soap as a product made mainly of the alkali salts of fatty acids, cleaning through those salts, and sold only as soap. Lye soap is the original version of exactly that.
The short version
Lye soap is true soap. Fat plus alkali becomes soap plus glycerin. Made properly, no lye remains in the finished bar.