Soap is a chemical reaction, not a recipe.
That reaction is saponification. You take a fat and a strong base, you put them together, and the fat splits into soap and glycerin. The word comes straight from the Latin sapo, soap. Every real bar of soap ever made, anywhere, is the result of this one reaction. Tallow soap is no different.
For tallow soap the fat is grass-fed beef tallow. The strong base is lye. And the lye part is where most people flinch, so deal with it first.
The lye question, answered straight
Yes, there's lye in the making of every soap. Sodium hydroxide, to be exact.
No, there's none left in the finished bar.
Lye isn't an ingredient that sits in the soap. It's the thing that triggers the reaction, and the reaction consumes it. The fat and the lye change each other into something new, soap and glycerin, and a properly made, fully cured bar has no lye left in it. None. It wasn't neutralized or diluted. It was chemically transformed and used up.
This is true of all real soap. There is no soap without lye, has never been, because that's the reaction. A bar that claims to be "lye-free" was either made from a pre-made base where somebody else handled the lye, or it isn't soap. When you see lye on a label, that's a maker telling you the truth about how soap is made.
Cold process, start to finish
Cold process is the slow, traditional method. The name says it. No outside heat is used to force the reaction along.
It goes like this.
Start with clean tallow. The beef fat is rendered down, melted low and strained, until it's a clean, stable fat ready to work with.
Make the lye solution. Sodium hydroxide is dissolved into water. This part gets hot on its own and gets handled with care.
Combine and bring to trace. The melted tallow and the lye solution are brought together at a controlled temperature and mixed. Within minutes the mixture thickens and emulsifies to the point soapmakers call trace, the first sign the reaction has caught. That's the cue.
Pour and leave it. The soap goes into molds and it's left alone. No cooking, no stirring. The reaction makes its own heat from here, an exothermic process that warms the soap from the inside, and it works through on its own.
Unmold, cut, then cure. Once it's firm it comes out, gets cut into bars, and goes onto a rack to cure for four to six weeks.
That's the method. The hard work is in the patience, not the equipment.
What the cure actually does
People assume the long cure is the soap still becoming soap. It isn't, mostly.
Saponification is roughly ninety to ninety-five percent done in the first day or two, and essentially finished within a week. After that the reaction is over. The fat is soap.
The four to six weeks of curing is about two slower things. Water evaporating out, and the soap crystals hardening and settling. A bar cut and used too early is soft, harsh, and short-lived. A bar given its full cure is harder, milder, and lasts far longer in the shower. Time isn't a delay in the formula. Time is part of the formula.
Why cold process, and not the fast versions
There's more than one way to make soap, and they don't give you the same bar.
Hot process cooks the soap to drive the reaction to completion in under an hour. Faster, but the heat tends to leave a rougher, more rustic bar.
Industrial soap goes further. It's often boiled, and here's the part that matters: the glycerin gets pulled out and sold off separately, because glycerin is valuable on its own. What's left is a harder-working, harder-on-skin detergent bar with the good part removed.
Cold process keeps the glycerin in. Glycerin is a humectant, which means it pulls moisture toward your skin. It's made for free in every batch as a natural byproduct, and the only reason to remove it is to sell it. So we don't. That retained glycerin is a big part of why a real cold-process bar feels the way it does and a stripped detergent bar doesn't.
Why tallow makes this work, and why it's hard
Tallow is high in stearic and palmitic acid, the saturated fats that give you a hard, dense, long-lasting bar. That's a gift. It's also a single fat, which means there's no second oil to soften the margin for error. One-ingredient tallow soap is more demanding to make well, not less.
The other quiet detail is superfat. The recipe is built with a small excess of tallow, a little more fat than the lye can consume. That guarantees no lye is left over, and it leaves a touch of unsaponified tallow in the bar so it cleans gently instead of harshly. It's the difference between a bar that respects your skin and one that scours it.
One fat. The slow method. The glycerin left where it belongs. That's the standard, and that's how the bar gets made.