Cold process vs hot process soap
Same oils, same lye, two very different afternoons — and one difference that decides it for most people.
Both methods use exactly the same weight of lye for the same oils. Hot process cooks the batter until saponification finishes in the pot, so the bar is usable in about 7 days rather than 28 days, at the cost of a rustic finish and almost no ability to swirl. Cold process pours smooth, takes detail, and waits.
Cold process and hot process are the same chemistry run at two different speeds. You mix the same oils with the same weight of the same lye. In cold process you pour that mixture into a mould and let the reaction finish slowly over the following days. In hot process you cook it until the reaction has finished before it ever reaches the mould.
Everything else follows from that one decision. Because the cook drives off water, a hot process batch has to start wetter — 367 g against 290 g on a one-kilogram batch, about 77 g more. Because the batter is already soap when it goes into the mould, fragrance added at trace would simply be cooked away, so it goes in at the end. And because the batter is thick cooked soap rather than liquid, it is spooned rather than poured, which is why hot process bars look the way they do.
The choice is mostly about what you want out of the bar. If you want to swirl, cold process is the only real option. If you want to use it this week, hot process wins. The lye calculator covers cold process and the hot process calculator handles the wetter batch and the after-cook additions.
Side by side
A one-kilogram beginner blend worked both ways. Read the first row first: the lye weight is identical, which is the thing most comparisons get wrong.
The three differences that decide it
The main reason people cook a batch. Hot process is safe to use in about a week because the saponification already finished in the pot.
On a one-kilogram batch. A hot process recipe run at cold process water will not spoon out of the pot.
Cook at 3% and stir this in at the end. That oil never meets live lye — the one thing cold process cannot do.
What actually differs
Worth saying plainly because so much writing implies otherwise. Any recipe works either way, and a lye calculator does not need to know which method you are using.
Hot process runs wetter — around 28% lye concentration against 33% — because the cook takes water back out. See water discounts explained.
In hot process, after the cook and below about 80°C (176°F). Fragrance added earlier flashes off, and anything added for a skin benefit is simply cooked.
Cold process batter pours, so it takes fine mould detail and holds a swirl. Hot process batter is cooked soap being pressed into a mould, and it looks like it. Some people prefer that; nobody achieves a clean swirl with it.
Cold process is short and then unattended. Hot process needs you at the cooker for an hour, stirring down a pot that puffs up like curdled mashed potato before it settles.
Working either way
It teaches you what trace looks like without a hot pot in the way, and almost every published recipe assumes it. Hot process is easier to pick up afterwards than the other way round.
Cook times are a guide to when to start checking. Touch a cooled scrap to your tongue — finished soap tastes of soap, unfinished soap stings. That test beats any timer.
It is safe in 7 days but noticeably better at 21. The extra fortnight is water leaving the bar, and a drier bar lasts far longer in the shower.
If you are paying for a delicate oil as the superfat, cold process will not necessarily leave that oil in the bar. Hot process will, because you stir it into finished soap.
Size a hot process batch
The wetter lye and water for a cook, the stages to watch for, and the superfat and fragrance weights to stir in at the end.
Open the hot process calculator →Print the safety checklist
Lye handling, kit and the order of operations, on one page you can pin up where you work. Free.
Estimates, not advice. Every figure on this page is computed from published reference values and standard measures, so your own inputs, equipment and conditions will move it. Nothing here is professional advice — talk to a qualified professional before acting on it.