Hot process and cold process soap start with the same ingredients. The difference is heat, and it changes how long the soap takes, how it looks and how it is made. Here is a plain-English guide, and why every J.D. Bauer bar is hot process.
If you have shopped for handmade soap, you have probably seen "cold process" on one label and "hot process" on another. Both are real, traditional ways to make soap from scratch. Neither is a shortcut. They are two paths to the same place, and each one leaves its own fingerprints on the finished bar.
Side by side
Hot process vs. cold process
| Hot process | Cold process | |
|---|---|---|
| Heat | Cooked over steady heat, often in a slow cooker or double boiler | Mixed warm (often around 80–120°F), then left to react on its own |
| How fast it becomes soap | Fast. The heat speeds the reaction along, so it finishes in the pot. | Slow. The reaction continues in the mold over the next day or two. |
| Batter | Thick, like mashed potatoes, and turns glossy and jelly-like | Smooth and pourable, like thin pudding |
| Look of the bar | Rustic, textured, handmade-looking tops | Smooth, with room for fine swirls and layers |
| Ready to cut | Usually within a day | Usually after a few days |
| Cure time | About 1–2 weeks is recommended, but it matters less | About 4–6 weeks for a hard, long-lasting bar |
| Delicate additives | Milk and fruit purées can scorch; some scents fade in the heat | Easier to add milk and fresh ingredients at low temperature |
First, how any soap is made
All true soap comes from the same basic reaction. Fats and oils are mixed with lye, and the two react to form soap. Soapmakers call this saponification. When a recipe is measured correctly, the lye is used up in the reaction, and what is left is soap.
The question is how you get the reaction to finish. That is where hot and cold process go different ways.
What is cold process soap?
In cold process, the oils and lye solution are mixed while they are warm but not cooked, often somewhere around 80 to 120°F. The maker blends them until the batter thickens to a stage called "trace," which looks like thin pudding. Scent and color go in, and the batter is poured into a mold.
From there, the reaction finishes on its own over the next day or two. After the bars are cut, they cure for about four to six weeks while water evaporates and the bar hardens. Because the batter stays fluid, cold process is the method behind smooth bars with swirls and layers.
What is hot process soap?
Hot process starts the same way, with oils and lye blended together. Then the batter goes into steady heat, often a slow cooker or a double boiler. The heat speeds up the reaction so the soap finishes in the pot instead of in the mold.
As it cooks, the batter thickens to something like mashed potatoes, swells and turns glossy and translucent. When the cook is done, scent and botanicals are stirred in and the thick soap is pressed into the mold by hand. Bars can usually be cut within a day. A rest of about one to two weeks is still recommended so they harden, but the long cure matters much less.
How each one is made
Hot process
- 1 Mix. Oils and lye solution are blended, just like cold process.
- 2 Cook. The batter goes into steady heat and is stirred now and then.
- 3 Watch it change. It thickens, swells and turns glossy and translucent as it becomes soap.
- 4 Add the botanicals. Scent and texture go in once the cook is done.
- 5 Pack the mold. The thick soap is pressed into the mold by hand.
- 6 Cut and rest. Bars can be cut the next day, then rest to harden.
Cold process
- 1 Mix. Oils and lye solution are blended at warm, not hot, temperatures.
- 2 Reach trace. The batter thickens to a pourable pudding.
- 3 Add the botanicals. Scent, color and other additives go in at trace.
- 4 Pour and design. The fluid batter is poured, swirled or layered.
- 5 Wait. The reaction keeps going in the mold for a day or two.
- 6 Cut and cure. Bars are cut, then cure for about four to six weeks.
Soap has been cooked for centuries
Cooking soap is an old idea. For centuries, soapmakers boiled oils or fats with an alkali in big kettles and vats. The traditional Castile and Aleppo soaps we wrote about in our Castile soap guide were boiled, too. Hot process is the small-batch, home-kitchen cousin of that tradition.
How long does each one take?
This is the biggest practical difference. A cold-process bar needs weeks on a rack before it is at its best. A hot-process bar is finished soap when it leaves the pot and only needs a short rest to firm up.
Typical timeline
From batter to shower-ready
Hot process
Cold process
Typical ranges from common soapmaking guidance. Every recipe and maker is a little different.
What does each one look like?
You can often tell them apart by eye. Cold-process bars tend to be smooth, with sharp edges, crisp layers and fancy swirls. Hot-process bars have a more rustic, hand-pressed look, because the soap goes into the mold thick instead of being poured.
How to spot a hot-process bar
✓Rustic top
A hand-pressed, textured surface rather than a glassy-smooth one.
✓Slightly marbled look
Botanicals and color tend to sit in the bar unevenly, in a natural way.
✓Solid from day one
A hot-process bar is already finished soap when it goes into the mold.
Pros and cons
Neither method is better across the board. Each one has trade-offs.
Hot process
Good at:
- Shorter wait from batch to bar
- Handles thick, heavy ingredients
- Rustic, old-world look
Harder at:
- Fine swirls and smooth designs
- Milk and fruit purées, which can scorch
- Some delicate scents, which can fade in the heat
Cold process
Good at:
- Smooth, detailed designs
- Milk and other fresh ingredients
- No cooking step
Harder at:
- Patience: about 4 to 6 weeks of curing
- Ingredients that speed up or seize the batter
- Soft bars if used too early
Why J.D. Bauer makes hot-process soap
Every soap in our lineup is hot-process, handmade in small batches in Redlands, California, on a Castile base with olive oil at the center. The method fits what we make. Our bars lean on heavy, characterful ingredients: real Norwegian pine tar, Dead Sea mud, sea salt, ground oatmeal. Hot process gives those ingredients a sturdy, rustic bar to live in, and it fits the old-world stories behind each soap.
Hot process gives heavy, old-world ingredients a sturdy, rustic bar to live in.
Pine tar is a good example. It is thick, sticky and known for being difficult in both hot and cold process. Most pine tar soaps use 10 to 20%. Ours uses 30%.
Thick, sticky pine tar is notoriously hard to work with. We use 30% anyway.
A nod to the Gauls, who Pliny the Elder said made soap from fat and ash.
Which should you buy?
If you love bold designs and smooth, glossy bars, look for cold process. If you like a rustic, handmade-looking bar and ingredients you can see and feel, hot process is worth a try. Either way, a good handmade bar comes down to the recipe and the care that goes into it.
Ready to try one? Browse all J.D. Bauer handmade soaps, or read the stories behind every bar.