Convert Hot Process Soap Recipes to Cold Process (and Cp to HP too!)

Soapmakers ponder if they can turn a cold process recipe into a hot process one—and the answer is yes, you absolutely can. Every oil and butter that behaves beautifully in cold process will also behave beautifully in hot process. The chemistry doesn’t change, only the method does. Once you understand where those differences live—mostly in water content and technique—you can move between the two as easily as you’d switch from baking a cake to cooking one on the stovetop. The same ingredients, just a different path to the finish line. (And yes, you can cook a cake on the stove top, and even over a camp fire! But that’s another discussion❤️).

I’ll also answer the reverse question, because it comes up just as often: can you turn a hot process recipe into a cold process soap? Again, yes. You can. The only real limitation is how you handle your superfat.

This topic comes up a lot in my classes, especially when people discover the Liz Ardlady shampoo bar recipe. Liz makes her version hot process, but the moment someone sees that, they ask: can I make it cold process? Absolutely—you can. I talk about this inside the Shampoo Bar Course and you can even read the first lesson for free if you wondering how to wash your hair with shampoo!

So let’s dig into the differences, the conversions, and a few myths that need to quietly melt away when it come to hot process vs cold process soap formulations.

Cold Process and Hot Process—What’s Really Different?

Both methods start the same way: oils, lye, and water combine to start saponification, the chemical reaction that turns fats into soap. In cold process, you pour that mixture into a mold while it’s still in the early stages of saponifying, then let it finish in the mold over the next 24–48 hours. In hot process, I apply heat (usually in a slow cooker) to drive saponification to completion before the soap ever touches the mold.

So the real difference isn’t in what’s inside your pot—it’s in how you push the reaction along.

Cold process soap is smooth, fluid, and easy to swirl because the batter stays workable longer. It’s my go-to for complex designs, fine lines, and gradient blends. Hot process, on the other hand, gives you that rustic texture, quick turnaround, and a certain satisfaction that comes from watching your soap “cook.” Once the reaction is done, you can even add your superfat oils after the cook, giving you more control over which oils stay unsaponified (and thus give a unique skin feel to your lather).

Convert Cold Process Recipes to Hot Process

When you take a cold process recipe and turn it into hot process, the formula itself—the oil blend, lye calculation, and superfat percentage—remain the same. The water however, needs to be adjusted. Hot process soap cooks for a while, and that heat evaporates moisture. If you don’t start with a higher water content, you can end up with a thick, dry and crumbly mess that’s hard to move around or mold.

• In cold process soap, I often use a very low to moderate water level (anywhere from 1:1 up to 2:1).
• For hot process, I increase that to 3:1 water to lye (take the lye amount and multiply it by 3).

That extra water compensates for evaporation during the cook and gives you a smoother, more manageable texture when you’re ready to mold it. For more details on my preferred water/liquid ratios for cold process soaps, information on why I prefer ratios (and dispise the term “water discount”) check out this article.

A Quick Note on Fluid Hot Process

You might have heard people talk about “fluid hot process” soap—the kind that pours almost like cold process and can even be swirled. That’s achievable, but it takes more formulation finesse than I want to dive into here. The short version is that you’ll need a higher water/liquid content and a little help from ingredients like yogurt, milks, or lactic acid. They help the soap stay fluid by softening the texture during the cook. It’s wonderful once you get the hang of it, but it’s a beast all its own and deserves its own deep-dive guide. For now, just know that you can absolutely make your existing recipes work as traditional hot process.

Converting Hot Process Recipes to Cold Process

Now for the reverse. Let’s say you find a beautiful hot process recipe—like that Liz Ardlady shampoo bar—and you want to make it cold process. Again, the oils and lye amounts stay the same. What changes is how you handle the superfat, and the water.

In hot process, I can reserve my superfat by holding back a specific oil until after the cook. Since the lye has already done its work by then, that reserved oil won’t saponify, meaning I know exactly which oil becomes the superfat. In cold process, saponification happens in the mold, and you can’t control which oils the lye reacts with. You still set your superfat percentage in your recipe, but it’s distributed randomly among all the oils. That’s not a bad thing—it’s just different chemistry.

The rest of the process is straightforward. You’ll reduce your water slightly back to typical cold process levels, blend to trace, pour, and insulate. You’ll likely have a smoother texture and more design options. If you were using an ingredient that behaves better in hot process (like a heat-sensitive essential oil), just remember it might behave differently when added to your CP formulation. I.E. accelerating essential oils can sieze a CP recipe where they wouldn’t bother a HP recipe.

Hot Process Cure Time Myth

So, you think your HP soap is going to cure faster than your CP do ya? Sorry. Not true

This one deserves to be said clearly: hot process soap does not have a shorter cure time than cold process. The cure phase isn’t about finishing saponification—it’s about water evaporation and bar hardness. Because hot process uses more water to begin with, it can actually benefit from a slightly longer cure, sometimes by a few extra days. The soap may be technically “safe to use” right after the cook, but if you give it time to cure, it will last longer, lather better, and feel smoother on the skin. Treat your hot process soaps with the same patience as your cold process ones. The results are worth it.

When to Choose CP vs HP Soapmaking Methods

I reach for cold process when I want design flexibility—swirls, layers, embeds, and detailed patterns. It’s also the best method for keeping botanicals and delicate natural colors bright, since you’re not applying prolonged heat. It’s also faster for me. I typically don’t heat my oils (I soap at room temp) so the prep time for CP is usually faster than the prep for HP. But, not by much.

I go for hot process when I want precision with my superfat oils, or when I’m formulating with rancid oils or other re-batching methods. Hot process can be more forgiving with certain essential oils or herbs that might otherwise react poorly during the longer (and more temperamental) saponification of cold process.

Soap Chemistry Doesn’t Play Favorites

Every fat, acid, and lye molecule behaves the same way whether you’re heating it or not. What matters is how much control you want over the timing. That’s the heart of it. Once you see that, the decision between hot and cold process stops being a rulebook issue and becomes a soaper’s choice.

If you want to take an existing recipe and explore it both ways, go for it. Adjust your water, think through your superfat, and accept that the finished look and feel will shift a little. That’s part of the fun. You’ll learn how your ingredients behave in both settings, and you’ll come away with a deeper understanding of your soap’s chemistry.

So is CP or HP soap better than the other? Absolutly not.

Whether you’re converting recipes or designing from scratch, knowing how to move between cold process and hot process gives you flexibility. It opens up a world of experimentation. With a few adjustments, you can make it fit your workflow, your tools, and your favorite style of soapmaking.

If you want to dive deeper into soap formulation for both methods, check out the Ultimate Natural Soapmaking Course. I go into the details of formulation, water content, and superfat balance, plus include recipes and more guidance for creating natural soaps that fit your style. The course is included in all Soapy Friends membership levels, and members also get live instruction and Q&A time in our monthly Soapy Chats.

See you in the next class!