Cold Process vs. Hot Process: What's the difference?

When I started making soap about 7 years ago, I started the way a lot of folks do. Melt and pour soaps were an easy starting point that gave me the opportunity to be picky about ingredients without having to learn the chemistry behind great soap. After a couple years that wasn't satisfactory anymore, and I started looking into traditional cold process and hot process soaps. When Adam and I bought Shampoochie® we got the amazing chance to learn to make cold process soap from someone who'd been making it for decades. The difference in how much control I had over ingredients, lather, color, and other qualities was night and day, and I personally never went back to melt and pour (I may dedicate a whole post to this sometime, because melt and pour is a hot button topic online, but I'll just say I loved getting started that way and greatly appreciate the artistic expression that can be achieved with melt and pour soap).

If you ask what my favorite type of soap to make is, I'll struggle to answer. Hot process soap and cold process soap have some major differences, even though they're generally made with the same ingredients. Both start with high quality oils and fats, which are combined with sodium hydroxide or lye to saponify them into solid soaps. For soaps intended for use on skin, there is always a 'superfat' or excess of oils compared to lye, so that some of the oils do not get saponified. This makes soap less drying or harsh. Both cold process soap and hot process soap require (or at least greatly benefit from) a curing period after they're made to let the soap continue to lose water and create a harder bar. Both types of soap can utilize similar ingredients from base oils to colorants to essential oils or fragrances.

Cold process soap is the most well known type of handmade soap (even though folks may not realize that's what it is), and that's mainly because of the seemingly endless possibilities with design and ingredients. Cold process soap is made by melting your solid oils (like palm oil, coconut oil, shea butter, cocoa butter, beef tallow, lard, and more), mixing in your liquid oils (things like olive, almond, apricot, avocado, cottonseed, hempseed oils, etc.), and allowing both your oil mix and your lye solution (sodium hydroxide mixed with water) to cool to anywhere between 90°F and 120°F, then mixing the oils and lye solution to begin saponification. The oils are mixed to 'trace', or a slightly thick consistency, and colorants, fragrances, or other additives are put in. Once the soap is at the desired trace, swirled patterns and pours can be created when pouring the cold process soap into the mold. After the soap solidifies it's cut and set to cure for 4-8 weeks, depending on the recipe. The following three soaps are cold process soaps (Sacra Terra, Indigo Meadow, and Into the Woods), and you can see the fun swirled designs and colors in them. The cold process soap from Reedhive Collective is all made with a similar base recipe and a certain superfat percentage. With cold process soap, the superfat is just a percentage of the total oils that doesn't get saponified. This is the reason you won't see or hear us advertising our cold process soaps as 'moisturizing'. Cold process soap can't really be moisturizing, by nature. It's a wash off product with minimal oils left over after it's made and used. It's beautiful, it smells amazing, and at Reedhive Collective you can guarantee that the ingredients are better for you than the grocery store stuff, but it's main purpose is to get you clean. If you feel surprised to read this, knowing full well the handmade soap makes your skin happier, know that you're not crazy, but it comes down to the quality of the ingredients and the lack of detergents and surfactants, and literally anything synthetic if your soap is from Reedhive Collective. (We use only essential oils for scents, and our colorants are clays and plant powders.)

Hot process soap, on the other hand, has a very cool chance to be a little more purposeful and moisturizing due to how it's made. The process starts the same, with melting and mixing your base oils and making your lye solution, but from there it takes a turn. As the name suggests, for hot process soap you want heat all the way through. Instead of letting things cool, you add all the ingredients when they're toasty hot and mix them until they reach an instant-mashed-potatoes-reminiscent texture. The batter then cooks in the crockpot (did I mention it's made in a crockpot? Or two depending on your batch size...) for as long as it takes to start becoming fluffy and then translucent. This is where the superfat difference comes into play. In hot process soap, you choose your superfats because they're added after the cook. Once the soap is cooked, you can add whichever oil or combination of oils you'd like as your superfat (and once it's a cooled a smidge we also add our colorants and essential oils), and that oil doesn't get saponified like the rest of them. Because you get to choose the type and amount of those superfats, we tend to find hot process soaps can be a bit moisturizing or serve a purpose (like our Herbal Ink, which has a castor oil, rosemary essential oil, and lavender essential oil superfat that scalps tend to love). Hot process soap also doesn't require the same curing time and can technically be used as soon as it's cut (however we normally give ours a couple weeks or more before selling just to be sure we give our customers a long lasting bar). The one downside to hot process soap (and it's pretty subjective) is that pretty, smooth, swirly designs are difficult and sometimes downright impossible. That doesn't stop us from loving these rustic, skin-loving soaps. Below are a couple favorites (Herbal Ink, Hot Tea Garden, and Waldbaden, my personal favorite.)

Did you know this about cold process soap and hot process soap? The chemistry is so fun and fascinating! If you need help choosing a soap, send us an email or a message and we'll help you narrow it down!