One of the most common question in natural soap making is “What is the difference between cold and hot process soap making?”
While they have their differences, cold and hot process are similar in 3 different areas:
Both require that you prepare a lye solution (which means dissolving NaOH or KOH beads/flakes in distilled water). In some cases, milk, tea and fruit juices can be used.
Both require that the oils are prepared – which includes melting fats and mixing them with liquid oils.
Both require that the lye and the oils are combined and at that time, the emulsion takes place (there is no lye in the finished product).
With that aside, each has their own technique, and that is where the differences come in:
COLD PROCESS SOAP
Cold process, just like hot process soap, is made with oils and lye. You combine the lye with the water (milk, tea, or even fruit juice) and in a separate container, combine the oils – when the temperature of each (oils & lye liquid) when they are within 10-15 degrees of each other. Once you combine, you use an immersion/stick blender to bring the mixture to trace, and pour into your mold.
Your essential oils or fragrances (if used) are added to the soap during trace – which happens after you mix the oils and lye, and use your immersion blender to combine.
Cold process soap goes through gel phase, or sometimes avoids gel phase (if you are making a milk soap or a soap with a high sugar content). That saponification happens over the next 24-48 hours.
Soap heats from the inside out.
Cold process soap is insulated during the gel or non-gel phase (non-gel phase would be putting in the freezer or fridge to prevent the sugars from heating up too much).
Cold process gives you more flexibility to do swirls and embeds, designs within the soap batter before you pour into the mold.
Cold process soap batter is much thinner when it is poured into the mold, and the mold is usually insulated.
Soap made through the cold process method will appear to look more shiny and polished, versus hot process soap, which tends to look more rustic.
The disadvantage of cold process soap is that once poured into the mold, it takes anywhere from 24-48 hours for that saponification process to occur. The soap is then removed from the mold, sliced into bars and cures for 4-6 weeks.
The longer the cure time, the harder the bar, as the water/liquid within the soap has an opportunity to evaporate.
HOT PROCESS SOAP
Similar to cold process soap, hot process soap is also made with oils and lye – but instead of stick blending until it hits trace (a thick batter) and being poured into a mold, it is mixed and heated in the slow cooker, somewhere between 140 degrees F and 176 degrees F. That slow cooker helps that batter go through saponification until the soap reaches gel phase (1-2 hours).
Unlike cold process soap, the essential oils or fragrance/additives are added after gel phase.
Hot process soap batter is thick and lumpy, and is poured into the soap mold after saponification has finished. From there it hardens into a bar.
Hot process soap has a rustic appearance – it doesn’t look as polished and finished as cold process soap.
It is difficult to do swirls and designs in hot process soap – although it’s not impossible per-se, it’s a challenge.
Cold process soap heats from the inside out, while hot process soap heats from the outside in.
Unlike cold process soap, that lasts for 4-6 weeks, hot process soap can be used immediately after it hardens. However, by giving it a week or so, the bar can harden up considerably and allow an opportunity for the water to evaporate and will make a harder, longer-lasting bar.
There are many advantages of hot process, one being the shortened cure time (you can use it almost immediately after they have hardened) but many avoid hot process because the look isn’t as clean and polished as cold process soaps.
Many people have said though that the scents tend to last longer in hot process since they are added after saponification – that chemical reaction between the lye and oils can sometimes have a difference on the scent you wish to use.
All in all, there are lots of pros and cons with both – and neither is better than the other. In the end, it’s truly a matter of personal preference.