What is cold process soap and how cold process soap making works

TL;DR: Cold process soap is created through a chemical reaction (saponification) between oils and sodium hydroxide. The result is soap and glycerin — no additives, no heating. The process takes 24–48 hours in the mold and then 4–6 weeks of curing.

Every bar of cold process soap was created by a chemical reaction that takes place right in your bowl. This is not cooking, remelting or mixing ready-made bases. The oils you prepared and the sodium hydroxide you dissolved in water transform into something entirely new when they meet — soap and glycerin. This process is called saponification and it happens at your kitchen table just as it does in an industrial facility.

This page will guide you through the entire process from the basics: what happens chemically, why soap needs to cure, what the individual production phases look like, and what to prepare for on your first attempt.

What are the three soap making methods and what is the difference between them?

Mýdlo lze doma vyrobit třemi způsoby a každý z nich je vhodný pro jiný typ výrobce. Nejjednoduší je melt & pour, nejoblíbenější je cold process a pokročilejší mydláři pracují s hot process.

Melt & pour (M&P) is the simplest method. You buy a ready-made soap base — transparent, white or goat milk — melt it, mix in colors and fragrances, pour into molds. No hydroxide, no risk. The result is done in an hour. The drawback: no control over the composition, the base is commercially made soap and ingredients are more expensive. Suitable for children, gifts and creative experiments.

Hot process (HP) uses the same chemistry as the cold process, but you "cook" the whole process with direct heat - in the oven, slow cooker or microwave. The soap is ready to use sooner (2-7 days), but the surface tends to be rougher and more rustic. Adding sensitive ingredients after cooking is safer because the hydroxide is already consumed. The method is useful for experimenting with colors and textures.

Cold process (CP) is the method this entire website is dedicated to. You mix oils and sodium hydroxide at a controlled temperature, pour into the mold and let them react over hours and days. The soap then cures for 4–6 weeks. Advantages: complete control over ingredients, smoothest surface, widest design possibilities (swirls, layers, embeds). Disadvantage: time and the need to work with lye.

How does saponification work?

Saponification is a fundamental chemical reaction between fat (or oil) and a strong base (hydroxide) that produces soap and glycerin. Simply put: fat consists of a glycerol molecule with three fatty acid chains attached — hence the name triglyceride.

Sodium hydroxide breaks this molecule apart: glycerol is released as free glycerin and each fatty acid chain bonds with sodium from the hydroxide, creating a soap molecule. The soap molecule is interesting because it has a dual character: one end loves water (hydrophilic), the other loves fat (lipophilic). This allows it to grab dirt and oils during washing and rinse them away with water.

Glycerin is produced as a byproduct and remains in homemade soap — this is one of the reasons it is more natural and conditioning than industrial soap, from which glycerin is usually removed and sold to the cosmetics industry. Saponification is not instant — the reaction begins immediately upon mixing, but full completion takes 24–48 hours in the warmth of the mold. That is why fresh soap must not come into contact with skin — the hydroxide may not have fully reacted yet.

When to use NaOH and when KOH?

For making solid soap using the cold process method, you always use NaOH — sodium hydroxide (caustic soda). It is available in pharmacies or drugstores under the name "waste cleaner“ (100% NaOH, without additives). In foreign sources you will see it as lye or caustic soda.

KOH — potassium hydroxide (potassium lye) is used exclusively for making liquid soap and soap paste. It is more expensive, harder to find and dosage calculation is more complex (purity is typically 90%, not 100%). These two substances cannot be interchanged — if you add KOH to a recipe designed for NaOH, the soap will set as a paste or remain liquid.

Sodium hydroxide is a dangerous substance — see the dedicated page Lye safety. It causes chemical burns and reacts with moisture. However, with proper handling it is completely safe and soap makers work with it daily without incidents.

How does the production process work step by step?

Cold process soap production consists of several phases in sequential order. Here is an overview — you will find the detailed first recipe on the page Your first recipe.

Workspace preparation: Protective equipment, scale, molds, thermometer — everything prepared in advance. Once you start with lye, do not leave the workspace.

Weighing and preparing the hydroxide: Sodium hydroxide is weighed into a dry container. Water is weighed separately — always pour lye into water, never the other way around (exothermic reaction releases heat up to 80–90 °C and splashing would be dangerous). The solution is left to cool.

Melting and preparing oils: Solid oils (coconut, palm, tallow, cocoa butter) are melted, then liquid oils mixed in. The resulting oil mixture temperature should be 35–45 °C.

Tempering both components: The lye solution and oils should be at a similar temperature when mixing (within 10 °C difference). Exact temperatures depend on the recipe and technique. Both components are then processed simultaneously for even mixing.

Mixing to trace: The lye solution is poured into the oils and mixing begins. Alternate stick blender pulses with hand stirring until the mixture reaches trace — visible marks on the surface. Takes 5–20 minutes depending on recipe and temperature.

Adding additives: At light trace, heat-sensitive additives are added (essential oils, natural colorants, honey, milk). At medium/heavy trace, solid decorative elements are added. Proper timing affects the final appearance.

Pouring into mold and resting: The mold is covered with a lid or plastic wrap and wrapped in towels or a blanket. Temperature in the core rises during the first 24 hours — this is the desirable gel phase that improves colors and texture.

Unmolding and cutting: After 24–48 hours (depending on the recipe) unmold the soap and cut it. If still too soft, leave it longer. Soap at this stage may still contain residual alkaline substances.

Curing: The soap sits on a rack in a well-ventilated place for 4–6 weeks. During this time, excess water evaporates and the alkaline process completes. The soap pH drops from strongly alkaline (12–14) to acceptable (9–10).

What is trace and how do you recognize it?

Trace is a state of a mixture where the oils and the leaching solution have begun to emulsify - they are mixed so much that they no longer separate. The name comes from the English to trace — when you draw a spoon across the surface, it leaves a visible mark. Trace is divided into three stages, each suitable for different techniques.

Light trace — the mixture has the consistency of thin pudding or soup. The mark is barely visible, disappearing within seconds. Ideal for multi-color techniques, swirls and complex patterns. It fades too quickly only when an essential oil that accelerates the reaction is mixed in. At this stage, you have the most flexible working time.

Medium trace — consistency of thick pudding. The mark holds for several seconds. The most versatile stage for most recipes — thick enough to keep colors separate, but still liquid enough for pouring and surface decorations. This is the ideal point for beginners.

Heavy trace — consistency of thick cream or pudding. The mark is immediate and permanent. Suitable for piping, rustic tops and recipes where complex design is not desired. Caution: pouring into the mold is more difficult, the surface may be rough. This stage is chosen with care.

False trace — a trace imitation caused by solidification of fats (coconut, palm), not emulsification. If you see an apparent trace but the mixture is noticeably warmer than room temperature, heat it and remix. False trace will ruin your design if you confuse it with the real thing.

A stick blender reaches trace within 2–5 minutes. Hand stirring takes tens of minutes to hours — today it is only used by those who want specific results (for example with creamy recipes).

Why is it called the gel phase?

Gel phase occurs in the first 4–8 hours after pouring into the mold. Heat from saponification causes the interior to become transparent or translucent with a gelatinous texture — this is a normal and desirable state. Soap that has gone through gel phase has optimal properties: deeper colors, smoother surface and harder structure after unmolding.

Soap that has gone through gel phase has three distinct advantages over soap without it: colors are deeper and brighter, the surface is smoother and glossier, and the soap is harder after unmolding. If you want vivid colors and a professional look, gel phase is desirable. It is caused by gradual temperature increase inside the mold.

How to promote gel phase: Wrap the mold in towels or place it in an oven preheated to 50 °C for 1 hour (then turn off and leave in the closed oven). Silicone molds retain gel phase better than wooden ones. For a more even gel phase, ensure good thermal insulation of the mold.

When to deliberately prevent gel phase: When using milk (goat, rice, oat) or natural sugars, where heat can cause sugar scorching and darkening. In these cases, place the mold in the freezer for 1 hour after pouring to keep the temperature low and protect delicate ingredients.

Partial gel — gel phase occurred in only part of the soap (typically in the center) — leaving visible darker circles. Aesthetically less attractive, but the soap is perfectly fine. Prevent it by consistently tempering the entire mold and uniform insulation.

How long and why does soap need to cure?

Fresh soap after 48 hours is technically "finished" in the sense that the saponification has taken place. We still won't use it - it needs to age for at least 4-6 weeks. During the aging process, three critical processes occur that improve quality.

Water evaporation: During production we add approximately 35–38% water (relative to NaOH). This water must evaporate. Fresh soap has a water content of about 30–35%, cured soap 15–20%. Less water means harder and more durable soap, richer lather and longer life in the bathroom.

Crystallization of soap molecules: During curing, soap molecules arrange into more consistent crystalline structures. The result is firmer, denser soap with better lather. The process is slow and requires time — nothing can be rushed.

pH drop: Freshly poured soap has a pH of 12–14. After 48 hours it is still 11–12. Only cured soap has a pH of 9–10, which is acceptable for skin (healthy skin has pH 4.5–5.5, so soap is alkaline but temporarily). A pH strip test or phenolphthalein drops show whether the soap is ready.

Practical test: the tip-of-the-tongue test. If I were to recommend only one test to soapmakers in the Czech Republic, it would be this: lightly touch the tip of your tongue to the ripe soap. If you feel a "prick&#8220like a battery, the soap contains free hydroxide and is not yet ready. If you don't feel anything, it's ready. Yes, it's a strange test, but it's safe (a small amount of ripe soap won't harm your tongue) and 100% reliable.

How long to actually wait? Curing time varies by recipe. Recipes with high coconut content (50%+) require at least 4 weeks. Standard recipes (30% coconut, rest olive and other oils) cure for 4–6 weeks. Castile soap (100% olive oil) requires 6 months to 1 year. Recipes with tallow are ready in 4 weeks but even better after 6–8 weeks. Don't be discouraged by the wait — soap on the shelf needs no care, just good ventilation.

Frequently asked questions

Do I need to use a precise scale? Yes, without exception. Sodium hydroxide is a strong base — every extra gram produces caustic soap, every gram less means residual fat in the recipe (this is intentional as superfat but must be controlled). Kitchen scales with 1 g precision are the minimum, 0.1 g precision is ideal for small batches.

Where can I buy sodium hydroxide? Pharmacies (as drain cleaner or under the name NaOH), drugstores (look for drain cleaners in bead or flake form with 100% NaOH content), soap making e-shops. Important: buy only 100% NaOH, not mixtures with additives for drain pipes.

What should I do if lye gets on my skin? Immediately rinse with cold water for 15–20 minutes. Do not wipe, only rinse. Acid (vinegar, lemon juice) for neutralization is a myth — it causes additional irritation. See Lye Safety for more details.

Can I use any cookware? No. Aluminum, tin and zinc react with sodium hydroxide and release toxic hydrogen. Use only: stainless steel, plastic containers (PP — polypropylene or HDPE — high-density polyethylene), glass (caution — glass bowls can crack with rapid heating). The safest are polypropylene (PP) containers marked with number 5.

Can I use essential oils? Yes, but mindful of the recommended percentage (usually 2–3% of total oil weight) and behavior at trace — some essential oils (cinnamon, clove, eucalyptus) dramatically accelerate thickening. For first recipes, I recommend lavender or peppermint, which are gentle and predictable.

Why does my soap have a white coating on the surface? Soda ash (sodium carbonate) is a white coating on the soap surface that forms from oxidation of the surface by atmospheric CO₂. The soap is perfectly fine, just less attractive. Prevent it by covering the mold with plastic wrap immediately after pouring or spraying alcohol (70% IPA) on the surface.