Cold Process vs. Boiling: A Comparison of Soap-Making Methods

The cold process and the boiling method (hot process) are the two most commonly used techniques in soap production. Both are based on the saponification reaction, in which fats react with sodium hydroxide (NaOH) to form soap and glycerin. The differences between these two methods—along with the advantages and limitations of each—enable informed decision-making when choosing natural soap.

This article is not a product endorsement. Its purpose is to present the scientific framework of soap-making methods. The information contained herein does not make any therapeutic claims.

Saponification: The Common Chemistry Behind Both Methods

Saponification is the process by which fats with a triglyceride structure react with a strong base (NaOH or KOH) to form fatty acid salts (soap) and glycerol. In a simplified equation:

Triglyceride + NaOH → Soap (sodium salt of fatty acid) + Glycerin

This reaction is exothermic; that is, it spontaneously generates heat. While the internal heat generated is considered sufficient for saponification in the cold process, an external heat source is used in the boiling method to accelerate the reaction (Spitz, 2016).

Every oil used in soap making has its own specific saponification value (SAP value). This value expresses, in milligrams, the amount of NaOH required to completely saponify 1 gram of oil. For example, while the SAP value of olive oil is approximately 0.1345, that of coconut oil is 0.1910.

What is the cold process method?

In the cold process method of soap making, oils are typically combined with an alkali solution at temperatures ranging from 27–50 °C. When the mixture reaches a pudding-like consistency known as “trace,” it is poured into molds. The saponification reaction continues inside the mold at room temperature for 24–48 hours (Typology, 2024).

Curing Process

Soaps removed from the molds are left to cure in a cool, shaded environment for 4–6 weeks. During this time, two critical processes occur: the soap hardens as water evaporates, and the pH stabilizes. Soap used before curing is complete may irritate the skin because unreacted alkali residues may still be present.

Advantages of the Cold Process Method

Since the process takes place at low temperatures, it helps preserve the unsaturated fatty acids in the oils. A study published in the journal *Foods* in 2018 found that in soaps produced by cold saponification, oleic acid (C18:1), linoleic acid (C18:2), and α-linolenic acid (C18:3) were preserved at rates of 60–100% without saponification (Prieto Vidal et al., 2018). This imparts natural moisturizing properties to the soap.

In addition, the cold process better preserves the fragrance profile of essential oils. Since essential oils added after the trace stage are not exposed to high heat, evaporation loss is kept to a minimum.

Limitations of the Cold Process Method

The most notable limitation is the 4–6-week curing requirement. Furthermore, superfat control is less precise compared to the boiling method—because it is impossible to predict in advance which superfat oils will react during saponification.

What is the boiling method (hot process)?

The boiling method is a traditional soap-making technique that has been practiced in Anatolia for centuries. Oils are combined with NaOH and water in large kettles and boiled at 80–100 °C while being continuously stirred. Saponification is completed inside the kettle—the soap separates from the solution and rises to the surface.

The olive oil production centers in Nizip (Gaziantep), Antakya, and the Aegean Region are the most established practitioners of this method. Nizip soap received a geographical indication registration in 2013 and is exported to 25 countries with an annual production of approximately 50,000 metric tons.

Modern hot process

Unlike traditional boiling, in the modern hot process, the soap paste is cooked at 60–70 °C for 1–2 hours using a slow cooker or a double-boiler setup. In this method, the temperature is lower than in traditional boiling but significantly higher than in the cold process.

Advantages of the boiling method

Since saponification is completed inside the kettle or cooker, the soap is theoretically ready for use as soon as it is removed from the mold. However, in practice, an additional curing period of 1–2 weeks is recommended to allow the soap to harden and for the water to evaporate. Since superfat oils can be added after saponification is complete, it is possible to control which oils remain unsaponified—this provides an advantage in terms of achieving the desired moisturizing profile.

Limitations of the Boiling Method

High temperatures can lead to the loss of heat-sensitive components (particularly the volatile terpene components of essential oils). Since the soap batter is dark and thick, it is difficult to create fine details and decorative designs (such as layered pours and marbling) during the pouring stage. The surface of the final product is typically rough and has a “rustic” appearance.

Comparison: Cold Process vs. Boiling

What happens during the curing process?

Curing is a critical stage that determines soap quality and applies to both methods—though it is much longer and more decisive in the cold process.

pH change

The pH of freshly poured cold-process soap ranges from 12 to 14 (strongly basic). During curing, saponification is completed and free alkali decreases; the pH of mature soap typically drops to the 9–10 range. Although this level is higher than the skin’s natural acid mantle (pH ~5.5), it is considered safe for brief contact—it is rinsed off.

Crystal Structure and Hardening

During curing, soap molecules arrange themselves into ordered crystalline structures (lamellar phases). As the water evaporates, the bar becomes denser and harder. A harder bar melts more slowly, lasts longer, and produces a richer lather. For this reason, a minimum curing period of 4 weeks is a direct determinant of the soap’s performance.

What should consumers know?

The fact that a soap is “natural” is not, by itself, an indicator of quality. The production method significantly determines the soap’s effect on the skin. Points to consider:

Glycerin content: The presence of saponified oil names such as “Glycerin” or “Sodium Olivate” on the label indicates that natural glycerin remains in the soap. In industrial saponification products like “Sodium Tallowate,” glycerin is typically removed.

Curing information: Reputable cold-process soap makers specify the curing period. Phrases like “minimum 45 days” or “6 weeks” indicate that the manufacturer has mastered the soap-making chemistry and adheres to quality standards.

INCI: According to the EU Cosmetics Regulation (EC/1223/2009), all cosmetic products—including soaps—must include an (INCI) list. This list lists the product’s ingredients in descending order of concentration. For detailed information on reading the “INCI” list, please refer to the guide at ChaileapediaINCI.

Why does Chailea prefer the cold process method?

All of Chailea’s bar soaps are produced using the cold process method. There are several key chemical reasons behind this choice: Chailea to preserve the bioactive components of essential oils such as St. John’s wort oil, tea tree oil, and lavender oil used in its recipes; to ensure that natural glycerin remains in the soap; and to allow the crystalline structure to fully mature during the 45-day curing period.

Each soap bar is cured for a minimum of 45 days. During this time, the pH level decreases, the bar hardens, and the soap develops its distinctive scent, texture, and lather. Tea Tree Soap and Activated Charcoal Soap, produced using the cold process method, are examples of these recipes.

The Glycerin Issue: Why Is It Important?

Glycerin (glycerol) is naturally produced alongside soap during the saponification reaction. Glycerin is a powerful humectant—it draws moisture from its surroundings to support hydration of the skin’s outermost layer (stratum corneum).

In the cold process, glycerin remains in the soap bar and becomes part of the final product. In traditional industrial boiling methods, however, glycerin is leached into the solution during the “salting out” stage and sold as a separate product—this is one of the reasons industrial soaps often feel more drying.

In the modern hot process (HP) method, the washing step is omitted, so the glycerin remains in the soap. The difference lies between traditional boiling and the modern HP method—this is where the terminology confusion arises.

Frequently Asked Questions

What is cold process soap?

The cold process is a production technique in which oils are combined with sodium hydroxide (NaOH) at low temperatures (such as 27–50 °C) to allow the saponification reaction to complete within the mold. Saponification takes 24–48 hours, but the soap requires a curing period of 4–6 weeks before it is ready for use.

What is the most significant difference between the boiling method and the cold process method?

The most fundamental difference lies in the temperature and the location where saponification is completed. In the cold process, saponification takes place in the mold, whereas in the hot process, it occurs in a kettle or cooker. This difference directly affects the curing time, the surface texture, and the degree to which heat-sensitive ingredients are preserved.

Why is glycerin in soap important?

Glycerin is a humectant that forms naturally during saponification. It helps maintain the skin’s moisture. In the cold process, glycerin remains in the soap; in traditional industrial boiling, it can be removed through a brine rinse.

Why is cold-process soap left to cure for 4–6 weeks?

Two key processes occur during the curing period: the mold hardens as excess water in the soap evaporates, and saponification is fully completed, bringing the pH level down to a safe range for use. If curing is skipped, the soap may be both soft and potentially irritating.

A Historical Perspective: From Anatolia to the World

The history of soap production dates back to Babylonian tablets from around 2800 BCE. However, a modern understanding of the chemistry of saponification was only made possible in the 19th century through Michel Eugène Chevreul’s studies on fatty acids. By elucidating the structure of triglycerides, Chevreul laid the scientific foundation for the saponification reaction.

In Anatolia, soap production has been carried out using the boiling method for centuries. Antakya bay leaf soap, Nizip olive oil soap, and Gaziantep soap are living examples of this tradition. These soaps are produced by boiling lye (NaOH) and salt in large copper or steel cauldrons over a wood fire. After boiling, the soap paste is spread out in an open area, allowed to harden under natural conditions, and then cured for 3–6 months while arranged in a dome shape.

The cold process, on the other hand, gained popularity in the second half of the 20th century, particularly with the rise of the artisanal soap movement in North America and Europe. Today, the vast majority of boutique soap makers worldwide prefer the cold process—primarily because it protects essential oils and natural ingredients from heat and offers greater creative design possibilities.

There is no “right” or “wrong” between the two methods. While traditional boiling is suitable for industrial-scale production and certain types of oils (especially high-acid oils like pomace oil), the cold process is ideal for producers aiming to maximize the preservation of ingredient quality, even in small-scale production.

References

1. Spitz, L. (2016). Soap Manufacturing Technology (2nd ed.). AOCS Press.

2. Prieto Vidal N, Adigun OA, Pham TH, et al. (2018). The Effects of Cold Saponification on the Unsaponified Fatty Acid Composition and Sensory Perception of Commercial Natural Herbal Soaps. Molecules, 23(9), 2356. PMID: 30223479. doi:10.3390/molecules23092356

3. EU Regulation (EC) No. 1223/2009 on Cosmetic Products.

4. Turkish Cosmetics Regulation (Official Gazette No. 25823, dated May 25, 2005).