Cold Process Soap: Everything You Need to Know About Making Your Own Soap

Cold process soap making is one of the oldest and most rewarding ways to make handmade soap. It allows you to choose your own oils, butters, colours, fragrances and additives, giving you complete control over what goes into your soap.

But what exactly is cold process soap? Why do we use sodium hydroxide? What happens during saponification? And why does the soap need to cure for several weeks?

Let’s take a closer look at the fascinating science and art behind cold process soap making.

What Is Cold Process Soap?

Cold process soap is soap made by combining fats or oils with sodium hydroxide (lye). The chemical reaction between the oils and sodium hydroxide is called saponification.

Unlike hot process soap making, cold process soap is not deliberately cooked at high temperatures to speed up the reaction.

Once the oils, lye solution and other ingredients have been combined and the mixture reaches trace, the soap is poured into a mould and allowed to harden and continue curing.

Although sodium hydroxide is essential for making solid soap, properly formulated cold process soap does not contain free sodium hydroxide when the saponification and curing process has been completed.

The Science Behind Cold Process Soap

At the heart of soap making is a chemical reaction called saponification.

Most vegetable oils and animal fats contain triglycerides. When these fats react with sodium hydroxide, they are converted into:

Soap + glycerin

The simplified reaction is:

Oils/Fats + Sodium Hydroxide → Soap + Glycerin

This is why sodium hydroxide is not simply an ingredient that can be left out of traditional cold process soap.

You cannot make true cold process soap by simply mixing oils together. The lye is required to chemically transform the oils into soap.

What Is Sodium Hydroxide?

Sodium hydroxide, commonly called lye or caustic soda, is a highly alkaline substance.

It must be handled carefully because it can cause serious burns when it comes into contact with skin or eyes.

However, sodium hydroxide is also an essential part of traditional solid soap making.

The important principle is:

You cannot make true soap from oils without an alkali.

During saponification, the sodium hydroxide reacts with the fatty acids in the oils. A correctly calculated recipe is designed so that the sodium hydroxide is consumed during the reaction.

This is why accurate formulation is extremely important.

Why Can’t You Just Guess the Amount of Lye?

This is one of the most important rules in soap making.

Different oils require different amounts of sodium hydroxide to turn them into soap.

For example, coconut oil, olive oil, shea butter and castor oil all have different SAP values.

A soap maker therefore needs to calculate the correct amount of sodium hydroxide for the specific combination and weight of oils being used.

A lye calculator should always be used when formulating a cold process soap recipe.

Never estimate the amount of sodium hydroxide by eye.

What Is Superfatting?

Superfatting means formulating a recipe with slightly less sodium hydroxide than would theoretically be required to saponify 100% of the oils.

This leaves a small proportion of unsaponified oils in the finished soap.

For example, a recipe might be formulated with a 5% superfat.

Superfatting is commonly used to create a milder-feeling bar and to provide a margin against minor formulation variations.

However, more superfat is not automatically better. Too much unsaponified oil can make a bar softer, greasy or more prone to rancidity.

The Oils Used in Cold Process Soap

One of the wonderful things about cold process soap is the enormous variety of oils and butters that can be used.

Each oil contributes different properties to the finished bar.

Olive Oil

Olive oil is popular for creating mild, conditioning soap.

Soap made with a high percentage of olive oil can take longer to harden and may require a longer cure.

Coconut Oil

Coconut oil contributes cleansing power and large, fluffy bubbles.

Too much coconut oil can produce a bar that feels very cleansing and potentially drying for some skin types.

Palm Oil

Palm oil can contribute hardness, stability and longevity to a bar.

If palm oil is used, many soap makers choose responsibly sourced palm oil.

Shea Butter

Shea butter can contribute hardness, creaminess and a luxurious feel.

It is often used at relatively modest percentages rather than making up the entire recipe.

Cocoa Butter

Cocoa butter produces a hard, long-lasting bar and contributes a creamy lather.

Castor Oil

Castor oil is usually used in small quantities because it helps support stable, creamy lather and can improve the quality of bubbles produced by other oils.

Sweet Almond Oil

Sweet almond oil can contribute conditioning properties and a pleasant skin feel.

Avocado Oil

Avocado oil is valued by many soap makers for its conditioning qualities.

There are many other oils and butters that can be incorporated into soap recipes, but every change to the oil blend affects the final soap and therefore requires the recipe to be recalculated.

Why You Should Never Substitute Oils Without Recalculating

This is an extremely important point for beginners.

If a recipe calls for 500 g of olive oil, you cannot simply replace it with 500 g of coconut oil.

The two oils have different SAP values and produce very different soap.

Changing an oil means changing the formulation.

Always recalculate the recipe whenever you change an oil or butter.

What Is Trace?

Trace is the stage where the oils and lye solution have emulsified sufficiently for the soap batter to begin thickening.

It is called “trace” because the batter can leave a visible trail or mark on the surface.

There are different stages of trace.

Light Trace

The batter is still relatively fluid but has emulsified.

This is often the ideal stage for intricate swirls and designs.

Medium Trace

The batter has become noticeably thicker.

It can be useful for certain embeds, layers and textured designs.

Thick Trace

The batter is considerably thicker and may resemble pudding.

It can be useful for textured tops, but it may become difficult to pour or create detailed designs.

Knowing how quickly a recipe traces is an important soap-making skill.

What Is Emulsification?

Emulsification is the point where the oils and lye solution have been thoroughly combined and are no longer separating.

The soap batter needs to remain properly emulsified so that the saponification process can proceed evenly.

An immersion blender is commonly used to help achieve emulsification and trace.

However, over-blending can cause the batter to become thick very quickly.

The Basic Cold Process Soap-Making Process

While every recipe is different, the general process follows these stages.

1. Weigh Your Ingredients

Use an accurate digital scale.

Soap should be measured by weight, not volume.

This includes oils, water, sodium hydroxide, fragrances and additives.

Accurate weighing is essential for safe and consistent soap making.

2. Prepare Your Equipment

Before beginning, have everything ready.

Typical equipment includes:

• Accurate digital scale
• Heat-resistant container for the lye solution
• Stainless steel or suitable soap-making container
• Thermometer
• Immersion blender
• Silicone spatula
• Soap mould
• Measuring and mixing equipment
• Safety goggles
• Chemical-resistant gloves
• Protective clothing

Avoid aluminium when working with sodium hydroxide because it can react with lye.

3. Prepare the Lye Solution

This is one of the most important safety steps.

Always add sodium hydroxide to water — never water to sodium hydroxide.

The reaction produces considerable heat and fumes.

Work in a well-ventilated area and keep children and pets away from the work area.

Never lean directly over the container while mixing the lye solution.

Allow the solution to cool according to the requirements of your recipe.

4. Prepare the Oils

Solid oils and butters are usually gently melted.

Liquid oils are then added according to the formulation.

The oils should be at an appropriate temperature for the recipe before combining them with the lye solution.

5. Combine the Lye Solution and Oils

Once the temperatures and conditions are appropriate, the lye solution is carefully combined with the oils.

The mixture is then blended until emulsified and the desired trace is reached.

6. Add Colour and Fragrance

Once the soap batter reaches the appropriate stage, colourants, fragrance oils, essential oils or other suitable additives can be incorporated.

Not every fragrance behaves the same way.

Some fragrances can accelerate trace dramatically, while others may cause the soap batter to thicken very quickly or behave unpredictably.

Always check supplier information and test unfamiliar fragrances in a small batch.

7. Pour Into the Mould

The soap batter is poured into the chosen mould.

This is where soap making becomes particularly creative.

You can create:

• Layers
• Swirls
• Ombre effects
• Two-tone designs
• Textured tops
• Embeds
• Natural-looking bars
• Seasonal designs

8. Insulate or Allow the Soap to Cool

Depending on the recipe and desired appearance, the mould may be insulated or left uncovered.

Soap can heat up significantly after pouring because saponification continues inside the mould.

Some recipes may even enter a stage called gel phase.

What Is Gel Phase?

Gel phase occurs when the soap becomes hot enough internally for the soap batter to develop a translucent, gel-like appearance.

A soap that goes through gel phase can develop a slightly different appearance and may have stronger, more vibrant colours.

Soap does not have to go through gel phase to become soap.

Whether you encourage or avoid gel phase is largely a matter of recipe and personal preference.

Cutting the Soap

Once the soap has hardened sufficiently, it can be removed from the mould and cut into bars.

The timing depends on the recipe.

Some soaps can be cut within a day, while softer recipes may require longer.

Cutting too early can result in misshapen bars, while waiting too long can make some recipes difficult to cut cleanly.

Why Does Cold Process Soap Need to Cure?

One of the biggest differences between cold process soap and many commercially manufactured soaps is the curing period.

Freshly made cold process soap may be firm enough to remove from the mould and cut, but that does not mean it is finished.

A typical cold process soap is cured for approximately 4–6 weeks, although some recipes benefit from a longer cure.

During curing:

• Excess water evaporates.
• The bar becomes harder.
• The soap becomes longer-lasting.
• The lather generally improves.
• The bar becomes milder-feeling over time.

Some high-olive-oil recipes can benefit from considerably longer curing periods.

Cure Time Is Not the Same as Saponification

This is another common misunderstanding.

Saponification begins when the oils and sodium hydroxide are mixed.

The cure period is the time after the soap has been made during which water continues to evaporate and the physical characteristics of the bar improve.

So:

Saponification = chemical reaction

Curing = physical maturation and water loss

They are related but they are not the same thing.

What Happens If You Use the Soap Too Soon?

A freshly made bar may be technically soap, but it is usually much softer than a properly cured bar.

It may:

• Melt away quickly.
• Produce less desirable lather.
• Feel harsher than expected.
• Become soft in the shower.
• Have a shorter lifespan.

Patience is one of the most important ingredients in handmade soap making.

How Should Cold Process Soap Be Stored During Curing?

Place freshly cut bars somewhere:

• Cool
• Dry
• Well ventilated

Bars should have enough space around them for air to circulate.

A wire rack is ideal.

Do not store curing soap in an airtight container.

The purpose of curing is partly to allow water to evaporate.

What Causes Soda Ash?

Sometimes a white, powdery coating develops on the surface of cold process soap.

This is commonly called soda ash.

It is usually cosmetic rather than a sign that the soap has failed.

Soda ash is more likely to occur under certain conditions, including some formulations and exposure to air during the early stages of the soap-making process.

It can often be reduced through formulation and process adjustments.

Why Does Soap Crack or Develop a Strange Texture?

Cold process soap can behave differently from batch to batch.

Possible causes include:

• Excessive heat
• Fragrance acceleration
• Incorrect water concentration
• Certain additives
• Incorrect mixing
• Pouring at unsuitable temperatures
• Insufficient emulsification
• Overheating during gel phase

This is why keeping accurate batch records is so useful.

Record your:

• Oil quantities
• Lye quantity
• Water quantity
• Temperatures
• Fragrance
• Colourants
• Additives
• Trace
• Mould
• Cure time
• Final observations

Your soap notebook can become one of your most valuable tools.

Common Cold Process Soap Problems

Soap Batter Separates

Separation can indicate that the oils and lye solution have not remained properly emulsified.

The recipe, temperatures, mixing and additives should all be reviewed.

Soap Traces Too Quickly

Possible causes include:

• Certain fragrance oils
• High percentages of hard fats
• High temperatures
• Excessive blending
• Certain additives

A fragrance that behaves perfectly in one recipe may behave very differently in another.

Soap Is Too Soft

Possible causes include:

• High water content
• A high percentage of soft oils
• Cutting too early
• Insufficient curing
• Formulation problems

Allowing adequate cure time often makes a significant difference.

Soap Is Crumbly

A crumbly bar may be associated with formulation problems, excessive hardness, insufficient liquid, overheating or other processing issues.

The entire recipe and process should be reviewed rather than simply adding more oil to the next batch.

Soap Has Orange Spots

Orange spots can sometimes indicate DOS — dreaded orange spots — which are associated with rancidity.

Possible contributors include:

• Oxidised or old oils
• Certain oil choices
• Excessive heat
• Water quality
• Contamination
• Storage conditions

Using fresh, good-quality oils and storing them correctly can help reduce the risk.

Can You Add Botanicals to Cold Process Soap?

Yes, many soap makers incorporate botanical ingredients.

Examples include:

• Calendula
• Lavender
• Chamomile
• Rose
• Oatmeal
• Coffee
• Herbs
• Clays

However, adding plant material can affect the soap’s appearance, texture and shelf life.

Fresh ingredients containing water can also introduce additional formulation and preservation considerations.

More is not necessarily better.

Can You Add Essential Oils?

Yes, suitable essential oils can be used in cold process soap.

However, essential oils are highly concentrated substances and should be used at appropriate levels.

Not every essential oil is suitable for every application.

Some have specific usage restrictions or safety considerations.

Always consult reliable supplier information and appropriate fragrance/essential-oil usage guidelines before adding them to soap.

Can You Use Fragrance Oils?

Yes.

Fragrance oils specifically intended for soap making can produce wonderful scents.

But fragrance oils can behave very differently in cold process soap.

A fragrance may:

• Accelerate trace
• Discolour the soap
• Cause ricing
• Affect the final colour
• Remain stable and easy to work with

Always check the manufacturer’s usage recommendations and test unfamiliar fragrances.

What Is Ricing?

Ricing occurs when small lumps or particles form in the soap batter after fragrance is added.

It can look similar to cooked rice.

Some fragrances are more likely to cause this reaction.

The good news is that ricing does not necessarily mean the batch is ruined, but it can make the soap difficult to work with.

What Is Seizing?

Seizing occurs when soap batter suddenly becomes extremely thick.

Instead of flowing like a liquid, it can become almost solid in the container.

This can happen because of:

• Certain fragrances
• Very high temperatures
• Certain additives
• Recipe composition
• Over-processing

A soap maker needs to work quickly and carefully when using ingredients known to accelerate trace.

Can You Make Cold Process Soap Without Lye?

No.

This is one of the most common myths about soap making.

You can make melt-and-pour soap without handling lye yourself, because the soap base has already undergone saponification.

But if you are making soap from oils and fats yourself using the cold process method, an alkali is required.

For solid soap, that alkali is normally sodium hydroxide.

Cold Process vs Melt and Pour Soap

Cold Process Soap

• Made from oils/fats and sodium hydroxide
• Requires careful lye handling
• Gives complete control over the formulation
• Requires curing
• Allows extensive customisation
• Creates a true handmade soap from scratch

Melt and Pour Soap

• Uses a pre-made soap base
• No handling of sodium hydroxide during the soap-making process
• Does not require a traditional 4–6 week cure
• Easy for beginners
• Can be melted, coloured, fragranced and poured into moulds
• Excellent for simple DIY soap projects

Neither method is “better” for everyone.

They are simply different soap-making methods.

Essential Safety Rules

Cold process soap should always be treated as a serious chemical-making process.

Remember:

Always wear safety goggles.

Always wear appropriate gloves.

Work in a well-ventilated area.

Keep children and pets away.

Never add water to sodium hydroxide.

Always add sodium hydroxide to water.

Never use aluminium equipment with sodium hydroxide.

Use an accurate digital scale.

Never guess the amount of lye.

Always use a reliable lye calculator.

Label your lye solution and ingredients clearly.

Never leave an active lye solution unattended.

If sodium hydroxide comes into contact with skin or eyes, follow appropriate emergency first-aid guidance and seek medical assistance when necessary.

Why Do People Love Making Cold Process Soap?

Cold process soap is much more than simply making a bar of soap.

It combines:

Science + creativity + chemistry + craftsmanship

You can design your own recipes, experiment with oils and butters, create beautiful colours and patterns, develop unique fragrances and make soap specifically for your own preferences.

And there is something incredibly satisfying about turning simple oils into a beautiful, usable bar of soap.

Final Thoughts

Cold process soap making may seem complicated when you first start, but once you understand the basic principles, the process becomes much easier to understand.

The most important things to learn are:

1. Understand how saponification works.
2. Learn how to calculate your lye correctly.
3. Weigh everything accurately.
4. Understand your oils and their properties.
5. Learn to recognise emulsification and trace.
6. Understand how fragrances behave.
7. Follow proper lye safety.
8. Give your soap enough time to cure.
9. Keep detailed batch records.
10. Never stop learning.

Cold process soap making is a wonderful combination of creativity and chemistry — and once you understand the science behind the bubbles, every batch becomes an opportunity to learn something new.