Cold Process vs Hot Process Soap: An Artisan's 10-Year Comparison

Ask ten soap makers whether cold process or hot process soap is better and you’ll probably get eleven answers. I’ve used both methods over the years, and they each have their own charm, quirks and ideal uses.

In this post, I want to give you an honest, experience-based comparison of cold process vs hot process soap making – grounded in what the science says, but framed in the reality of long evenings at the soap pot, curing racks and customer feedback. We’ll look at how each method works, how they differ in texture and appearance, and what that means for your skin and your bathroom shelf.

What Is Cold Process Soap?

Cold process soap is made by mixing oils with a lye solution at relatively low temperatures, then pouring the raw soap into moulds and allowing it to saponify and cure over several weeks.

As our detailed article on the art of cold process soap making explains, this method avoids external heat, allowing the natural properties of oils and additives to remain intact.

Key features:

• No external cooking – the reaction generates its own heat

• Longer cure time – typically 4–6 weeks

• Smooth, refined texture – ideal for intricate swirls and delicate designs

• Fragrance flexibility – essential oils are usually added at a cooler stage, helping delicate notes survive

From a chemistry point of view, resources like Chagrin Valley’s explanation of soap making chemistry and research on cold saponification confirm that colder, slower methods can preserve more unsaponified fatty matter and plant antioxidants in the finished bar.

What Is Hot Process Soap?

Hot process soap starts like cold process – oils and lye are combined – but then the mixture is cooked with gentle heat, often in a slow cooker or double boiler. This accelerates saponification so the soap is functionally ready sooner, though some cure time is still helpful.

Educational guides such as the School of Natural Skincare’s quick comparison of cold process vs hot process soap and BrambleBerry’s breakdown of hot process vs cold process methods describe the main differences: hotter temperatures, faster saponification, a more rustic texture, and earlier unmoulding.

Key features:

• Cooked to "gel" in the pot – you can see it thicken and change

• Shorter time to usable bars – some makers use hot process soap within days

• Rustic, textured appearance – less suited to fine swirls, more to rugged charm

• Fragrance added late – essential oils are usually stirred in after the "cook" to reduce scent loss

In practice, hot process can feel more forgiving if you want speed and are less focused on visual finesse.

How Do They Compare in Skin Feel?

From a skin perspective, the core chemistry is the same: both methods rely on saponification of oils with lye. What changes are the conditions under which that happens and how many unsaponified components remain.

The Cosmetics paper on cold saponified soaps suggests that cold process can preserve slightly higher levels of unsaponified fatty acids and certain plant compounds compared with more aggressive processing.  These components often contribute to a creamier, more conditioning feel on the skin.

At the same time, a recent study in Cosmetics investigating natural soaps prepared by cold and hot processes found that all soaps tested were alkaline, with pH values around 9–10, and that changes in skin surface pH after washing were relatively modest when compared side by side.

So, in my experience:

• Cold process bars tend to feel a touch more refined and "silky", especially when cured for a full 4–6 weeks

• Hot process bars can feel just as cleansing and conditioning if formulated thoughtfully, but they often have a slightly more rustic lather and texture

If you're working with sensitive or dry skin, I lean towards cold process with longer cure times and rich oils – the same philosophy that runs through guides like our post on discovering the best soap bars for sensitive skin and our moisturising bar soap ingredient guide.

Texture and Appearance: Smooth vs Rustic

If you love soap art – fine swirls, layers, embeds – cold process is the clear winner. The batter stays fluid for longer, allowing complex designs and glass-smooth tops.

Hot process, on the other hand, thickens into a mashed-potato-like texture in the pot, is spooned or scooped into moulds rather than poured, and often has a charming, rustic look with textured tops and visible "waves".

Some of my early hot process batches looked like loaves of homemade bread – not elegant, but very lovable. Over time, I realised that for The Humble Shepherd's aesthetic and the soft, spa-like feel we wanted to evoke (especially in posts like creating your own spa experience at home), cold process gave me the creative control I needed.

Fragrance and Additives

Essential oils, clays, botanicals and milks all respond differently to heat. Here's how I think about them in each method:

Cold Process

• Essential oils are added at "trace" when the batter is warm but not boiling

• Delicate notes like citrus, florals and light herbals are better preserved than in very hot methods

• Natural colours from clays and plant powders stay more stable and predictable

Hot Process

• Essential oils are usually added after the cook, once the soap has reached gel and cooled slightly in the pot

• Some makers feel this "locks in" scent, but the overall high heat exposure can still be challenging for very fragile oils

• Additives like milks and sugars can be trickier to manage without scorching

In both cases, I rely on conservative usage rates and guidance from reputable sources when working with essential oils, similar to the careful approach we describe in our post on the benefits of essential oil soap.

Time and Workflow: Patience vs Speed

If there's one undeniable advantage of hot process, it's speed.

Cold process:

• Soap is usually firm enough to unmould in 24–48 hours

• Needs 4–6 weeks of curing for best mildness and longevity

Hot process:

• Saponification is largely complete by the end of the cook

• Bars can technically be used sooner, though a week or two of drying still improves hardness

Guides like Heavenly Honey Company’s overview of cold vs hot process and Vedaoils’ comparison of the two methods highlight this time difference – cold process is the long, slow path; hot process is the fast track.

As a small artisan balancing life, family and a curing rack full of future bars, I eventually chose to lean heavily into cold process because I love the rhythm of slow making. It fits with the same slow-living energy we talk about in our articles on simple daily skincare routines and mindful home rituals.

Safety Considerations in Both Methods

Regardless of method, lye safety is non-negotiable. Sodium hydroxide is highly caustic, and both government and specialist resources emphasise the need for protection:

• The UK government’s general information on sodium hydroxide outlines emergency steps if it contacts skin or eyes.

• The US CDC’s medical management guidelines for sodium hydroxide stress how quickly burns can occur.

• Practical soap-making guides like BrambleBerry’s lye safety guide and the School of Natural Skincare’s lye safety tips translate that into on-the-bench advice: goggles, gloves, long sleeves, and respect.

Hot process doesn’t remove the need for safety; you’re simply doing more of the reaction under heat in a pan rather than in the mould.

So Which Is "Better" – Cold or Hot Process?

After a decade of making, my honest answer is:

Choose cold process if you:

• Want smooth, elegant bars and intricate designs

• Are happy to wait several weeks for the best cure

• Prefer the idea of minimal heat and potentially higher levels of unsaponified nourishing components

Choose hot process if you:

• Value speed and function over aesthetics

• Like a rustic, homemade look

• Want to be sure saponification is largely complete before the soap goes into moulds

At The Humble Shepherd, we work almost exclusively with cold process because it aligns with our values: slow craft, gentle skin feel, and a spa-like experience in everyday life. The same philosophy runs through everything from our zero-waste bathroom article to ingredient-focused posts like coconut oil soap.

Frequently Asked Questions

1. Is cold process soap gentler than hot process soap?

Not automatically, but often yes in practice, because many cold process artisans focus on rich oils, modest cleansing and long cure times. Research comparing cold and hot process soaps shows both are alkaline and can be made mild; the key is thoughtful formulation rather than the method alone.

2. Does hot process soap still need to cure?

Functionally, hot process soap is mostly saponified by the end of the cook, but a period of drying and maturing still improves hardness and longevity. I usually recommend at least 1–2 weeks before regular use.

3. Which method is better for sensitive skin?

For very sensitive or reactive skin, I tend to favour well-formulated, fully cured cold process bars with simple ingredients and minimal fragrance – the type we talk about in our guide to soap bars for sensitive skin.

4. Can you mix methods in a business?

Absolutely. Some makers use hot process for utility or kitchen soaps and cold process for body bars. At The Humble Shepherd, I've chosen to specialise in cold process because it gives me the finest control over texture, design and the gentle feel I want our soaps to be known for.

If you’re curious to experience the difference thoughtful cold process can make, you’re warmly invited to sign up to our waiting list on the Shop Now page. When new small batches finish their slow cure, you’ll be among the first to hear.