Making molds for soap

When I first dipped my toe into the world of soap making, I was obsessed with the chemistry—the oils, the lye, the magical transformation of saponification. But I quickly hit a wall that had nothing to do with formulas: I was tired of my soap looking like everyone else’s. The day I poured my first batch into a custom mold I built myself—a simple, clunky wooden box—was the day I truly became a soap artist rather than just a soap maker.

My Journey with Soap Mold Making

I still remember the disaster that started it all. I had spent hours formulating a perfect lavender and goat milk batch, only to pour it into a flimsy cardboard shoebox lined with a trash bag. The sides bowed out, the bag wrinkled, and I ended up with a massive, misshapen lump that looked more like a doorstop than a beauty product. It was heartbreaking, but it forced me to look at the “bones” of my craft.

Lisa Mandel

I started small, scavenging my kitchen for Pringles cans and yogurt containers, before graduating to building my own wooden loaf molds. Eventually, I took the plunge into RTV silicone. My first silicone attempt was a mess—I didn't mix the catalyst well enough, and I was left with a sticky, half-cured goo covering my favorite antique brooch I was trying to replicate.

But when I finally peeled back a perfect custom mold to reveal a bar of soap that looked exactly like a honeycomb? That feeling of pure creation was addictive.

What This Craft Really Entails

Making your own soap molds is a distinct sub-skill of the soaping world, often called “mold making” or “casting.” While many people are content buying mass-produced silicone trays, creating your own molds allows you to define the exact shape, size, and texture of your finished bars. It bridges the gap between carpentry, sculpture, and chemistry.

At its core, this craft is about negative space. You aren’t building the object; you are building the absence of the object. Whether you are screwing together pine boards to create a “loaf” that will be sliced later, or pouring liquid silicone over a sculpted clay model to create a cavity, you are engineering the vessel that will hold your liquid gold. It’s a technique that has evolved from the crude wooden ash-boxes of early settlers to today’s high-tech 3D printed masters used for silicone casting.

Ever wondered why your expensive essential oils seem to vanish or your bars warp as they cure? The answer might not be your recipe, but the insulation and shape of the mold you’re pouring into.

This craft is best suited for intermediate soap makers who are comfortable with their soap batter’s trace and behavior. If you are still struggling to get your soap to harden, you might want to stick to store-bought molds for now. However, for those ready to scale up production or create a signature brand look, making your own molds is the logical next step. It’s similar to a painter stretching their own canvas—it gives you total control over the foundation of your art.

Essential Materials and Tools

The materials you need depend heavily on whether you are building a rigid mold (wood) or a flexible one (silicone). Below is a breakdown of the essentials for both paths.

Key Techniques and Skills

Mastering mold making requires a mix of precision and patience. Here are the specific techniques you’ll need to develop:

• Leak-Proofing: Learning to seal the edges of your mold box with hot glue or clay. Liquid silicone will find microscopic holes and leak out, ruining your project and your table.
• Vacuum Degassing: For silicone, removing air bubbles is key. While professionals use vacuum chambers, beginners can use a “high pour” technique—pouring a thin stream from high up to break bubbles.
• Registration Keys: Creating notches or keys in multi-part molds so the two halves lock together perfectly, preventing “steps” or seams in your final soap bar.
• Lining Wood: The hospital corner fold technique for freezer paper. This ensures your wooden mold yields sharp, professional corners on your soap loaf.
• Cure Inhibition Awareness: Knowing which materials (like sulfur-based clay or latex gloves) will chemically stop platinum-cure silicone from ever hardening.
• Draft Angles: Ensuring your master object has no “undercuts” that will lock it inside the mold forever. The shape must taper slightly to allow release.
• Temperature Management: Understanding how wood insulates soap to force a “gel phase,” versus how silicone can sometimes result in partial gel if not insulated.
• Volume Calculation: Using water weight to calculate the exact volume of your mold so you don’t waste expensive silicone or soap batter.

Be incredibly careful with sulfur-based modeling clays. If you use them to sculpt a master, platinum-cure silicone will refuse to set, leaving you with a permanent, sticky mess that cannot be cleaned.

Skill Level and Time Investment

Creating a simple wooden mold is a weekend afternoon project, but mastering custom silicone casting is a journey. Here is a realistic timeline.

Advantages and Challenges

Why go through the trouble? I asked myself this when I was covered in sawdust one Saturday. But the benefits usually outweigh the frustration.

Benefits:
• Total Creative Control: You are no longer limited to the same shapes every other soap maker uses.
• Cost Savings: A DIY wooden mold costs a fraction of the price of a professional one.
• Scalability: You can build molds to fit your exact batch size, reducing waste to zero.
• Brand Identity: A unique shape becomes your signature—customers will recognize your soap instantly.
• Durability: A well-made wooden mold can last decades; high-quality silicone lasts for hundreds of pours.
• Versatility: Silicone molds you make can often be used for other crafts like resin or beeswax (with care).

• Tension Control: Not yarn tension, but “wall tension.” Silicone that is too thin will bow out when filled with heavy soap batter, resulting in bulging bars.
• Initial Cost: High-quality RTV silicone is expensive per pound. One mistake can be a significant financial loss.
• Storage: Wooden molds are bulky and rigid; they don’t stack or nest as easily as commercial silicone liners.
• Maintenance: Wooden molds require constant re-lining (unless you use a silicone liner insert), which adds time to every single batch.

Real Project Applications

One of the most successful projects I ever did was for a local bee sanctuary. They wanted soaps to sell in their gift shop. I didn’t want to use a generic “bee” mold that five other vendors would have. I sculpted a custom honeycomb texture using actual beeswax sheets pressed onto a clay block, then cast a silicone mold from that master. The result was a bar that looked authentically organic, not cartoonish. It sold out in a week.

Another practical application is the “Tall & Skinny” loaf mold. This is a trendy shape in the soaping community that gives you a high, elegant canvas for intricate swirls. Commercial versions can be pricey. I built my own using scrap plywood for practically pennies. It produces a loaf that is 2.5 inches wide but 4 inches tall. This vertical space allows for drop swirls and layer designs that simply aren’t possible in standard wide molds. I use it for my holiday collection every year.

Did you know you can make “embed” molds? These are tiny molds for creating small shapes (like hearts or stars) that you then place inside a larger loaf of soap. It’s a great way to use up leftover silicone.

The Learning Experience

The learning curve for mold making is steep but short. Most beginners start by making a wooden box. The most common mistake here is not accounting for the thickness of the wood when measuring the inside dimensions. You might aim for a 10-inch loaf and end up with an 8.5-inch one because you forgot the end boards take up space. Sound familiar? It happens to almost everyone.

When moving to silicone, the breakthrough usually comes when you learn to “dam” your mold box properly. My first few attempts leaked liquid silicone all over my workbench. I learned that hot glue is your best friend—you need to seal the edges of your master model and your containment box aggressively. Once you get that seal right, the process becomes incredibly satisfying. Resources like manufacturer tutorials are invaluable, even though they focus on industrial casting; the principles apply perfectly to soap.

“You can’t force the release. If you have to pry your mold open with a screwdriver, you didn’t design the draft angles right.” — A lesson I learned the hard way.

Comparison with Similar Crafts

Making molds for soap is distinct from other casting arts. Here is how it stacks up.

Common Questions from Fellow Crafters

Q: Can I use construction silicone (caulk) to make molds?

A: You can, using a method with dish soap and water, but I generally advise against it for professional results. It smells like vinegar, shrinks significantly, and often cures with a rough texture. For a rustic look, it’s fine; for professional bars, stick to RTV-2 casting silicone.

Q: Why is my silicone mold sticky after 24 hours?

A: This is called cure inhibition. It likely touched something that poisoned the catalyst. Common culprits are sulfur in modeling clay, latex in gloves, or residue from UV resin. Unfortunately, there is no fix—you have to scrap it and start over.

Q: Do I really need to line my wooden mold?

A: Yes! Raw wood will absorb the water from your soap batter, causing the soap to stick permanently. You will never get it out in one piece. Plus, the raw lye can damage the wood over time. Freezer paper or a silicone liner is mandatory.

Q: Can I use these molds for food afterwards?

A: No. Once a mold has been used for soap (especially with fragrance oils), it is no longer food safe. Keep your chocolate molds and your soap molds strictly separate.

Never try to speed up the cure of a silicone mold by putting it in the oven unless the manufacturer specifically says so. You risk warping the master model or creating dangerous fumes.

Q: How long will a DIY silicone mold last?

A: A high-quality platinum-cure silicone mold can last for years and hundreds of pours. Tin-cure silicone is cheaper but tends to become brittle and tear after a few years of shelf life.

Q: Can I use plastic food containers as molds?

A: Absolutely! This is how most of us start. Just check the recycling number. #5 (PP) is heat resistant and works great. Avoid brittle plastics that might crack when you try to pop the soap out.

My Personal Results and Insights

Tracking my own progress has shown me just how valuable this skill is.

Pro Tip: If you use PVC pipe to make round soaps, line the inside with a flexible plastic cutting mat rolled into a tube. It makes getting the soap out effortless instead of a wrestling match.

Final Thoughts and My Recommendation

Making your own molds is a game-changer, but it isn’t for everyone. If you just want to make soap for holiday gifts twice a year, stick to buying a sturdy silicone loaf mold online. It’s cheaper and easier. However, if you are bitten by the creative bug and find yourself dreaming of shapes you can’t find in stores, or if you are trying to launch a business with a unique visual identity, then learning to build your own molds is indispensable.

I highly recommend starting with wooden loaf molds. They are forgiving, cheap, and teach you the mechanics of volume and lining. Once you master that, try a simple silicone block mold of a found object. The satisfaction of peeling back that rubber to reveal a perfect negative is a thrill that never gets old. It transforms you from a consumer of supplies into a creator of tools, and that is a powerful place to be as an artisan.