Rosemary & Marjoram Herbal Infusion: Making Botanical Bar Soap with Infused Olive Oil
From aromatic herbs and olive oil to a handmade botanical bar
There is something satisfying about taking something that grows in the garden and finding a practical way to work it into something you can actually use.
Rosemary and marjoram are two herbs that have been used around the kitchen and garden for generations. They're aromatic, distinctive and packed with the kinds of plant compounds that make herbs so interesting to explore.
In this Boner's Botanicals experiment, we're taking that idea one step further.
We're infusing olive oil with rosemary and marjoram, then using that botanical-infused oil as part of a cold-process soap formulation.
The result is a handmade bar of soap with a story that begins with the plant itself.
And there's some interesting science behind the infusion.
Watch Us Make It
In the video above, we take you through the process of making our rosemary and marjoram infused olive oil and then turning that botanical oil into handmade bar soap.
But before we make soap, let's talk about what we're actually putting into the oil.
Why Infuse Herbs Into Olive Oil?
An herbal infusion is essentially a way of allowing compounds from plant material to migrate into another substance.
In this case, the receiving medium is olive oil.
The process is not mysterious.
Plant material contains a wide variety of chemical constituents. Some are volatile and aromatic, while others are more readily extracted into particular solvents or oils.
When dried herbs are placed in oil, compounds that are compatible with that oil can gradually move from the plant material into the surrounding oil.
Researchers have demonstrated this process with aromatic herbs and olive oil.
In one study, dried rosemary was infused into olive oil and researchers measured changes in the resulting oil, including its phenolic compounds and other chemical characteristics. Rosemary-infused olive oil showed increased polyphenol content compared with the control oil.
Another study examined the enrichment of olive oils with aromatic and medicinal herbs and found that compounds originating from the plants could migrate into the oil during maceration. Olive oils infused with rosemary showed increased phenolic content compared with untreated oil.
So when we put rosemary and marjoram into olive oil, we're not simply adding a pleasant smell.
We're creating a botanical infusion.
Rosemary in Olive Oil
Rosemary is particularly interesting from an extraction standpoint.
The plant contains numerous volatile and phenolic compounds, including compounds associated with rosemary's characteristic aroma.
Research into rosemary-infused olive oils has demonstrated that plant-derived compounds can move into the oil during infusion. Studies have measured changes in phenolic content and other characteristics of the resulting oil.
That doesn't mean every rosemary infusion will have the same chemical profile.
The result depends on factors such as:
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the rosemary variety
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whether the herb is fresh or dried
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particle size
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herb-to-oil ratio
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infusion time
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temperature
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oil composition
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storage conditions
In other words, an herbal infusion is a process—not a single chemical recipe.
And Then There's Marjoram
Marjoram brings another interesting layer to the experiment.
The botanical name for sweet marjoram is Origanum majorana.
Researchers have specifically studied marjoram infused into extra-virgin olive oil.
In a 2024 study, researchers used dried marjoram in extra-virgin olive oil and examined the resulting oil's volatile and phenolic profiles.
They identified multiple terpene compounds that had migrated from the marjoram into the olive oil, along with phenolic compounds associated with the plant. The researchers also observed changes in several measured characteristics of the flavored oil.
That's fascinating because it gives us direct scientific evidence that we're not simply making an oil that smells like marjoram.
Plant-derived compounds can actually migrate into the oil.
Two Herbs. One Oil.
Rosemary and marjoram also make an interesting pairing.
Both are aromatic herbs in the mint family, but they have distinctly different aromatic profiles.
Rosemary tends to bring a sharper, resinous, evergreen character.
Marjoram is generally softer, warmer and more herbaceous.
Together, they create a botanical combination that feels right at home in a natural soap.
And because we're infusing the olive oil itself, the herbs become part of the story of the soap rather than simply being sprinkled on top at the end.
From Infused Oil to Soap
Once the botanical infusion is ready, we can move on to the soap.
This is where chemistry takes over.
Cold-process soap making is based on a chemical reaction called saponification.
Oils and fats contain triglycerides.
When those triglycerides react with sodium hydroxide under controlled conditions, the molecules are converted into fatty acid salts—what we commonly call soap—along with glycerol.
In other words:
Oil + sodium hydroxide → soap + glycerol
The sodium hydroxide isn't simply sitting inside the finished bar as the same material we started with.
It participates in the chemical reaction that creates the soap.
That transformation is the foundation of traditional cold-process soap making.
What Happens to the Olive Oil?
Olive oil is particularly interesting as a soapmaking oil because its fatty acid profile is dominated by oleic acid.
During saponification, the triglycerides in the olive oil are converted into their corresponding fatty acid salts.
The exact characteristics of the finished bar depend on the entire formulation—not simply the fact that olive oil was used.
The proportions of different oils influence properties such as hardness, cleansing, lather and longevity.
Research examining cold-saponified natural herbal soaps has demonstrated that the fatty acid composition of the finished soap depends in part on the oils used in the formulation. The researchers also detected unsaponified fatty acids remaining in the finished soaps they studied.
That's one reason soap formulation is more than simply mixing ingredients together.
It's chemistry.
Does the Herbal Infusion Survive Saponification?
This is where we need to be careful.
It would be tempting to say:
"All of the beneficial compounds from rosemary and marjoram survive the soapmaking process."
We don't have enough evidence to make that claim.
Saponification is a chemically active process involving a strong alkali, and different plant compounds have different chemical stability.
Some compounds may survive.
Some may change.
Some may be reduced or lost.
Volatile compounds can also behave very differently from less volatile compounds.
So rather than promising that every compound in the original infusion survives into the finished bar, we can make a much more defensible statement:
The botanical infusion becomes part of the soapmaking oils, but the chemical composition of the finished soap is not necessarily identical to the original herbal-infused oil.
That's the scientifically honest answer.
And honestly, it's more interesting.
Why Use an Infused Oil Instead of Just Adding the Herbs?
There are several reasons a soapmaker might choose to infuse the oil itself.
First, it provides a controlled way to incorporate botanical material into the oil phase of the formulation.
Second, it allows the soapmaker to explore the extraction of oil-compatible plant compounds before the soapmaking process begins.
Third, it gives the finished soap a story.
The rosemary and marjoram aren't simply decorative ingredients.
They were part of the preparation from the beginning.
The process becomes:
Herb → Infusion → Oil → Soap
That's the part we love at Boner's Botanicals.
We're not just making a product.
We're exploring what happens when we work with the plant itself.
Why Dried Herbs?
For oil infusions, dried plant material is often preferable to fresh material because fresh plants contain substantial amounts of water.
Water introduces additional considerations when plant material is stored in oil, including microbial and stability concerns.
Drying reduces the water content of the plant material before infusion.
That doesn't automatically make every homemade herbal oil shelf-stable or safe for every purpose, but it is an important consideration when preparing botanical infusions.
And it is another reason that the condition of the plant material matters.
The Art of the Infusion
Making an herbal infusion isn't simply a matter of throwing some herbs into a bottle and waiting.
The variables matter.
Herb-to-oil ratio
More plant material doesn't automatically mean a better infusion.
The amount of herb affects the concentration and extraction environment.
Particle size
Smaller particles provide more surface area for the oil to contact.
But extremely fine plant material can also make filtration more difficult.
Time
Longer isn't necessarily better.
Extraction changes over time, and different compounds move into the oil at different rates.
Temperature
Heat can accelerate some extraction processes, but excessive heat can also affect sensitive compounds and accelerate oxidation.
Light and oxygen
Both can influence the stability of oils and plant compounds.
That's why storage conditions matter once the infusion is finished.
A Botanical Soap With a Little History
There is also something appealing about using herbs that have such a long history of human use.
Rosemary has been cultivated and used for centuries as an aromatic and culinary plant.
Marjoram has its own long history as a culinary and aromatic herb.
We're taking plants with deep roots in human culture and putting them to work in a very modern handmade-product process.
The result isn't a recreation of an ancient soap.
It's something different:
traditional plants + modern soapmaking + a little chemistry.
What Makes This a Botanical Soap?
We use the term botanical soap because the formulation incorporates plant-derived material as part of the process.
In this case, the olive oil itself has been infused with rosemary and marjoram before becoming part of the soap formulation.
That distinction is worth making.
A soap doesn't have to contain chunks of leaves or flowers floating through the bar to have a botanical story.
Sometimes the most interesting part of the plant is what you can't see.
From Garden to Bar
This is one of our favorite ideas at Boner's Botanicals:
Find something interesting in nature.
Figure out how it works.
Find a practical way to work with it.
Make something useful.
Rosemary and marjoram give us a perfect example.
They start as plants.
We transform them into an herbal infusion.
That infused oil becomes part of a cold-process soap formulation.
And eventually, after saponification and curing, we end up with a handmade bar of soap.
The finished bar may look simple.
But behind it is a chain of processes involving plant chemistry, extraction, fatty acids and saponification.
That's what makes handmade soap so much more interesting than simply picking up a bar from a shelf.
The Bottom Line
An herbal-infused oil is more than oil with herbs sitting in it.
It is an extraction process.
Research has demonstrated that compounds from rosemary, marjoram and other aromatic plants can migrate into olive oil during infusion.
When that infused oil becomes part of a cold-process soap formulation, the chemistry changes again.
The triglycerides undergo saponification, producing soap and glycerol, while the exact fate of individual plant compounds depends on their chemistry and the conditions of the process.
And that is what makes this experiment so interesting.
We're taking something that begins in the garden and following it all the way to the finished bar.
Plant → Infusion → Oil → Saponification → Soap
That's herbal soapmaking at Boner's Botanicals.
A Note From Boner's Botanicals
This article is intended for educational purposes and describes our own soapmaking and botanical-infusion process.
"Botanical" does not automatically mean safe for every person or every application, and the presence of a plant-derived ingredient should not be interpreted as a medical claim.
When making soap with sodium hydroxide, always follow appropriate soapmaking safety procedures, use accurate measurements, and consult a reliable soapmaking or chemical-safety reference before attempting the process yourself.
Sources & Further Reading
Rosemary-infused olive oil
Research has demonstrated the migration of rosemary-derived compounds into olive oil and measured changes in phenolic content and other chemical characteristics of the resulting flavored oils.
Herbal enrichment of olive oil
Researchers studying olive oils infused with aromatic and medicinal herbs found that plant-derived compounds could migrate into the oil and alter its phenolic profile. Rosemary-infused oils showed increased phenolic content in the study.
Marjoram-infused olive oil
A 2024 study investigated Origanum majorana in extra-virgin olive oil and identified terpene and phenolic compounds originating from marjoram in the infused oil.
Cold-process herbal soap
Research examining cold-saponified natural herbal soaps provides useful information about saponification, fatty acid composition and the characteristics of finished herbal soaps.
Boner's Botanicals
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Natural history, wildlife, plants, and practical ways to work with nature.
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Affiliate disclosure: Boner’s Botanicals may earn a commission from qualifying purchases through this link, at no additional cost to you.
Affiliate disclosure: Boner’s Botanicals may earn a commission from qualifying purchases through this link, at no additional cost to you.
