Why Glycerin Rivers Aren’t Just About Water: The Science of Soap’s Most Misunderstood Feature

Ah, glycerin rivers—the delight of some soap artists and the bane of others. If you’ve ever cut into what you thought would be a pristine white bar and found it veined with translucent streaks, you’ve met the infamous “glycerin river.” But contrary to popular belief, these rivers aren't caused by “too much water” alone, nor are they purely a cosmetic fluke. There’s actual science behind their formation, and understanding it can help you either avoid them completely—or harness them for spectacular design.

Let’s break down what really causes glycerin rivers, how you can influence them, and why they’ve become one of the most fascinating topics in soapmaking chemistry.

What Are Glycerin Rivers, Anyway?

First off, let’s clear up a common misconception: glycerin rivers do not actually contain rivers of glycerin. The name is misleading. They appear as semi-translucent streaks or crackle-like textures in bars of soap—usually in areas containing titanium dioxide (TD) or similar white pigments. These rivers tend to follow random or slightly directional paths, giving a marbled or veined appearance that some find elegant and others try desperately to avoid.

What we call “glycerin rivers” are actually the result of localized differences in the crystalline structure and water content of the soap, often exaggerated during gel phase. They are not pockets of glycerin, and the illusion they create is based on the refractive and reflective properties of soap crystals under specific conditions of heat, moisture, and pigment concentration.

The Science Behind Glycerin Rivers

The most compelling experiments on this topic come from Kevin Dunn (author of Scientific Soapmaking) and Clara Lindberg (of Auntie Clara’s Soap Blog). Their research and artistry have helped uncover the conditions that produce these rivers and the chemical and physical processes involved.

The Three Key Conditions:

In all documented cases of dramatic glycerin rivers, two specific conditions must be present:

• High water content: Lye concentrations below ~31% (e.g., 1:2.2 NaOH-to-water ratio or higher water levels)

• Full gel phase: The soap must gel completely and stay in gel long enough to allow crystal migration and growth

  A third condition, that is not required, but significantly impacts the prescence of these rivers is:

• Presence of titanium dioxide (TD) or another dense, pigment-like additive

  
  

When one or more of these conditions is missing, the rivers usually do not form. For example:

• Soap that skips gel phase entirely, even with TD, often remains smooth

• Low water recipes—even with TD and heat—rarely produce visible rivers

• Soaps without colorants can still undergo subtle phase separations but generally don’t display defined rivering
  

This means glycerin rivers are not an inevitable byproduct of “bad” technique. They are a result of very specific variables working together.

Water Content Thresholds

Dunn’s team determined that a NaOH concentration below 31%—which roughly corresponds to a lye-to-water ratio of 1:2.2 or more—was a strong predictor for river formation. Soaps made with lye concentrations of 29% or lower (e.g., 1:2.4) produced more dramatic rivers, while higher lye concentration formulas (e.g., 33%) suppressed the effect.

This threshold is important: the more water in your recipe, the more likely you are to experience phase migration during gel, allowing pigment pockets or crystalline shifts to form visible lines or patterns. Simply put: more water equals more migration potential.

Time and Temperature: Prolonged Gel Equals Bigger Rivers

The longer your soap stays hot—and especially the longer it stays in gel phase—the more pronounced the rivers become. Dunn’s Time Test proved that even identical batches of soap would yield dramatically different results depending on how long they remained in the oven.

The batch that gelled for four hours showed thick, defined rivers. The batch that gelled for only one hour showed faint or no rivers. Time in gel phase allows more mobility within the soap structure, increasing the possibility for separation and crystallization that causes the river effect.

The Glycerin Myth and Amplification

Although all saponified soap contains glycerin as a natural byproduct, additional glycerin added to TD slurries or soap batter was shown to significantly amplify river formation. Dunn's experiments added 5, 10, and 20 grams of extra glycerin to otherwise identical layers. The more glycerin present, the larger and deeper the rivers became.

This doesn’t mean glycerin causes the rivers directly, but it does highlight glycerin’s role in altering water distribution and migration during gel.

Why Titanium Dioxide (TD) Stands Out

Titanium dioxide is a very dense, white pigment that acts differently under high heat and high water conditions than the surrounding soap matrix. As the soap enters and maintains gel, differences in moisture concentration and crystal formation occur at the TD sites.

TD reflects light differently than other parts of the soap, and under gel-phase transformation, the texture and appearance of the pigment-rich zones change. This leads to translucent streaks or outlines around pigment concentrations.

Important to note: Titanium dioxide creates pastel or tinted effects, because it essentially acts as a “missing white.” When combined with a pure colorant, TD creates opacity and softness in hue. This same scattering ability can exaggerate the effect of rivers when it becomes unevenly distributed during gel.

Other pigments—especially dense oxides and clays—may cause similar effects, but TD is by far the most notorious and widely used.

Cold Process vs. Hot Process

Glycerin rivers are more frequently seen in cold process, however, hot process soap is not immune. When hot process soap is cooked with a high-water formula and subjected to additional heat or placed in a mold and wrapped tightly, it can still undergo the kind of structural shifting that produces rivers.

In fact, many HP soapmakers report glycerin rivering when:

• They use water percentages over 38% TOW

• They cook and then mold with insulation

• They pre-mix pigments into glycerin or a high-water sugar syrup before adding post-cook

  
  

Because hot process soap is more mobile post-saponification than cold process in its gel state, proper dispersion of pigments (especially TD) is essential to controlling whether rivers appear or not.

How to Avoid Glycerin Rivers

Want a perfectly smooth, evenly colored soap bar with no trace of rivers? Here’s how to stack the odds in your favor:

• Keep water content lower: Aim for higher lye concentrations to reduce the total amount of water

• Use water discounts: Especially helpful in CP

• Avoid full gel: Use shallow molds, cool environments, or even refrigerate your soap post-pour

• Mix TD in oil, not water or glycerin: This reduces water migration and helps create a more even dispersion

• Use TD (and similar colorants) sparingly: The less TD, the fewer opportunities for rivering to occur

• Add colorants at emulsion: Early mixing helps ensure complete integration

How to Encourage or Enhance Glycerin Rivers

Want to lean into this effect as a design choice? Here’s how to create beautiful, intentional glycerin river crackles:

• Use high water: Lye concentration under 31% (e.g., 1:2.4 ratio or higher water %TOW)

• Ensure full gel phase: Wrap or oven-process soap at 160ºF (70ºC) for 2–4 hours

• Use glycerin in your TD slurry: 1:1 ratio of TD to glycerin will deepen and spread river effects

• Pour layers with different water concentrations: This creates contrast and purposeful direction

• Brush glycerin between layers: You can even “carve” glycerin patterns before layering to create artistic designs

• Let your soap stay hot longer: More gel time = more dramatic rivers

Beyond the Basics: Experimental Soapmaking

Want to take your skills to the next level? Try these advanced techniques:

• Create gradients using water layering: Dunn’s “Gradual Rivers” bar used 10 layers, each with a slightly higher water percentage. The higher the water, the more intense the rivers.

• Combine colorants: Add ultramarine blue or other oxides with TD to see how rivers affect pigmented zones differently

• Use sculpted molds: The glycerin river effect can emphasize 3D surfaces, especially when brushed with glycerin in valleys

Final Thoughts

The truth about glycerin rivers is that they’re not a sign of bad technique or a random accident. They’re the result of specific and measurable variables interacting inside your soap batter. When you understand these variables—water, pigment, temperature, and time—you gain control over your designs.

So go ahead. Make them bigger. Make them vanish. Carve rivers into valleys and let them flow with intention.

That’s not just good soapmaking. That’s soap chemistry.

Dive deeper with The Ultimate Guide to Cold Process Soap and The Ultimate Guide to Hot Process Soap. Want to master everything from water discounts to pigment placement? We cover it all.