Glycerine rivers in cold process soap are a visual effect where translucent, slightly shiny or gel-like streaks or "rivers" appear throughout the soap, often in areas with heavy colourants like titanium dioxide (TiO₂). Despite the name, they are not actual rivers of glycerine but are more about how ingredients behave during saponification and the gel phase.

The Chemistry Behind Glycerine Rivers in Cold Process Soap:

1. Gel Phase & Temperature Gradients:

   • During gel phase, the soap heats up and becomes more translucent due to the crystalline structure of the soap changing as the water and soap molecules interact.

   • If temperature is uneven, some parts of the soap gel while others don’t, leading to textural and visual contrast — this can contribute to the river-like effect.

2. Water Content:

   • High water content slows the saponification reaction and extends the gel phase.

   • As the water evaporates unevenly during curing, pockets of more concentrated soap or additives (like TiO₂) can visually separate from the rest of the batter, causing the appearance of “rivers.”

3. Titanium Dioxide (TiO₂):

   • TiO₂ is a common white pigment used to lighten soaps. It tends to clump or behave differently under high water and heat.

   • In gel phase, TiO₂ can migrate or cause localized overheating, leading to streaks that resemble glycerine rivers.

4. Glycerine as a Byproduct:

   • Glycerine is naturally created during saponification as triglycerides (fats) react with lye (NaOH), producing soap and glycerine.

   • Though these rivers are not pools of free glycerine, the name comes from the shiny, slightly translucent appearance resembling pure glycerine.

How to Reduce or Prevent Glycerine Rivers in Soap Making.:

• Reduce water content in your lye solution (e.g., use a 1.8:1 or 2:1 water-to-lye ratio).

• Avoid overheating: Don’t force gel phase, or use a water discount to reduce internal heat buildup.

• Insulate evenly if you want a full gel, or don’t insulate at all to keep it from gelling.

• Mix colourants well and consider dispersing TiO₂ in oil rather than water.