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Why Is My Soap Lighter Around the Edges and Darker in the Middle?

I recently got this question from one of my students about discolored cold process soap.  It's a great question about a problem I also had during my initial soap making years! 

"We made some honey oatmeal soap yesterday that looks great on the outside.  It really looked good until we started to slice it. It's lavender on the outside, but wet and brownish inside.  Were we just over eager and sliced too soon or is something else likely wrong. If we let it sit a couple days before we slice the rest will it be better? Or did we just mess up this batch?"

Photo used with permission (c) Rich 2015.

Thanks, Rich, for the question and the photo!  As you can see, each bar has a darker oval spot in the center with a lighter color around the edges.

The first time I cut into a batch of soap that looked like this, I was disconcerted.  I remember thinking, did I do something wrong?  Is this soap still safe to use?

What happens when soap turns out dark in the middle and lighter around the edges? 
The soap went through an incomplete gel.
 During the soap making process, your soap goes through some pretty amazing changes.  One of these is called the "gel phase."  It happens in those magical hours after your soap batter is poured into the mold and sets quietly undisturbed.  If you never peak at your soap during these hours, you might completely miss the gel phase.

You see the start of saponification (the process of oils, lye, and liquid turning into soap) in your pot when the soap comes to trace, but that's just the beginning. As your soap batter sits in the mold, it continues to generate heat as the saponification process  takes place. 

In the mold, the soap generates heat as saponification continues.  As the soap heats up, it begins to turn translucent.  It starts in the middle and works its way to the outside of the soap.  This is the "gel phase," and if you've never seen it, you've got to peak at your soap a few times in the mold.  It looks really cool!

As the soap cools, it becomes opaque again, starting from the outside and working its way back toward the middle.

Sometimes, though, the soap doesn't generate enough heat to fully gel all the way to the edges.  When this happens, you get the problem pictured above -- soap with a dark center and a lighter edge.

The soap is still safe to use. 
Rest assured, this is just a cosmetic issue.  Your soap is still completely safe to use even with what we soapers call a "rind" (a light edge).

It will still lather and clean just as well.  It maybe doesn't look as "pretty" as you had hoped, but it's still fine to use.

  Here's how you can avoid it:
If you don't mind your soap having a rind, and it will stress you out trying to ensure your soap doesn't develop one, I would just happily soap away and not worry about it! But if you don't want an unexpectedly two-toned soap, especially if you're hoping to sell your bars, there are a few steps you can take to avoid it.

Ensure your soap gels completely.  To do this, make sure your soap retains enough heat during the time it sits in the mold.  Put a lid on your mold and wrap it in several towels or an old blanket to insulate.  Keep it in a warm place.  You will probably have to experiment to see just how much help your soap needs to obtain a complete gel, as this depends on the recipe you're using, the temperature you soap at, and the temperature of your room.

Don't let your soap gel at all.  I know, this is contradictory advice!  But remember how the middle of the soap (the soap that has gelled) gets darker?  Gelled soap is generally darker than soap that doesn't gel.

There are some instances where you don't want that darker soap.  Milk soaps, for example, tend to get very brown if they gel because the sugars in the milk caramelize.  Which is exactly what happened to Rich's soap pictured above.

In this case, you want to keep your batter cooler so it will not gel.  Soap at a cooler temperature (say 80-90 degrees).  Do not insulate your mold.  Some soapers also put there mold in the fridge or freezer (just don't let the soap actually freeze!)
Deep loaf or column molds hold heat more than slab molds do, so take that into account as well.

Soap making is all about experimentation.  The more batches you make, the better you'll get.  Happy soaping!

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