Dark Outer Ring
I have recently made a batch of soap using the room temperature method.
I followed your instructions to a tee. It seemed to turn out wonderfully but I found when I cut the soap the outside was dark and the inside was lighter in color.
The next morning when I came into my kitchen the cut soap was all the same color. I thought how great it worked!
I waited two weeks to cure and then cut the soap again to make sampler soaps and noticed it was still lighter in the middle.
I make quite a bit of cold process soaps and have never ran into this. How long should I cure this soap so that the color will be the same through and through? Answer:
The discolouration you are describing sounds very much like that which occurs from some fragrance or essential oils.
Upon cutting you notice a darker ring around the exterior surfaces of the soap but the inner core is a much paler colour that darkens upon exposure to oxygen.
All fragrance oils that contain a vanilla component will discolour in this manner. You'd be amazed at how many fragrance oils contain vanilla in them.
You ask also how long to cure the soap. Cold process, including room temperature soap should be cured at a minimum of 4 weeks before use.
The colour will never be consistent all the way through. It will however, maintain a dark outer rim even with use. This shouldn't pose any problems since the only way to see the discolouration is by cutting the soap.
Hope this eases your mind,