I dug up some stuff I've written before. Here ya go:
"Glycerin rivers" and "stearic spots or streaks" are misleading terms. The more generic terms "streaking" and "mottling" are really more accurate. Any soap can show rivers, streaks, spots, or mottling; these patterns are just more obvious in soaps with pigment colorants.
Probably the worst name of the two is "glycerin rivers" because it is so incorrect. A high concentration of glycerin would make soap soft and goopy, and the soap should wear away more quickly in those areas due to the softness and water solubility of the glycerin. Ask yourself -- have you ever seen a "glycerin river" that behaved like that??? "Stearic streaks" or spots can be formed by non-stearic soaps as well as stearic soap, but that term is closer to reality.
I have no illusion that people are going to use more accurate names, however. "Glycerin rivers" and "stearic spots" will continue to confuse and mislead soapers for decades to come.
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Soap is made of many different kinds of soap molecules. As the soap cools in the mold, some parts of the soap may crystallize (harden) before other areas do. That affects where a colorant such as titanium dioxide ends up in the soap. Colorants are more likely to concentrate in the soaps that crystallize last. Also some of the soaps themselves are more opaque (stearic, palmitic) and some are more translucent (oleic, linoleic), which in itself can cause mottling and streaking even without added pigments.
Water content is not really what causes mottling and streaking, although I can see why one would think it is. What water content DOES do is affect whether the soap is likely to gel or not at relatively low temperatures. High water soaps go to full gel at much lower temps than low-water soaps, so they are thus more likely to show mottling and streaking than soaps that don't gel.
Mottling and streaking is more likely if the soap reaches a full gel state and is allowed to very slowly cool, so the different soaps (stearic soap, oleic soap, palmitic soap, etc.) can crystallize at different times.
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The old soap makers (1800s to early 1900s) intentionally made mottled soaps that were popular with the customers of the day. A mottled soap could only be made using a pure soap that had not been "filled" or adulterated with too much water, clay, or other cost-cutting additives, so the mottling was proof the soap was pure and high quality. The soap maker would add a coloring agent to a finished soap, pour the soap into "frames" (large molds), and carefully control the rate of cooling.
The stearic and palmitic soaps would solidify into pale colored clumps first, essentially concentrating the color into the remaining liquid oleic and linoleic soaps. When the oleic soaps solidified, the color would be trapped within these areas and make rivers or veins of darker color around the stearic clumps. The size and appearance of the mottles were controlled by the oils in the recipe, the way the finished soap was handled, and the rate of cooling in the frames.
"...When [soap] is permitted to cool rapidly the colouring matter remains uniformly disseminated throughout the mass; but when means are taken to cause the soap to cool and solidify slowly a segregation takes place: the stearate and palmitate form a semicrystalline solid, while the oleate, solidifying more slowly, comes by itself into translucent veins, in which the greater part of the coloured matter is drawn. In this way curd, mottled or marbled soap is formed..."
Source:
http://www.1911encyclopedia.org/Soap
