Definition: Once they oils and lye are mixed together, they begin to saponify - the chemical reaction that makes soap. The chemical reaction gives off heat (exothermic). If you're using small, single bar molds, you may only notice the soap getting warm and then cooling off and hardening. But if you use any sort of loaf, log or slab mold, the soap mixture will retain enough heat to creat what's called a "gel stage." The soap mixture will get so hot that it almost liquifies and turns translucent. It really does look like a gel. This is perfectly normal. Once the chemical reaction is done, it will cool back down and harden. In flat or slab molds, you may only get the gelling in the center, leaving the edges cool and opaque. In a log mold, like the one pictured, the gel stage will take place in the "core" of the log.
There is some debate as to whether soap needs to go through a gel stage. It really doesn't. The gel stage does seem to hurry up the overall saponification though. Without a gel stage, the final soap is a little more opaque, a little less translucent...and needs some more time to cure before you can use it.
