In days of hand stirring (which took hours!), trace was a sign that the soap was finally ready to pour into the mold. Some people describe the "trace" as a little line, ridge or mound of soap that after you've stirred the soap and drizzled it back into the pot, takes a second or two to disappear back into the mix.
How to Test for Trace
To test for trace, dip a spatula or spoon into the mix and dribble a bit of it back into the pot. If it leaves a little "trace" behind, you're there.
Trace is a sort of "point of no return" in the soap making process. Technically, "trace" is when your soap has reached "emulsification"--when the oils and water have mixed and are not going to separate. Once your soap "traces," the mixture will not separate back into the original oils and lye-water. The soap does not have to be thick just yet, it just needs to be well mixed with no streaks of the remaining oil. That's the key thing to know.
Trace in Modern Soap Making
But with today's soap-making world of stick blenders, trace is less of a consideration. It happens in a matter of 30-60 seconds.
When there is a small ridge of soap across the middle of a bar of soap, this is from the dribble off of the stirring spatula as the color was being added to the batch. This is what trace looks like.
Some soap makers prefer to pour their soap into the mold at "light trace," that is, immediately after trace is reached and the soap is still very liquid. Others prefer a more "heavy trace," that is, pouring the soap after trace has occurred, and the soap has thickened considerably.
Soapmaking dates back to about 2800 BCE in ancient Babylon, where an excavation unearthed a clay tablet bearing an inscription for a soap-like substance.