@A-Polly wrote: "...Holly's Soapmaking on YouTube; she used only 10 percent salt, which is an amount often dissolved into the liquid for a soleseife bar, but she added it to traced batter instead. ..."
If you want to make a salt/spa bar with large salt crystals rather than a soleseife/brine bar with tiny salt crystals, then, yes, you can add salt to the traced batter per Holly's method. Or you can add the salt to the finished lye solution. Either way will work to achieve the same goal, although adding salt at trace is probably easier to do.
Brine/soleseife bars are made by dissolving enough table salt (NaCl) in PLAIN water to saturate the water. This brine will be about 25% salt by weight of solution (25 g NaCl per 100 g brine). Then you add NaOH to this brine to make the lye solution.
The interesting thing is, very little of the NaCl dissolved in the plain water will stay dissolved after you add the NaOH -- only about 3% by weight of solution (3 g NaCl per 100 g of water-NaCl-NaOH solution). That's why the clear salt-and-water brine turns milky when you add the NaOH.
If you leave this mixture alone for awhile, you'll see a layer of tiny salt crystals settle out. They're so tiny they become invisible in soap and can't possibly scratch. That's the essential difference between salt/spa bars and solseife/brine bars -- the size of the salt crystals.
If you do the other way around (add salt to the lye solution), nearly all of the salt crystals will not dissolve and you'll end up with a salt/spa bar instead.
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