This assumes, of course, that you're trying to simulate Fox Talbot's early work. Indeed, he did use NaCL early on but was unhappy with the results (he tried several methods to varying degrees of success). Herschel came up with Hypo (now known as Sodium Thiosulfate) as a means to permanently fix out the excess silver and passed the info on to Talbot (Herschel was very involved with many of the early chemical processes). Both Talbot and Daguerre switched to Hypo when it became clear that sodium chloride was not up to the task.
Originally Posted by cliveh
In answer to the OP. Sodium Thiosulfate is the fixer you want to use. As you've already noticed, Ammonium Thiosulfite is far too active for many of the older processes. Salt prints, Albumens, and VanDykes, all work best with plain Hypo. For VanDykes I use 1 heaping tablespoon of Hypo crystals to 1 quart of water. Works like a champ!