Ad. [z] became [s] before [x] and [s], and after aspirates, [s] and [x]; [z{sx}|{pʰtʰkʰsx}z] > [s{sx}|{pʰtʰkʰsx}s]

Ad. [z] became [s] before [x] and [s], and after aspirates, [s] and [x]; [z{sx}|{pʰtʰkʰsx}z] > [s{sx}|{pʰtʰkʰsx}s]

Tolkien stated that [z] was unvoiced before and after S and H [x], as well as after aspirates (SD/421-2).

References ✧ SD/421-422

Phonetic Rule Elements

[{pʰtʰkʰsx}z] > [{pʰtʰkʰsx}s] ✧ SD/421 (Z > S)
[z{sx}] > [s{sx}] ✧ SD/422 (Z > S)