Starting in the Primitive Eldarin of the 1930s, many initial consonants could have an s-prefix (PE18/43, 93). For s-prefixed voiceless consonants, the resulting combinations [sp-, st-, sk-, spʰ-, stʰ-, skʰ-] became s-prefixed voiceless spirants [sɸ-, sx-, sθ-]. In the case of the s-prefixed aspirates, this was probably part of the larger trend of aspirates becoming voiceless spirants in Old Sindarin and Old Noldorin, but for s-prefixed voiceless stops, the changes were distinct and occurred later. A detailed discussion of the nature of these sound changes appears in Tolkien’s notes on the Old Noldorin use of the Feanorian Alphabet from the 1930s:

A marked change of ON was the aspiration of t, p, k after all consonants. Since sp, st, sk and mp, nt, nk has special letters, this did not receive spelling-recognition in ON, except that in very late ON absolute initial [that is, at the beginning of a phrase] sp, st, sk Q, !, A became occasionally written e 3 d, i.e. ph, th, kh, or ee (eR), 33 (3R), dd (d$) indicating a change > sf, sþ, sχ > ff, þþ, χχ. This did not occur initially [that is, at the beginning of a word inside a phrase] (and here sp, st, sk remained in Exilic). r 4 f remained unchanged in use, and here aspiration probably did not proceed to spirantalization until the Exilic period. The letters Q are transcribed sp etc., and r mp without reference to exact phonetics (initial Q ! A are sometimes transcribed sph, sth, skh) (PE22/28).

From this note, it seems that sp-, st-, sk- ultimately became long spirants ff-, þþ-, χχ- [ɸɸ-, θθ-, xx-] which then shortened to f-, þ-, χ- [ɸ-, θ-, x-]. I am of the opinion that the voiceless stops probably first became spirants (sf-, sþ-, sχ-) and then the s assimilated to the spirant; these changes were probably aided by the sf-, sþ-, sχ- produced from earlier (but rarer) [spʰ-, stʰ-, skʰ-]. David Salo instead proposed that the voiceless stops aspirated first, and then became spirants such as: [st-] > [stʰ-] > [sθ-] > [θ-] (GS/§4.65, 4.80, 4.85), but he did his analysis before the publication of the Feanorian Alphabet note above.

The timing of these sound changes is tricky to nail down. It seems that in the 1930s at least, Tolkien imagined this change first occurred in absolute initial position (the beginning of a phrase) as early as Old Noldorin, but did not proceed to words in other positions within a phrase until after the Exile of the Noldor. How Tolkien imagined these changes happening in Sindarin is unclear. For convenience, this lexicon lumps them into Old Sindarin along with sound changes related to the aspiration and spirantalization of other voiceless stops, though all these changes may still have been ongoing after the Old Sindarin period.

Examples of these changes are easy to find in both Sindarin and Noldorin:

• ✶spindē > S. and N. find “tress, lock of hair” (PE17/17, Ety/SPIN).
• ✶stenna > S. thenn “short”; ᴹ√STINTĀ > N. thent (PE17/185, Ety/STINTĀ).
• ᴹ✶skalnā > ON. skhalla > N. hall “veiled, hidden” (Ety/SKAL¹).

Conceptual Development: There is no clear evidence of these changes in Gnomish, but as noted by Roman Rausch in his Historical Phonologies of Ilkorin, Telerin and Noldorin around 1923 (HPITN/§4.1.1), nearly identical changes appear in Early Noldorin from the 1920s:

• ᴱ✶spalk(w)e > ᴱN. falch “cleft, ravine” (PE13/143).
• ᴱ✶stalga > ᴱN. thala “valiant” (PE13/153).
• ᴱ✶skar-wé > ᴱN. harw “wound” (PE13/147).

In the last example, likely initial [x-] became [h-] as was the case in later Noldorin and Sindarin as well.

@@@ PE22/148