On Sibilants

PE22/65 (3)

s is not strictly homorganic with the other dentals, but is now whatever its origins (these are various) the normal s, a voiceless 'blade' consonant, with the blade of the tongue in contact (and so possessing an i-timbre), but with the point touching the lower teeth. It has no tendency, even before [i] to palatalization to [š] or [ç].

PE19/71 (3.ii)

th became the interdental spirant [þ], more or less identical with the sound of th in English thin.

AppE

TH represents the voiceless th of English in thin cloth.

VT41/7-8

This in Quenya was earlier, as it remained in Vanyarin, a dental spirant (made with the tongue-tip behind the back of the upper front teeth, which makes the passage to s easier). [...] The shift from dental and labial þ and f to interdental þ and labio-dental f occurred first in Telerin. [...] Why the Ñoldor did not then proceed to adopt the interdental þ and so clarify the distinction between þ and s remains uncertain. [...] the T-series had two voiceless spirants allied to it, s and dental þ (somewhat similar) [...] in times of peace and calm thought the loremasters would no doubt have prevailed and the change to interdental þ have been accepted.