cirtha-

verb. to turn sour (tr.)

In the Gnomish Lexicon of the 1910s there was an intransitive verb G. cur- “turn, curdle, congeal (intr.) ”, as opposed to transitive G. curtha- “to curdle, make to cheese (tr.)” (GL/28), both clearly derived from the early root ᴱ√TYURU “curdle” (QL/50), where the primitive voiceless palatal stop [c] [[g|became [k] in Gnomish]] but became [ty] in Quenya (PE12/16). The verb cur- was initially not marked as intransitive and had a (deleted) past form caur (GL/28) which became the past form cauri elsewhere in the Gnomish Lexicon (GL/25).

In Gnomish Lexicon Slips modifying that document, cur- was glossed “to turn sour (tr. and intr.)” with past forms curthi or cŷr (PE13/112). In Early Noldorin Word-lists of the 1920s it was again split into a pair of verbs ᴱN. cirtha- [or possibly certha-] “to turn sour (tr.)” (PE13/140) and ᴱN. curann- “to curdle (intr.)” (PE13/141), the latter with past forms cyrenaint or agyraint and infinitive curreni.

Neo-Sindarin: For purposes of Neo-Sindarin, I would adapt Gnomish cur- as ᴺS. cor- “to turn sour, curdle, congeal; to make cheese” from the Neo-Root ᴺ√KYUR, where short u became o as was generally the case in Sindarin. I would further assume it was both intransitive and transitive as the verb was in the Gnomish Lexicon Slips.