The same pronunciation also applies to the chemist Curtius and all others with the same surname.
In words of Latin origin that end in -tium, -tius, -tion, -tial, -tiell and others with the general pattern of ti + vowel, the t underwent softening in various degrees in different European languages. In English in a word such as nation, the ti is effecticely pronounced as if it were a sh. In French, the t turns into an /s/ sound so the result is something like [nasjõ]. In German, instead of turning into a full fricative like in English or French the resulting sound is an affricate /ts/.
In writing, the /ts/ sound is generally represented by the letter z in German. (The sound commonly represented by the letter z in English, /z/, is represented by a single s in German – but not all dialects use it.) So to a German, instead of saying ‘an s has been added’ the typical thought process would be ‘t turned into z’ – or possibly, since tz is another possible spelling for the /ts/ sound, ‘a z was added’.
In the 1996 spelling reform, this pronunciation was reflected in the spelling of a couple of words such as Potenzial which may now be spelt with z instead of t.