This paper investigates the potential of a silent speech interface (SSI) based on Permanent Magnetic Articulography (PMA) to be used in applications involving unconstrained, phonetically rich speech. In previous work the SSI was evaluated on isolatedword and connected-digits recognition tasks with promising results. Furthermore, it was shown that PMA data is enough to distinguish between minimal pairs of consonants with the same manner and place of articulation but different voicing. The study presented in this paper extends previous work by investigating to what extent information about the speech production process can be recovered from PMA data. In particular, the three main aspects of speech production are investigated here: voicing, place of articulation and manner of articulation. The results show that PMA achieves comparable accuracy to using the audio signal for discriminating the place of articulation of consonants, but it provides little information regarding the voicing and manner of articulation.