You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
Alexander_Graham_Bell

Context:
By 1874, Bell's initial work on the harmonic telegraph had entered a formative stage, with progress made both at his new Boston "laboratory" (a rented facility) and at his family home in Canada a big success.[N 14] While working that summer in Brantford, Bell experimented with a "phonautograph", a pen-like machine that could draw shapes of sound waves on smoked glass by tracing their vibrations. Bell thought it might be possible to generate undulating electrical currents that corresponded to sound waves. Bell also thought that multiple metal reeds tuned to different frequencies like a harp would be able to convert the undulating currents back into sound. But he had no working model to demonstrate the feasibility of these ideas.

Question:
On what material did the phonautograph operate?

smoked glass