Said to be from Arabic صَفْرَاء (ṣafrāʔ, “yellow”) because of its being made of brass (Arabic صُفْر (ṣufr), whence Spanish azófar). The resemblance with Arabic صَفَّى (ṣaffā, “to filter”), مِصْفاة (miṣfāh, “colander”) is then coincidental.
Practice speaking and memorizing "zafra" and many other words and sentences in Spanish.