What is the motivation behind the choice of unglazed Porcelain as a Streak plate?

Question

What characteristics does Porcelain possess that makes it a good choice for the Streak test? Additionally, are there any other materials that could also be used for designing a Streak plate?

Answer 1

Glaze would prevent or seriously affect the ability of the plate to scrape material off of the sample and produce a streak. Because porcelain is harder than most naturally occurring minerals the action of scraping a sample across the plate results in parts of the sample being scraped from the sample and producing the streak. Any material that has the same hardness of a porcelain streak plate an dis white could be used. Porcelain is relatively cheap to produce, so the economics figures into the picture also.