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A blech (from the German by way of Yiddish word for tin or sheet metal) is a metal sheet used by many observant Jews to cover stovetop burners (and for some, the cooker's knobs and dials) on Shabbat (the Jewish Sabbath), as part of the precautions taken to avoid violating the halachic prohibition against cooking on the Sabbath.
Rabbi Fishel Jacobs's The Blech Book—The Complete & Illustrated Guide To Shabbos Hotplates gives the following guidelines:-
During Shabbos, the pots are removed according to need. After removal, it is permissible to return the pot onto the blech, following these guidelines:
The permissibility of blech (and unblech, below) and the acceptable manner of their use is questioned by several modern kashrut organizations; however, the use of a blech to reheat food on the Sabbath remains very popular among observant Jews.
An unblech, or K'Deira Blech (lit. "pot blech", commonly referred to as "water blech"), is also used to heat up pre-cooked food on the Sabbath, but utilizes different halakhic mechanisms from a standard blech. An unblech consists of a shallow metal pan filled with hot water and covered by another metal pan, and thus is akin to a pot of warm food for halakhic purposes. As such, it may be more flexible than a standard blech for halakhic purposes. However, the temperature of an unblech is limited by the boiling point of water and is not as hot as a typical blech.