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 knobs/dials too) on Shabbat (the Jewish Sabbath), as part of the precautions taken to avoid violating the halachic prohibition against cooking on the Sabbath.
(Following excerpt with permission of publisher of The Blech Book—The Complete & Illustrated Guide To Shabbos Hotplates, by Rabbi Fishel Jacobs.)
- The food [including water] intended for Shabbos use, should be completely cooked.
- The stove’s gas flames or electric coils are turned on. The blech is placed over these. Alternatively, the Shabbos hotplate, which needs no blech (when it is the type which has no knobs to adjust the heat level,) is plugged in.
- The pot is placed on the blech. Note: It is permissible to place another pot on this one.
- The pot on the blech, or another pot which has been placed on it, may be covered with a blanket, clothing, towel, cloth etc. to keep the heat from dissipating. One side of the pot should be left partially uncovered.
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 pot should be removed from the blech with the intention to replace it afterwards and held at all times, not leaned onto any surface. (A heavy or unwieldy pot may be partially leaned on a surface, while being held, if there is no alternative.)
- The food must be in the same pot, completely cooked, and has retained at least some of its original heat.
The permissibility of blech (and unblech, below) and the acceptable manner of their use is questioned by several modern Kashruth organizations ; however, the use of a Blech to re-heat 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.
- ↑ See, for example, Warming Food on Shabbos by Rabbi David Cohen, Rabbi David Cohen, stating in part "Even if food is fully cooked, hot, and not wrapped, Chazal legislated that it may not be put into the fire or onto a blech." But See Ovadia Yosef, Yehave Daat, Vol 2, # 45; and Pinchas Toledano, Fountain of Blessings, Vol. 2, Page 67.
- ↑ Blech Definition - What is a Blech - Glossary of Kosher Terms
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