Laws of Using a Shabbos Blech Before Shabbos (Hotplate or plata Shabbat)
Must I Always Use A Blech ?
The widespread custom is to always use a blech when leaving pots of food or water on a stove (gas flame or electric coil) for Shabbos use, and that the food in them be already fully cooked. This is commendable and avoids complications.
Are there conditions when a blech is not necessary?
According to the letter of the law, food or water which will be more than half cooked—or, in an emergency, a third cooked—by nightfall may be left on the stove without a blech before Shabbos.
Additionally, if raw meat was added to the pot immediately before Shabbos entered, a blech is not necessary.
In these instances, extra caution must be exercised against doing anything which would hasten the cooking.
Finally, food so well done that continued contact with heat only overcooks it does not require a blech to be left on the stove before Shabbos. Here, too, there is no reason to increase the heat.
However, it is preferable to follow the custom to always use a blech, and that the food be completely cooked prior to the onset of Shabbos.
Is a blech needed when the stove is on its highest flame?
Yes. Even though it is impossible to raise the flame, a blech is still necessary (in accordance with all the above.)
How is the blech used?
On Friday, before sunset, the gas flame or electric coil is adjusted to the desired heat level. The blech is then placed on this gas flame or electric coil.
Next, the pot is placed on the blech, making sure it contains enough water or moisture to last until being served, particularly if it is intended for the fol- lowing day.
Before Shabbos, is it permissible to place a pot on another pot already on the blech?
Yes. A pot may be placed on another, which is already on the blech, before Shabbos.
Must I cover the knobs on the stove?
Some opinions do not require covering the knobs of the lit flames or coils, others do. (The following photos illustrate both opinions.)
May I raise the flame after placing the blech?
No. Once the blech has been placed, the flame should not be raised.
May I lower the flame after placing the blech?
After the blech has been placed, it is best not to lower the flame.
Is it permissible to cover the pot while on the blech?
Yes, it is permissible to cover the pot with blankets, clothing, towels, cloths etc. to keep the warmth from dissipating. (The following photos illustrate: a pot on a pot and pots side by side—partially covered.)
However, this should be done in a way that avoids covering the pot snugly and completely on top and all sides. Rather, at least one side should be left partially uncovered.
There is no blech available. What now?
In this case, a pot containing food which is half-cooked—or in an emergency a third cooked—when Shabbos begins, may be left on an uncovered flame. (For further details, see earlier in this chapter.) Note that using a blech is always preferred.
Alternatively, aluminum foil, which acts as a blech, may be placed over the flame, in which case it is best to use a thick type which will not burn through during Shabbos.
Note that a pot may never be replaced onto an uncovered flame on Shabbos.
There is no blech, and the food is not cooked. What now?
Food or water less than half cooked—or, in an emergency a third cooked—left on an uncovered flame must be removed before Shabbos arrives.
I left an uncooked pot on an uncovered flame. What now?
Food or water less than half cooked—or in an emergency a third cooked—left on a stove not covered with a blech, may not be eaten until Shabbos has ended, specifically, until the amount of time has passed motzei Shabbos which would be needed to cook this food. This is whether the food or water was forgotten or left there thinking it was permissible.
How is a crockpot used?
Placing food, before Shabbos enters, into a crockpot whose temperature can not be adjusted, has the same function as placing it on a blech.
A crockpot whose temperature can be adjusted must have a preventive- reminder inserted, as with a stove.
How is a hotplate with variable temperature used?
A hotplate whose temperature can be varied needs to be covered with a blech or aluminum foil, just as with a stove.
A hotplate whose temperature can not be varied does not need to be covered with a blech.
How is a stove with electric coils used?
A stove’s electric coils, even when they are below a glass surface, are considered like an open flame. All the laws regarding a gas flame apply here as well.
May I leave food in an oven before Shabbos?
Yes. Before Shabbos, food may be left in an oven, whether on or off, for removal and use on Shabbos. It is best if the food is completely cooked when Shabbos begins.
However, according to the letter of the law, the food’s being at least half cooked—or, in a tight spot, a third—suffices40 to leave it inside an oven from before Shabbos for use on Shabbos morning or even Friday night41.
Adding raw meat immediately before Shabbos allows leaving food in an oven for use on Shabbos morning.