Practical summary of how to prepare the Blech Erev Shabbos:
It is forbidden to leave less than half cooked food on an open flame into Shabbos. This law created the use of a Blech. The blech effectively covers the fire and allows one to leave even less than half cooked foods on the fire into Shabbos. In most cases the foods are in any event fully cooked and a blech is hence not needed. It is customary however to place a blech over the fire even in such a case, as this allows one to be allowed to return foods to the fire on Shabbos in case of need.
Covering knobs: It is proper to cover the knobs of the stove in addition to covering the fire. Today there are blechs available that contain a side metal addition which is used to cover the knobs. Alternatively one can place tinfoil over the knobs.
Electric plates: If one uses an electric plate to keep his food hot over Shabbos then if the electric plate does not have alternate settings of heat, it does not need to be covered even if the food will not be half cooked before Shabbos. If it does have alternate settings of heat then it must be covered for one to be allowed to leave less than half cooked food on it into Shabbos.
Ovens: One may leave food which is at least half cooked in a lit oven into Shabbos. One may not leave less than half cooked foods in an oven unless the inside of the oven is properly covered. One must beware not to open an oven while it is off if it is temperature based.
When to set up the blech: The blech is to be set up together with the food with enough time for the food to heat up if the food were to be cold. Meaning even if the food is currently hot it is to be placed on the Blech with enough time for it to become hot if it were to be cold. In a case of need one may set up the Blech any time before Shabbos even if the food is cold.
If one does not have a blech or electric plate: If one does not have a blech or electric plate for Shabbos he may still leave all his food on an open flame before Shabbos, so long as the food is half cooked by the time Shabbos enters.
What status of oven do Crock-pots, electric plates, and water boilers have today?
Adjustable temperatures: If they have adjustable temperatures they have the same status as stoves and ovens, and must be covered in order to leave less than half cooked foods on them before Shabbos.
One setting: If they have only one setting, either on or off and it is thus impossible to higher or lower the temperature, they never need to be covered. However there are Poskim which are stringent in this matter regarding water boilers. [To note that boilers with Shabbos setting are considered to have adjustable temperatures and hence the water must be cooked to the point of Ben Drusiay before Shabbos begins.]
Attached to a timer: If the electric plate, crock pot etc is attached to a timer which turns on and off constantly on Shabbos, then even if the electrical device itself contains no adjustable temperatures seemingly it has the same status as an electric device that does contain adjustable temperatures, and hence must be covered in those cases where Ketumah is required.
How does one achieve “Ketumah” on today’s stoves, oven, crock-pot, and electric plate, in order to allow leaving uncooked food on them from before Shabbos?
Stoves: Today it is a widespread custom to cover the stove top with a strong aluminum sheet called a Blech, and to also try to cover the knobs which control the flame.
Ovens: Some Poskim rule all the walls of the oven must be covered. Others rule it suffices to cover one wall, and the knobs which turn the flame. Practically one should avoid using an oven for less than half cooked foods. According to all one may use a tin oven insert which surrounds the food from all sides.
Shabbos oven: Those ovens which contain a Shabbos setting must likewise be covered in order to leave less than half cooked food on them before Shabbos. However there are Poskim which say that if it is placed on this setting it does not need to be covered.
Electric plates with adjustable temperatures: Is to be covered with a sheet of tinfoil.
If the flame is turned on to its highest level, must it still be covered if the food is not half cooked?
Is there any minimum thickness which the blech must be?
No. However there are those which are stringent to use only a thick blech in order so it somewhat diminishes the heat of the flame.
May one cover the fire with tinfoil?
Ideally this is a legitimate form of covering, although there are opinions which doubt its validity. For this reason if one has nothing else to use but tinfoil he should try to fold the foil to make it thick, or to also cover the knobs of the stove.
May one use a blech that has holes in it?
If the holes are small, it is a valid covering. However if the holes are large enough to allow the flame to enter through it, then it is not considered a valid covering.
Must the entire area of the stove, or electric plate, or crock pot be covered?
Only the areas that are under the pots of food need to be covered. [Thus by a crock pot that the walls of the pot touch the entire inside, all of the inside must be covered, although as already explained in the previous question, if there are small holes, this does not pose a problem.]
Must the knobs of the stove etc be covered?
From the letter of the law it suffices to cover the flame alone. Nonetheless, there are Poskim which rule that the knobs should also be covered. According to all it does not suffice to only cover the knobs and not the flame.
May one adjust the flame after having placed the blech on it?
Some Poskim rule that one should not adjust the flame, either higher or lower, once the Blech has been placed. However it is allowed to extinguish a flame entirely.
 Shabbos Kehalacha Vol. 1 page 299
 Although electric stoves are covered by metal or fiber, and hence are physically similar to a gas stove with a blech, nevertheless since the metal is constantly there, without one doing any action, it does not serve as a reminder against raising the flame. In a similar case of doing Chazara to a heater in 253/26 Admur brings a dispute regarding if we consider the constant metal covering deemed as Ketumah, and he rules to be like the stringent opinion.
 There is a discussion amongst Poskim regarding if in truth the suspicion of “Shema Yechta” applies today by our ovens and stoves being that one can only higher the flame, while the suspicion back then was not that one may come to add more wood to the flame but that he may blow on the coals. The Igros Moshe [1/93] thus sides that today the suspicion is no longer applicable, however nevertheless even he rules that the flame must be covered, and so rule all the Poskim today. [Shabbos Kehalacha Vol. 1 page 296]
 On the one hand it has an opening on its side as does the ovens in Admur’s time. On the other hand there is only room for one pot on top, which is the main logistic ramification between the heat of the two ovens. See however Igros Moshe 4/74-26 which rules that if an oven is thermostat based, and it hence prevents the flame from becoming too hot, then it always has the status of a Kirah.
 Shabbos Kehalacha Vol. 1 pages 298-300
 Although electric plates, boilers and crock pots are covered by metal, and hence are physically similar to a gas stove with a blech, nevertheless since the metal is constantly there, without one doing any action, it does not serve as a reminder against raising the flame. In a similar case of doing Chazara to a heater in 253/26 Admur brings a dispute regarding if we consider the constant metal covering deemed as Ketumah, and he rules to be like the stringent opinion.
 Now although it is possible for one to remove the plug from the socket, nevertheless we do not suspect one may come to lower the flame, but rather only to raise it.
 Igros Moshe 4/74-23
 Perhaps this is for the reason mentioned by the questioner there, that one may come to remove water from the boiler on Shabbos prior to it being fully cooked, and doing so is forbidden due to hastening the cooking. However such a logic seems puzzling as we have no precedent that the Sages ever suspected for such a matter, and from the clear rulings it is implied that the opposite is the case.
 Authors note
 Such as one set up his electric plate to turn on for half hour intervals in order to keep his food warm, and not burn the food.
 As we suspect one may come to adjust the time in a forbidden way in order so the food be ready on time.
 The covering of the fire which hence allows one to leave uncooked food on the flame over Shabbos.
 Shabbos Kehalacha Vol. 1 pages 302-304
 Although this form of covering is not exactly similar to the concept of “sweeping or covering the coals” being that the latter effected a diminishing of the heat as opposed to the former, nevertheless many Poskim permit this to be used, including the Kaf Hachaim, Igros Moshe [1/93], and many of today’s Poskim.
Their reasoning is:
- It does somewhat diminish the heat.
- The main point is to show recognition that there is a decree, and this is accomplished also by placing a sheet of metal over the fire.
Other Opinions: The Ketzos Hashulchan [71/4] and Chazon Ish voice doubt regarding the validity of the Blech to achieve the state of Ketumah.
 Regarding why simply lowering the flame before Shabbos does not suffice as Gerufa: See Shabbos Kehalacha 8/14 that Gerufa means to completely remove the coals from under the pot and not to simply remove a portion of them.
 Shabbos Kehalacha page 312
 Chazon Ish 37/9, Minchas Yitzchak 3/28 and Igros Moshe 4/74-27
 Sheivet Haleivi
 This is similar to a tin box with an opening.
 This places the oven on a set temperature.
 As one can easily switch the oven from a Shabbos setting to a non-Shabbos setting, just as we suspect one may come to raise the flame. What the Shabbos setting is useful for is to allow removing food from the oven on Shabbos without worry that doing so may lead to the fire turning on, or staying on longer. [Shabbos Kehalacha 8/17]
 SSH”K 1 footnote 17
 It does not suffice to claim that a Shabbos setting serves as a reminder that one not raise the flame, as the Sages did not suffice with a mere reminder but rather with an act that diminishes the flame, as is evident from the concept of Ketumah. Perhaps however one can claim that placing it on a Shabbos setting diminishes the flame, and hence it can serve as a reminder.
 Shabbos Kehalacha pages 300-302
 Shabbos Kehalacha page 299
 Igros Moshe 4/74-25
 This is due to “Lo Plug” that we do not differentiate in the decrees of the Sages. However regarding diminishing the flame, this we do not suspect for. However by our gas ovens which are easily able to be diminished, perhaps the Sages would have likewise suspected for lowering the flame. [ibid]
 Shabbos Kehalacha Vol. 1 pages 304
 Shabbos Kehalacha Vol. 1 pages 304
 As at times tin foil is placed also during weekdays if one does not want the stove top to dirty, as well as that the foil burns off.
 Shabbos Kehalacha Vol. 1 pages 305
 Shabbos Kehalacha Vol. 1 pages 305-306
 Shabbos Kehalacha Vol. 1 pages 306
 So rules Shemiras Shabbos Kihilchasa, Igros Moshe [1/93] and other Poskim, based on the fact that the covering is only needed as a recognition.
 Igros Moshe ibid, and Sheivet Haleivi, in order so the area from where one raises the flame also have a recognition.
 Shabbos Kehalacha Vol. 1 pages 307-309
 As explained in Halacha 1
 Shabbos Kehalacha Vol. 1 pages 310
 Rav SZ”A [SSH”K 1 footnote 185]; See Shabbos Kehalacha 8/15 for an analysis on this subject.
 As doing so uproots the recognition made by the covering of the blech, as the entire purpose of the recognition is to show one is uninterested in the flame. Hence by adjusting the flame one uproots this recognition.