Covering the flame with a Blech:
It is forbidden to leave less than half cooked food on an open flame into Shabbos. This Halacha created the use of a Blech. Today, it is a widespread custom to cover the flame on the stove top with a strong aluminum sheet called a Blech, and to also try to cover the knobs which control the flame. [Thus, one may leave even less than half cooked food on a Blech into Shabbos.]
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.
Q&A on Ketumah
Does putting up a sign help to achieve Ketumah?
If the flame is turned on to its highest level, must it still be covered if the food is not half cooked?
Q&A on Blech
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 be covered?
Only the areas that are under the pots of food need to be covered.
Must the knobs of the stove 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.
Does it suffice to remove the knobs of the stove or lock the knobs or the oven in a way that it can’t be opened?
If the blech fell off before Shabbos, must one replace it?
If one had already placed the food on it there is no obligation to replace it even if the food is not cooked.
May one adjust the flame after having placed the blech on it?
Some Poskim rule that one may not adjust the flame, either higher [or lower], once the Blech has been placed. However if one noticed an extra flame lit under the Blech it is allowed to extinguish the flame entirely.
May one on Erev Shabbos place food on his electric plate which is not yet on but will turn on later with the timer?
See the laws of Chazara.
This matter is disputed amongst Poskim.
 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.
 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 of lowering the flame. Likewise perhaps they would suspect of lowering the flame and then raising it again. [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 Kehilchasa, Igros Moshe 1:93 and other Poskim, based on the fact that the covering is only needed as a recognition.
The reason: As the Sages did not suspect that one would add fuel to the fire and only suspected that one would stoke the currently existing flame. [Igros Moshe ibid]
 Igros Moshe ibid, and Sheivet Haleivi, in order so the area from where one raises the flame also have a recognition.
 Piskeiy Teshuvos 254:1; Chazon Ish 36:3; Az Nidbaru 4:26; Bris Olam Shehiya 14; Orchos Shabbos 2:18
 The reason: As this is similar to plastering the oven with clay. [ibid]
 Shabbos Kehalacha Vol. 1 pages 307-309; Pnei Shabbos 7:2; Arugas Habosem 57:8
 As explained in Halacha 1
 Shabbos Kehalacha Vol. 1 pages 310
 Rav SZ”A brought in SSH”K 1 footnote 55 and 185; Pnei Shabbos 1; Shabbos Kehalacha 8:15 [see there for an analysis on this subject]; Piskeiy Teshuvos 253:6
Other opinions: Some write that it is permitted to adjust the flame even if the Blech is already covering it as the mere fact that he decides to leave the Blech on the flame shows that he is not interested in it. [See Piskeiy Teshuvos ibid; Shabbos Kehalacha ibid footnote 37]
 Rav SZ”A ibid; See Shabbos Kehalacha ibid footnote 39
 The reason: 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. [See SSH”K ibid]
 Rav SZ”A in Miluim brought in Shabbos Kehalacha ibid
 See Halacha H!
 Shabbos Kehalacha Vol. 2 pages 34-36
 The Poskim of this generation dispute this matter. Rav Shlomo Zalman Aurbach learns that it is allowed and is not comparable to the law above regarding coals, in which the Michaber rules stringently, as it is not possible to insulate the pots there as it is with the coals. Rav Farkash agrees and brings proofs to this ruling. The Ketzos Hashulchan even goes as far as to rule that it may even be minority insulated according to the Beis Yosef, although as brought above, Rav Farkash argues on this ruling.