Gemara

(Rav): If one slaughtered with a Nochri's knife, he must peel off a layer from the areas cut;

(Rabah bar bar Chanah): It suffices to rinse them.

Suggestion: They argue about whether the place of Shechitah is Rose'ach (very hot, so that absorbed taste in the knife will enter the meat).

Version #1 - Rejection: All agree that it is Rose'ach. This is why Rav requires peeling off a layer. Rabah says that it suffices to rinse. Since the Simanim are busy exuding blood, they cannot absorb tastes from the knife.

Version #2 - Rejection: All agree that it is not Rose'ach. This is why Rabah says that it suffices to rinse. Rav requires peeling off a layer. The pressure of the knife causes its absorbed tastes to enter the meat.

Rav Acha bar Yakov and Ravina argue about a knife used to slaughter a Treifah. One obligates Hag'alah (Kashering in boiling water). The other holds that it suffices to wash it in cold water. The Halachah follows him.

Question: Even if it was used to slaughter a Kosher animal, it absorbs Isur, i.e. Ever Min ha'Chai (a limb of a living animal) before the Shechitah is completed!

Answer: It absorbs only when it gets warm, which is when the Shechitah finishes. Then, the meat is permitted!

Shabbos 40b (Beraisa - R. Shimon ben Gamliel): One may anoint her hand with oil, heat it in front of a fire, and anoint her son without any concern.

(Rav Yehudah): Yad Soledes Bo is forbidden. Less than this is permitted.

Question: How hot is Yad Soledes Bo?

Answer (Rachbah): It is hot enough to scald a baby's stomach.

Rishonim

Ran (Shabbos 19a DH Tanu and DH Soledes): The Gemara permits putting water near the fire to get warm, but not hot. Rashi explains that one may put it near the fire where it could cook, as long as he removes it before it gets Yad Soledes Bo. The Ramban disagrees, for the text of the Ge'onim and Yerushalmi permits only in a place where Yad Sholetes Bo, but not where Yad Soledes Bo. Soledes means 'drawn back.' The Gemara asked about what is Yad Soledes Bo, for it is different for different people.

Poskim

Shulchan Aruch (OC 318:14): One may leave water near a fire if it cannot reach Yad Soledes Bo, i.e. a place where a baby's stomach is scalded.

Chavos Ya'ir (104, cited in Pischei Teshuvah 94:2): Anything hotter than lukewarm, which is the temperature of saliva, is a Safek Yad Soledes Bo.

Darchei Teshuvah (105:51): Shoresh mi'Yakov says that one should be stringent only if it is Cham Heitev (properly warm). If not, it is cold. Yad Yehudah says that Rashi's Perush is primary. Yad Soledes Bo is so hot that one fears to keep his hand there, lest it get scalded if he would leave it there. Divrei Yosef says that one may rely on this for mid'Rabanan laws. If a Stam person's finger would not get scalded, we are not concerned, even if a baby's stomach could get scalded. The Mechaber said Stam that the Shi'ur is based on a baby's stomach, i.e. even for mid'Rabanan laws. Toras Yekusi'el says that the custom is gauge based on the hand; we do not question the custom. Eliyahu Rabah, the Drishah, the Pri Megadim and Machatzis ha'Shekel say that it is based on a baby's stomach. However, this Shi'ur is not known. Minchas Shai says that if one can eat food without blowing on it, it is not Yad Soledes Bo. Also this varies from person to person! Chavos Ya'ir is stringent for anything hotter than saliva. Bahag says that milk is cold, even at the time of milking, even though it is hotter than saliva. Even for an Isur Torah, one need not be concerned for anything no hotter than this. For an Isur mid'Rabanan, one may rely on Rashi, and permit if one can keep his hand there.

Pischei Teshuvah (7) Bechor Shor (Chulin 103) says that since we are unsure of the Shi'ur when a baby's stomach is scalded, for mid'Oraisa laws one should be stringent about a Kli Rishon until he has no Safek that it is not the mentioned temperature. Even for mid'Rabanan laws, this requires investigation. The custom is to estimate a heat that one cannot leave his hand there due to the heat; only this is Yad Soledes Bo. This is wrong.

Ben Ish Chai (2 Bo 5): We do not measure Yad Soledes Bo based on fingers, rather, based on a baby's stomach. How does one know this Shi'ur? Something is Yad Soledes Bo only if it is too hot to eat or drink.

Chelkas Yakov (YD 44:3): If water gets between 50 and 70 degrees (Celsius), this is Yad Soledes Bo, but it is not enough for hglk. Hag'alah requires boiling water, i.e. 100 degrees.

Igros Moshe (OC 4:74): It is proper to be stringent to be concerned lest 43 degrees is Yad Soledes Bo. There is a Safek until 71 degrees. That is surely Yad Soledes Bo, even to be lenient.

Igros Moshe (YD 2:52): Yad Soledes Bo is about 79 degrees, to be stringent.

Igros Moshe (YD 3:31): Surely, 74 degrees is Yad Soledes Bo.

Minchas Yitzchak (7:61): Sixty degrees is surely Yad Soledes Bo.

Minchas Shlomo (1:91:8): Bahag says that milk is cold, even at the time of milking, even though it is hotter than saliva. This is never more than 40 degrees. I think that this is why people are lenient up to 40 degrees. This does not contradict the Chavos Ya'ir, for when one is Choleh (sick), his temperature reaches 40 degrees. Perhaps Chavos Ya'ir intended to be stringent only for what is hotter than saliva of a Choleh.

Minchas Shlomo: I say that there is no concern less than 45 degrees. Amora'im argue about whether Beis ha'Shechitah is cold or Rose'ach. One version holds that all agree that it is cold. The Rema says that Iruy suffices for a Shechitah knife because the heat of Beis ha'Shechitah is not truly like a Kli Rishon that is Yad Soledes Bo. Even those who disagree hold that Beis ha'Shechitah is not truly Rose'ach. It is only slightly Rose'ach. The knife absorbs only at the end of Shechitah, for then Beis ha'Shechitah is Rose'ach. Before this, it is cold. The Poskim did not distinguish between birds and animals. The temperature of a healthy duck is almost 45 degrees. The busy that exudes from Beis ha'Shechitah is at least a degree more. If it is sick, it is at least two degrees higher. Since we hold that this is cold, and no one distinguished between healthy and sick, at least 48 degrees is cold. Even if we would find that they are no hotter at the time of Shechitah, and we will not judge from Shechitah of sick ducks, since we do not distinguish ducks from oxen, in any case at least all agree that 45 degrees is cold. The Tur (YD 105) brings from the Rashba that we do not distinguish heat of Beis ha'Shechitah from heat of a fire. The Ritva (Shabbos 42a DH ul'Inyan Beli'as) says that even according to the opinion that Beis ha'Shechitah is Rose'ach, it is not Yad Soledes Bo. Rosh Yosef says that if it were Yad Soledes Bo, all would agree that it is Rose'ach. They argue about whether absorptions occur even though it is not Yad Soledes Bo.

Rebuttal (Ashrei ha'Ish 2:23:102, citing ha'Gaon R. Y.S. Elyashiv, Shlita): The Amora'im do not argue about whether Beis ha'Shechitah is Yad Soledes Bo. This could be verified! Rather, they argue about whether it is hot enough to cause absorptions. I tested with my finger, and found like Igros Moshe says, that the Safek is between 30 and 71 degrees.

Note: Surely '30' this is a printing error. Presumably it should say '43', like it says in Igros Moshe. Even regarding 43 degrees, this arouses a question on hot summer days. If the entire house is close to 40 degrees, the entire kitchen can be Safek Yad Soledes Bo due to heat of the stove or oven. Why may one put a food that is not fully cooked (or any liquid that is not still hot) in the kitchen?

Yechaveh Da'as (6:21, b'Sof): If a hot water urn has a glass tube outside in which there is water to show how much water remains in the urn, when one takes water, he causes some of the water in the tube to enter the urn and get boiled. Shalmei Yehudah said that he measured and found that the water in the glass is about 50 degrees. Surely this is Yad Soledes Bo. Therefore, there is no concern at all. Also R. Y. S. Elyashiv and R. Binyamin Zilber permit.

Note: I did not find any Ashkenazi Poskim say that 50 degrees is surely Yad Soledes Bo.

Yachel Yisrael (20:8): Darchei Teshuvah proved from Bahag that Yad Soledes Bo is no less than 40 degrees. Minchas Shlomo said that it is at least 48, and in any case one may be lenient up to 45 degrees. Many Poskim are stringent for anything more than 40 degrees. They say that the Chazon Ish held like this. Igros Moshe says to be stringent for 43 degrees. Everyone should follow his Rebbi.