Why do we use a blech or hotplate to keep our food warm?

The most natural way to keep food warm is to store it in a hot oven or on an open flame and yet we know that it is not done on Shabbos. What is the reason for this?

In times of yesteryear, food was cooked in and on stoves fueled with wood and coals. In the course of cooking, the wood would become coals and eventually simmer down to glowing ambers. When leaving cooked food in these ovens before Shabbos, in order to remain warm for the night and day meals, there was the problem of cooling down, which would result in cold food. Chazal were concerned that people would stoke the dying coals to revive them and thereby heat the food. [1] Chazal devised a method to circumvent this problem.

What is the halachic problem with stoking coals?

The problems are twofold. Firstly, stoking coals is similar to lighting a fire, which is a melacha called mav’ir, a biblical prohibition. [2] Secondly, the food might have cooled down to a point that it is no longer considered cooked and by reheating it one is cooking the food. [3] The latter is of lesser biblical consequence but it is nonetheless severe.

How did they keep the coals hot?

They did not. That was not within their capacity. They devised a method to prevent stoking the coals. Two methods were developed whereby making the oven 'gerufa v'katuma'.  'gerufa'  means to rake the coals from the oven and 'katuma' means to sprinkle the coals with ash. By raking the coals from the oven before Shabbos one would not be able to stoke the coals because there were none left inside. By sprinkling ash on the coals one would demonstrate that one does not intend to handle the coals. [4]

Obviously sprinkling does not extinguish the coals, so what’s the point?

Indeed only a small amount of ash was sprinkled and yet Chazal wanted that action performed in order to demonstrate that one is ‘removing’ one’s thoughts from the heat source and that it will not be tampered with on Shabbos. The idea was not to totally eliminate the possibility of stoking but rather to act as a reminder not to stoke. Chazal were concerned that ‘in a moment of forgetfulness’ a person will stoke the coals, not that one would deliberately disregard Shabbos laws. Thus this small reminder would suffice.

How do modern stoves and grates relate to this?

We see that by modifying the coals one was permitted to leave pots of food on the heat source, so too by modifying our heat sources we will accomplish the same.

There are two common methods: using a blech – a flame-cover made of galvanized iron, (sometimes incorrectly described as “tin”) copper or stainless steel and a hotplate.

The blech serves as a reminder not to ‘stoke’ the fire. The hotplate, which is not equipped with a dial for adjusting the temperature, is used for keeping the pots of food warm. Since there are no dials, there is no concern that one will adjust the heat.

Must the gas dials be covered by the blech?

It is a machlokes haposkim (dispute amongst the great rabbis). Rav Moshe Feinstein ztz”l preferred the dials covered, as is common custom in most parts of the world. Rav Shlomo Zalman Auerbach ztz”l and Rav Eliashiv shlita hold that the dials need not be covered. One should follow one’s rav in this (and every other) aspect.

Would covering the dials suffice?

Since the idea of the blech is to prevent one from adjusting the heat it would seem that it would suffice to cover the dials and not the flames and yet we find the opposite. The answer is that the Rishonim (early poskim) wanted the fire modified in a detrimental manner [5] and that modification would serve as a reminder. Otherwise why would it not suffice to attach a ‘remind-me’ note to the dials saying “Shabbos – don’t touch”? [6]

To summarize - there are two prevalent ways to leave food warm before Shabbos, the blech and hotplate.

May one leave food in an oven before Shabbos to keep it warm?

The problem with modern day ovens is that it is quite difficult to bring them to the situation of  ÷èåîä – covering the flames or heating elements. One way would be to place a metal box inside the oven and place the pots of food inside it. Another might be to line the oven sides with aluminum foil and thus create a blech. Today, in North America and other places, there are stoves and ovens available that are “kosher for Shabbos”. It is not a simple matter and a rav must be asked.