Beitza 33a (2)
1- Our Mishnah presents a dispute regarding the permissibly of gathering slivers of wood.
Rav Yehudah rules that it is permitted to make slivers – toothpicks – out of materials that are soft enough to be used as animal food, such as straw or reeds.
2- Our Mishnah continues about the prohibition of creating a fire on Yom Tov, specifying the types of materials one can use to create a fire:
Rubbing wood, knocking stones, heated sand, and tiles which are banged against each other.
It also mentions creating fire from ‘water’ !
Rashi explains that one uses water as a magnifying glass to create a flame. He concludes ‘so I have heard from the Gaon of Lombardia, Italy’.
cooking on heated sand
See here an interesting forum about the teachers quoted by Rashi.
3- We discussed the general rule about cooking on Shabbos. The Gemara in perek Kira in Shabbos 39a, categorizes two types of heat sources:
1- Fire- אור (ur)
2- The sun – חמה
Cooking/baking/frying/ on a fire is of course prohibited Min Hatorah.
Cooking by a derivative of fire, such as placing raw food on a hot tray that was heated by fire, is also Min Hatorah. תולדות האור.
cooking with the sun
On the other hand, cooking by using the heat of the sun is fully permitted! Such as placing a pitcher of water exposed to the sun on very hot day. That is not considered בישול at all.
Reason being in that it is not ‘the usual way of cooking’, or because it is ‘not the way things were cooked in the Mishkan’.
sun oven
But cooking in a derivative of the sun’s rays is prohibited. So placing raw food on a hot try that was heated by the sun is prohibited MideRabonon. תולדות החמה.
4- What about cooking using a mirror that reflects the sun’s rays?
We mentioned the ancient Greek story of Archimedes (lived in the times of the, lehavdil, early תנאים) that set the ship of his foes on fire by using a the sun’s rays via a glass contraption.
For years it was considered a myth until some scientists at MIT were able to somewhat replicate this feat.
See here.
A sea of mirrors reflects sunlight toward a solar power tower, heating liquid, creating steam, turning a turbine, making electricity.
5- From here we went to discuss the issue of the Israeli דוד שמש.
“dud shemesh” not employing electric heating
Water is heated by absorbing the sun’s rays and then piped down to the house. If one were to place raw food in water heated by this solar system, would that be considered cooking by ‘the sun’ (permitted) or cooking in the ‘derivative of the sun’ (prohibited)?
This question generated a heated (pardon the pun) debate among the great Rabbis of the previous generation. See here for many opinions.
some dude cleaning his dud
6- We spoke about the opinions about using matches or as it was called צינדהעלצעל. Is lighting one considered crating a new fire or perhaps the phosphorus has ‘the fire in it already’ and it is like lighting from an existing flame.
The כתב סופר and others prohibited it and such is the Halachah. See here. (67)
http://beta.hebrewbooks.org/pdfpager.aspx?req=950&st=&pgnum=123
7- We mentioned cooking in a natural hot water spring like חמי טברי.
To be continued…