T-Fal has a cookware set known as "T-Fal Thermo Spot Heat Indicator Cookware". These pots have a special technology which indicates when the pan is ready for use. May one use such a pot on Yom Tov to cook? May one remove such a pot from a Blech on Shabbos when this will cause the indicator to go off?

  • I have been reading up on temperature color change technology and it seems that there are many who allow such color change items on Yom Tov. I can forward the answer I got from an Israeli rav when I posed a similar question if that helps.
    – rosends
    Commented May 4, 2015 at 15:49
  • @Danno: Sounds like an answer perhaps Commented May 4, 2015 at 16:38
  • I asked about a number of color changing technologies and the answer was sourced but not precisely about heating so I'd rather just forward the whole thing.
    – rosends
    Commented May 4, 2015 at 17:14
  • I have notes with many sources on this topic, including: Forehead thermometers, Coors Beer - Cool/Super Cool indicators, child safety spoons with heat indicator, coffee mugs with heat sensitive text, transitions (photogray) glasses - tinted only when in the sun and diapers with wetness indicators. However it is all in Hebrew. Can I just put them in as an answer or do I have to translate the sources to English first?
    – nagah
    Commented May 8, 2015 at 2:57
  • @nagah: The best way to do that is to put in a short English summary together with the notes. Commented May 8, 2015 at 13:00

1 Answer 1


I had hoped to have time to do a better summary, but this will have to suffice:

Regarding forehead thermometers, there are a relatively large number of responsa:

  • שו"ת מחזה אליהו סימן סה is of the opinion that it is forbidden both for writing as well as coloring. (His thermometer has the letters N and F that would light up if there was No Fever or Fever, respectively.)
  • ציץ אליעזר חי"ד סימן ל' as well as שו"ת יחוה דעת חלק ד סימן כ"ט permit using such thermometers.

Photogray Lenses (i.e. Transitions lenses):

  • שמירת שבת כהלכתה פי"ח הערה ע, אז נדברו ח"ב סימן ל, שו"ת אגרות משה או"ח ח"ג סימן מ"ה, שו"ת אז נדברו ח"ב סימן ל"א, בילקוט יוסף או"ח ט' סימן "א אות ל"ד Are all of the opinion that it is permitted to wear such glasses on Shabbos.
  • However there are opinions that it is forbidden. For example: אז נדברו ח"ב סימן ל, חלקת יעקב חאו"ח סימן קנ"ב and מנחת יצחק ח"ה סימן ל"ב אות ב among other hold that it is forbidden to wear these glasses on Shabbos.

Diapers with color indicator:

  • Star-K (I can no longer find the link on their site, they seem to have removed it, but see here) initially forbid using such diapers on shabbos, but subsequently retracted and permitted their use. See page 6 of link.

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