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In the question Pregancy Test on Shabbos a notion was raised in an answer that the chemical reaction is a problem due to tzovaya.

So what are the boundaries of this, in terms of chemical reactions being a problem, with the specific question of those glasses that dim in sunlight, and become dark like sunglasses. This is a reversible chemical reaction, and I have always wondered if it applies.

As a possible side note/question, what are the boundaries of tzovaya as would mixing vinegar and baking soda to make a volcano on Shabbos be allowed? A chemical reaction that makes a mess...

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    I suggest you split the volcano question off into a separate question: it really is quite separate. – msh210♦ Jun 20 '11 at 17:09
  • msh2010: Good point, leaves me with two excellent answers, and I cannot accept both. However, minor question, probably not worth doing it. – geoffc Jun 20 '11 at 19:12
  • With regard to the first part, don't we generally allow certain forms of Melachah if it isn't permanent (disappearing ink for medical uses in Israel, although still a problem MiDeRabbanan for others), and especially if it's part of the function of the thing (like a folding chair not being a violation of Boneh, or a self-winding watch)? This is a comment, not an answer, because I'm unsure. – Seth J Jun 20 '11 at 19:17
  • Also, someone wearing the glasses isn't causing the graying, although he could be responsible for it by walking outside. – Seth J Jun 20 '11 at 19:20
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Rabbi Moshe Feinstein allowed the photo grey lenses, though others disagree.

For more on the theory behind this, see this excellent article from the Star-K's Rabbi Mordechai Frankel (based on the ruling of Rabbi Moshe Heinemann). It discusses the related question of diapers that change color when wet.

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I saw an excellent comprehensive article on exactly this topic

It brings down all the opinions and reasons, but the bottom line is that these photo chromic lenses are permitted on shabbos.

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    cmb, welcome to the site and thanks for this sourced answer. I hope you stick around and enjoy the site; you may also wish to register your username, as that will afford you a better site experience. – msh210♦ Feb 10 '12 at 15:45
  • Great link! excellent info - thanx! the other articles there (in that column) by that author are also very interesting and comprehensive! – user1248 Feb 12 '12 at 14:43
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There is no problem of Tzoveya with food. Regarding photogrey glasses per this link it is allowed. http://www.revach.net/article.php?id=2497

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Tzovaya translates to coloring or dyeing something. I can't really see how a baking soda and vinegar volcano would be a problem at all with tzovaya.

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There is a really funny story regarding this question. Basically it boils down to an argument between Rav Elyashiv and Rav Shlomo Zalman Auerbach.

See here for the story: http://books.google.co.il/books?id=mxf2QDFNvcYC&lpg=PA265&ots=XhCH9pRwV7&dq=elyashiv%20auerbach%20eyeglasses&pg=PA265#v=onepage&q=elyashiv%20auerbach%20eyeglasses&f=false

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    Doesn't come up for me (gives an error message that the page is unavailable). Would you mind summarizing the story? – Alex Feb 10 '12 at 18:15
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    @Alex I see it. In short, R' Eliyashiv's daughter married the son of R' Shlomo Zalman Auerbach. She once came to an optometrist who suggested her to buy photo-gray glasses. Since her father in law and father disagreed on whether these glasses are permitted, she was unsure whether to buy them or not. When the optometrist asked her why she can't make up her mind, she said that while her father was of one opinion on the matter, her father in law had a different one. The optometrist looked at her and said "Who cares what your father or father in law think, go ask a posek!" – Shmuel Brin Feb 10 '12 at 18:44
  • Yes, I heard this story brought down by Rabbi Hanoach Teller who, if I recall correctly, was close with the Auerbach family and was one of Rav Auerbach's biographers. – Bruce James Dec 27 '12 at 19:42
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Harav Yitzhak Yosef Shelit"a is Matir (Y"Y KS"A 320:20).

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There are many reasons why photogray glasses are not Tzoveya (see "the 39 Melachos", by R.D Ribiat):

  1. Its not really dyeing
  2. Its regular use of the glasses (derech tashmisho)
  3. Its not permanent
  4. etc.
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