I received this question via email. I am not really a klaf expert, I was wondering if anyone could answer this question: Dear Rabbi Gutnick, I am writing to you because a good friend of mine has put the idea into my head that the klaf in my tefillin were not really tanned and therefore are not kosher. He referred me to Megilla 19a re diftera. From the research that I have done so far, it seems that the klaf that is used today is tanned only with a lime wash. On all of the tanning websites I’ve seen so far, they say that the lime doesn’t accomplish tanning but only the removal of the hair and some other pre-tanning effects. Would you be able to explain to me or refer me to a website that explains how the tanning process that is used today takes the hide out of the category of diftera? Thank you very much.