Avodah Zarah 34 - Items of Idol Worshippers that are Permitted
Rabbi Akiba visited Ginzak, where they asked him, "Is fasting for part of the day considered religious fasting?" He did not know. They asked, "Earthenware jugs of idol worshippers, are they prohibited or permitted?" He did not know. "In what garments did Moses serve during Tabernacle inauguration?" He did not know.
Rashi says that it is inconceivable that Rabbi Akiba would not know this, and prefers a version where it was Mar Ukva. But Tosafot reject Rashi's argument.
Rabbi Akiva went to ask in the Academy, and they told him, "If one was too busy to eat in the morning, he can formally accept a religious fast till the end of the day. The jugs of idol worshippers are permitted after twelve months. Moses served in white garments, since he was not entitled to kohen's garments, but could not use his regular garments either."
Fish brine prepared by an idolatrous householder is forbidden, because they would add wine to it. But if a professional fisherman made it, it is permitted, since he uses the fat of fish and does not add wine.
Art: James Abbott McNeill Whistler - Symphony in White