Reference for Beitzah 1:7
בֵּית שַׁמַּאי אוֹמְרִים, תְּבָלִין נִדּוֹכִין בְּמָדוֹךְ שֶׁל עֵץ, וְהַמֶּלַח בְּפַךְ, וּבְעֵץ הַפָּרוּר. וּבֵית הִלֵּל אוֹמְרִים, תְּבָלִין נִדּוֹכִין כְּדַרְכָּן בְּמָדוֹךְ שֶׁל אֶבֶן, וְהַמֶּלַח בְּמָדוֹךְ שֶׁל עֵץ:
Beth Shammai say: Spices may be pounded (on yom tov) with a wooden pestle, [but not with a stone one. With this wooden pestle, however, they may be pounded in the normal manner, without any variation; for their flavor dissipates if pounded the day before.], and salt (may be crushed) with [an earthen] jug or with a wooden pot ladle. [For (the crushing of) salt requires a variation (shinui), in that it should have been crushed the day before, its flavor not going lost.] And Beth Hillel say: Spices may be pounded in the normal manner, with a stone pestle, and salt, with a wooden one. [The halachah is that one who wishes to crush salt on yom tov turns the pestle on its side and crushes it with a shinui, and spices may be pounded in the normal manner without a shinui.]