can one pour hot water from a cup of hot water (that was poured from an urn on shaboos) over ground coffee in filter and make a fresh cup of coffee
joe man, nj
The Mishnah in Shabbos (145b) says that one is allowed to pour hot water on uncooked food on Shabbos but not on salty fish because that is considered completing its preparation (Gemar Melachah). The Mishnah is speaking about pouring hot water even from a Kli Sheni (e.g., a cup that hot water was poured into from the urn) (Mishnah Berurah OC 318:35). Rashi there says that Gemar Melachah means Bishul. We see from this Mishnah that there are certain foods which cook more easily than most foods (Kalei Bishul).
In addition, the Gemara in Shabbos (42b) says that salt cooks in a Kli Sheni. Based on this, Rebbi Eliezer of Metz (Sefer Yere'im, Shabbos 144b) writes that there are other foods that are Kalei Bishul, and therefore we should not put bread in hot water, even in a Kli Sheni (even though the bread was already baked; we will return to this point soon). The Magen Avraham and Mishnah Berurah (318:42) both say that all raw foods should be considered to be Kalei Bishul unless there is a clear proof from the Gemara that they are not (e.g., oil). The Chazon Ish (OC 52:19) says that we do not have to be concerned that a food is Kalei Bishul unless we see with our own eyes that it is cooking in a Kli Sheni; for example, bread and eggs. Everyone then agrees that tea leaves and roasted coffee are Kalei Bishul.
Therefore, seemingly, on Shabbos one may not pour hot water from a cup over ground coffee in a filter and make a fresh cup of coffee.
Coffee is roasted before it is ground. The roasting is equivalent to baking (Afiyah). Boiling the roasted coffee is cooking (Bishul). Therefore, making a cup of coffee is a case of cooking after baking (Bishul Achar Afiyah). It could be said that since the coffee was already roasted, the cooking afterwards is not considered a Melachah on Shabbos. As mentioned earlier, Rebbi Eliezer of Metz holds that cooking after baking is a Melachah. The Shulchan Aruch records his opinion (OC 318:5), but a second dissenting opinion is brought there that it is not considered a Melachah. In our case, however, it is a moot point because everyone agrees that by Kalei Bishul -- like coffee and tea -- cooking after baking is a Melachah (see Rema, ibid).
The whole discussion till now has been assuming that the hot water is hot (Yad Soledes, around 110 F and up) but not scalding (Yad Nichves). If it is scalding hot, then the Chayei Adam (cited by the Mishnah Berurah 318:48) says that all foods will cook in a Kli Sheni. Since most people use scalding hot water to make their coffee, even if it was not Kalei Bishul there would be a problem of Bishul.
This is not a Psak Halachah