On Passover we are required (on the first two evenings; see Ch.37) to eat MATZAH made for human consumption (see 454:2) out of wheat (preferably), barley, oats, rye, or spelt (453:1-2). Since CHAMETZ is forbidden, the only food made out of these types of grain that may be eaten on Passover is MATZAH. Most Jews have the custom of not eating food made from other grains or legumes on Passover (see 453:1); this includes mustard (see 464:1).
To prevent it from becoming CHAMETZ, the grain used for MATZAH should be clean (see 453:3) and should be protected against contact with water, preferably from the time it is harvested (453:4); in particular, it must not be washed before being ground (453:5). It should be stored in clean sacks (see 453:6) and transported carefully (see 453:7). The mill should be cleaned; the grinding should be supervised, and should preferably be done before the day that the flour is made into dough (453:8-9). The flour should preferably contain some of the shells of the grain (454:1).
The flour should not be mixed with hot water (see 454:3); the water should be drawn at the end of the day by a Jew and should stand overnight in a cool place (see 455:1-4). The amount of flour used to make the dough should preferably not be more than the volume of 43-1/5 eggs (see 456:1-3), and the MATZAH should not be too wide (see 454:1) or thick (460:4-5). Salt or spices should not be added to the dough (454:5-6;462:7). On separating the dough-offering see 455:1-2.
The water, flour and dough should not be transported out of doors uncovered, and the dough should be made indoors, away from open windows and from fire (459:1,5). The dough should not be allowed to stand without kneading even for a moment, and should be put in the oven within 18 minutes of the time it is made (see 459:2); on wetting it first (with cool water) see 459:3. The utensils used with the dough should be cleaned constantly so that no dough sticks to them, the water should be disposed of (459:4), and any remaining traces should be annulled (see 460:3). Flour should not be added to the dough or allowed to come in contact with the MATZAH (459:6).
The baking should be done by an adult Jew (460:1), should be supervised (460:2), and should be done quickly (460:4). The MATZAH should have a simple shape (460:4) and nothing should be baked with it (see 460:6-7). The oven should be heated to burn out any traces of CHAMETZ, and then should be lit evenly (461:1) so that the MATZAH bakes quickly (see 461:2) and completely (see 461:3). On MATZAH that is folded over or swells up, touches other MATZAH, or is baked with CHAMETZ see 461:5. The required MATZAH must not be stolen (see 454:4) and must not be cooked after being baked (see 461:4). Some have the custom of baking the required MATZAH on the afternoon of the 14th of Nisan, or the 13th if the 14th is a Sabbath (458:1).
Dough made with liquids other than water does not become CHAMETZ, but MATZAH baked from it is called “rich MATZAH” and must not be used to fulfill the requirement of eating MATZAH (462:1,3,6;463:1-2;466:5). Dough made with a mixture of water and other liquids becomes CHAMETZ quickly, and must be baked immediately (462:2); MATZAH should not be made from such dough except for use by old or sick people (462:4), and on the holiday such MATZAH should be made only in small quantities to avoid the need to separate the dough-offering from it (462:5).
Shulchan Aruch, Copyright (c) 2000 Project Genesis, Inc.