110:6 One must make sure that the vessel in which he does the kneading has no holes or cracks in which some dough might get stuck and leaven. While kneading, one may not place the vessel on cushions or pillows because of a concern that it might get warm. One must be careful to thoroughly clean the vessel every eighteen minutes, and also to wash his hands very well. Similarly, the boards and the rollers must be watched to make sure they don’t have any holes or cracks. At the very least, these must also be thoroughly washed every eighteen minutes, as well as the utensil that makes the holes. This is done to ensure that not even a little dough remains on them. The paddle used to put the matzos into the oven must also be inspected to make sure that it doesn’t have any cracks in which a little dough might get stuck and leaven. (It’s advisable to designate someone specifically to inspect all the utensils because it’s such a big job – Mishnah Brurah 459:33.)
110:7 If even a small amount of something strong – like salt or a spice – fell into the dough and was kneaded in, the whole dough is prohibited because of the warming effect. (If the salt or spice is removed and not kneaded in, the dough is still permitted – Mishnah Brurah 455:48. The sea salt that we use nowadays might not invalidate the dough after the fact – MB 455:42.) If a kernel of grain was found in the dough, one should a remove and discard a finger’s width of dough around it; the rest is then permitted.