If you have a few days, you may want to take a day trip from Madrid to the town of Talavera de la Reina, which is famous for its ceramic industry. Once when I was there with a few friends we asked in a factory shop if we could see how the ceramics were made, and we were shown around the back work area. It was really fascinating. We did not arrange this ahead of time, but I would not count on it working again, as this happened quite a few years ago, and it was very quiet at the time. Either it was a Sunday or a holiday, I bet. The artists weren't there. Still, maybe someone else has some insight as to how it could be arranged. If you go, you might also want to check out the little town of Puente del Arzobispo, which is close to Talavera. It is a small town, also devoted to ceramics. If you try to see the artists at work without arranging it previously, it MIGHT be easier there because it's all smaller than in Talavera.
Good luck, and have a wonderful trip!