Next time you see a piece of blue-and-white Chinese porcelain lovingly displayed in a friend's oh-so-tasteful home, keep in mind that the Chinese themselves considered it to be pure kitsch. That's why it's called export porcelain. The tasteful Chinese produced it to be foisted off on the rest of the world. Don't get me wrong, now. Your friend's Chinese export porcelain, if it's authentic, is beautiful, almost translucent. That's what's left today, the luxurious masterpieces, the pieces that collectors and connoisseurs have lovingly preserved for generations. What we don't know a lot about nowadays is everyday Chinese export porcelain. That is, we didn't know about it until recently. In the PBS "Nova" presentation of "Sultan's Lost Treasure," airing Tuesday, Jan. 16, at 8 p.m., viewers will discover a time capsule of a vanished commercial empire. At a time when the West was just emerging from the Middle Ages, Chinese traders ruled the waters from their homeland all the way to East Africa. One of their trading partners was the Sultanate of Brunei, a powerful monarchy on the island of Borneo. Recently, the 500-year-old wreckage of a Chinese junk was discovered at a depth of 200 feet off these shores. "Nova" explores the wreck, which is replete with popular consumer goods of the day - among them blue-and-white porcelain plates, cups and pitchers, along with giant stoneware jars. There are more than 15,000 such objects, many in a perfectly preserved state. The wreck, excavated by an international team of divers and archaeologists, is helping to rewrite the history of Asia from 1368 to 1644, the time of the Ming Dynasty.