Forgers know that porcelain is normally only sampled in the unglazed base. A large number of modern pieces have had their bases replaced with an ancient base. The fine dividing line is disguised with acrylic paint which will dissolve in acetone to reveal the line.
It is also worth noting that Yuan, Ming and other porcelain pieces are being artificially irradiated in Beijing and other regions of China to increase the apparent TL age. We have now developed a method to detect this through its effects on the TL signal.
Learn more about X-rays and artificial irradiation in our FAQs