One of the explanations for defects in paper and board defects in the corrugated box industry

A special dark line on paper. When the paper is laid flat, no transparent graphics are visible; when the paper is greeted with light, a clear pattern can be seen. For example, in the left-hand side of the 1990 version of the RMB 50 bill, there is a graphic of a steel worker, which is a watermark. The earliest watermarks were the end of the 8th century or the beginning of the 9th century, and were pioneered by the papermaking craftsmen of the Tang Dynasty in China. In the 13th century Italian craftsmen also woven letters, numbers or patterns on the papermaking network, leaving the paper industry with a watermark as a trademark. The modern papermaking is pressed by a facing stick on a rotary screen or a long wire paper machine and contacted with a wet paper sheet, and is divided into a black watermark, a white watermark, an yin and yang pattern watermark, a wet watermark, an imitation watermark, and the like. The most common is a black watermark used to print banknotes, passports, certificates, documents, etc. that require anti-counterfeiting effects.

Printthrough:

The ink on one side of the printed matter penetrates into the other side, causing a blurry picture. As a result, reading difficulties are caused, and prints are scrapped when severe. The reason for the print-through is that the selected paper is too thin and the opacity is too low.