Understanding the offset/wet ink transfer error

Wet ink transfer error or offset error is the most common error in the currency note printing world wide. Is the wet ink transfer really happens from a sheet to other sheet or it is something more interesting?? And how can such incidents happen during the printing process? 

Here's how : below is a simple illustration of the normal printing process, in which the printing plate for the portrait is a the top drum (Drum A) and as the paper goes in between the drums, the plate on drum A is pressed against drum B and the image of the portrait gets transferred to the paper.

Now image transfer happens when there is a momentary break in the paper feeding and as a result the image on drum A gets transferred to Drum B, as shown below :

And when the paper feed resumes, the wet ink on drum B gets printed to the bottom of the same paper. The image transferred will fade as more paper gets fed into the printing drums :

The image on drum A is a negative, when transferred to drum B, it will be a positive and final transfer to paper will result in a negative image.

Like others, this type of error can range from minor to a complete offset, with the later being more desirable to collectors. While a rational theory, note that the errors described or pictured here are not caused by wet sheet transfer, or ink transferring from still-wet sheets to other sheets!!

Hope this will provide some clarity on how such ink transfer can happen when banknotes are being printed.

Happy Collecting !

By Dr Kartik D Shah