Black gradient on copies of thick items

[CherylO-Xerox]

Hi Plank20, 

Thank you for using the Support Forum. Please take a look at this article Black Lines or Bands on Copies when Using the Document Glass.  The background suppression in particular may help with your situation.  Please consider contacting your support centre for further assistance.

[Plank20]

I have a Xerox WorkCentre 7120 but this is a potential issue with all photocopiers. When I place a standard paper on the document glass, copies come out perfectly fine. But when I place a thicker piece of paper to be copied, copies come out with black gradients on a side or corner (like this). How can I copy thick documents without these black gradients appearing?