The ink stays on the reverse side, but light passes through the sheet well enough that the image can be seen from the front. It depends on opacity, grammage, ink density and how much oil from the ink vehicle has wetted the fibers and made them translucent. Printers measure it by comparing front-side reflectance over a printed back with reflectance over an unprinted area.
Pressroom is complaining about show-through on the 40 gram stock, mostly where solids are printed on the back.
On newsprint, show-through gets worse for a day or so after printing because the ink oil keeps wicking into the sheet.
Do not mix it up with
- Strike-through: Strike-through means ink has physically penetrated toward the back; show-through can happen with the ink sitting entirely on the surface.
Related terms
Opacity
A sheet's ability to hide what lies behind it, expressed as a percentage from readings over black and white backings.
Strike-through
Penetration of ink or its oil vehicle through a sheet far enough to darken or stain the reverse side.
Printing opacity
Opacity calculated as single-sheet reflectance over black divided by the reflectance of an opaque pad of the same paper.