Here is a startling example of the harm that using a poor quality mountboard can do.
This is a picture brought in for re-framing.
The first picture is as it appeared after the old frame was removed. You can see that the mount has darkened along the cut sight-line:
Now, take off the old mount and you can see that it has seriously foxed (made brown) the paper underneath. This is due to the acids in the mount board. I'm not blaming the original framer for this, probably they used the only mount available at the time. Modern mounts board now comes in a variety of standards and wherever possibly I use conservation grade board which is treated to remove the acids and will not damage the artwork, even after many years.
Here is the completed, re-framed art. Not quite as pristine as the day it was drawn, but as least it won't deteriorate any further!