How does picture glass get dirty on the inside???

Can you see the document writing in the glass?

If you have something that has been framed for a few years, a haze can develop on the inside of the glass. 

 Sometimes, a "ghost" image of the picture will appear on the glass. 

 This is a transfer of some of the volatile chemicals from the printing ink to the glass...especially, if the glass is directly on the artwork; as with this example.

 So, with permanently framed art, it is advisable to open and examine it about every 10 years, to make sure it is in good condition. 

I call this a "spiff & clean" or more formally known as a "condition report". 

During the inspection for the condition report, the frame is opened and the glass is cleaned of any haze or ghosting that might be there.

Once opened, this is a good time for your framer to take a photo of the front and back of your art, for insurance purposes and valuation.