I have heard that glassine is not recyclable. What about paper enveloopes with glassine windows? Are some forms of glassine not recyclable?

Glassine that we sell and used for art applications is recyclable and biodegradable. The windows on envelopes are produced with a different plastic material, but it's our understanding that these envelopes can be recycled as the facilities processing these recycled materials can remove those non-recyclable elements from the system. But that's out of our wheelhouse.