Instruction: Answer the given question by extracting information from the supporting document.

Input:
Question: Are these filters made of glass or plastic?
Document: I'm retired and photography is a hobby for me, so I don't want to spend hundreds on professional quality filters. I've never had a problem with any Tiffen filter, they serve well. A side bonus is they are made in the USA.
I thought all the filters suppose to be packaged in hard protective plastic  cases. The filters I got were just placed inside the filter pouch with dust and finger print. I had to spend time to clean them.
These filters have been all that was promised and more. I get great photos with them, best use is landscape in my opinion.
Entire set of three filters with nice soft accessory case included. Everything was accurate and still in original box. Shipped faster than I thought. Great price too!
I've used Tiffen filters before and have always found them to be of exceptional quality. This pack is no exception. High quality filters which fit my Nikon lenses perfectly make my photographic efforts more fun. I purchased these to do some astrophotography - but here in the Pacific Northwest it may be a while before I see the sky ... however I'm sure these will do what I want them to do. Very, very happy with this purchase.
This was the first set of Tiffen filters I'd purchased, as well as my first set of neutral density filters. I recently got into photography, and I bought these for my Nikon D3000.So far, I have really enjoyed these filters. They have held up really well, and the included case for them makes it super easy to store them when not in use.I would definitely consider buying other Tiffen products in the future, as well as recommend this product to others.
Answer: I bought these as a gift so I don't know personally. The Tiffen website say this about the 52MM VARIABLE NEUTRAL DENSITY FILTER: "Made with high quality optical glass using Tiffen's ColorCore™ technology".B & H photo says this under the description of the Tiffen 52mm 3.0 Neutral Density Filter: "This filter is made using ColorCore technology, a process that involves laminating the filter substrate between 2 pieces of optical glass, grinding flat to a tolerance of 1/10,000th of an inch, and then mounting to precision metal rings."