From Sue Heim's website. She's yet another who truly knows her stuff and what she is talking about.
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https://www.hollywoodposterframes.com/index.php?p=page&page_id=protect_investment)
"The most important components of the frame job is what's in front of your piece and what is behind your piece.
Let's begin with what's in front.
First off, glass is a no-no! First of all, it can break, and if that happens, often your item is scratched, torn or ruined. Glass is also a heat conductor and accelerates fading and decay. Glass has one other very negative aspect and that is moisture entrapment. Framers often recommend spacers, but then do not dissuade customers from using glass. Also, even if you use spacers, movie posters are so large that they often touch the glass anyway nearer the center, thus defeating the purpose.
Framers often recommend conservation glass that typically has UV filtering but it is still a heat conductor that accelerates yellowing and brittleing. You can also get moisture entrapment and there is still a possibility of breakage, even with conservation glass, especially with these larger sizes. Glass, of any kind, is extremely heavy for any oversize piece. Conservation glass is also extremely expensive since it is very much a specialty product. When glass breaks it can shatter or just have a clean break in half. Either way, the likelihood is high of moderate to severe damage to your poster. Often times with such a large piece of glass breaking, the top half of the glass will slide behind the bottom half and scrape the face of the poster making it extremely difficult to repair. Thus, plexiglass is the recommended covering for your movie posters and valuable paper.
I have had customers tell me they don't like the look of plexiglass: it scratches, turns yellow, gets an opaque film over time. This is not high grade plexiglass! Just as some people call all tissues, Kleenex, many people call all plastics, plexiglass, and it is not. There is Lucite, styrene, low grade acrylics and many other forms of plastic sheeting, most of which have no archival qualities and least of all have no UV filtering which is the most important component of plexiglass for movie poster and paper preservation purposes.
Plexiglass also does not absorb dirt and grime from the air since typically it has been polished with an antistatic cream before it is installed in the frame. Actually, when polished, plexiglass will repel dirt and dust and very rarely even needs cleaning or repolishing.Just an added note, if you currently have something framed in your home in plexiglass or any form of plastic, never use any ammonia based cleaner on it as the chemical reaction with the plastic is what causes clouding. One of the other complaints I hear about plexiglass is that it warps and gives the poster a distorted look. Again, this is typical of the lower grades of plastics that are very thin and don't lie flat in the frame. A good piece of plexiglass should be the same thickness as glass, about one eighth of an inch thick.
One other note regarding the differences between glass and plexiglass that definitely is not crucial to the preservation of the poster is that glass has a green tint to it and a good grade of plexiglass is crystal clear. Lower grades of plastics have been made with inferior materials. Remember, the most important element of framing anything you want to preserve, is that it be framed in an acid-free environment. Plexiglass with extremely high levels of UV protection, in my experience, often has a yellow tint to it and this type of plexiglass would be used on extremely valuable items and perhaps items that are hung near high exposure areas. Plexiglass is available in clear and non-glare.
For many years I've had customers who have preferred a non-glare covering for their artwork. While non-glare glass is available there are inherent problems with this product. By nature of its design, when non-glare glass is placed over the artwork, it has a tendency to dissipate color and take the art slightly out of focus, especially if you are using a mat over your artwork. Most importantly however, it is glass, and still has ALL THE NEGATIVE ASPECTS OF GLASS.
A few years back, non-glare plexiglass became available which is as near true-view perfect as you can get, while still maintaining the high quality of UV. I immediately switched all of my posters that hung in highly lit areas to this non-glare plexiglass product and have been thrilled with the look ever since. Whichever type of plexiglass you choose, clear or non-glare, as long as you select high quality material with UV filtering, you will be helping to preserve the integrity of your own collectibles."
