Glass or acrylic for a framed print: weight, glare, UV and cleaning

This page compares the two sheets that can cover a framed pet portrait: glass and acrylic, the clear plastic sold under names such as Plexiglas and Acrylite. Framers call either one the glazing. It explains what every glazing must do, where glass is stronger and where acrylic is, what ultraviolet filtering can and cannot do, how to deal with reflections, and how to clean the sheet without harming the print. Most of the advice comes from the Canadian Conservation Institute's guide to glazing materials for framing works on paper, with added points from the Library of Congress and the Conservation Center for Art and Historic Artifacts. The main framing and hanging guide shows where glazing sits among the other layers of a frame.

What any glazing has to do

The Institute says the main job of a glazing material is protection, and it lists the basic properties every glazing should have. It should be inert, meaning it does not react with the print. It should be rigid, so it keeps its shape. It should be impervious to air and water vapour, so dust and damp stay outside. And it should be colourless, so the portrait's colours look the same through it. Two further properties are optional: a coating that cuts reflections, and a filter for ultraviolet (UV) light.

Both glass and acrylic can meet the basic list. They differ in weight, strength, static, scratching and how they behave in large sizes.

Glass: rigid, heavy and hard to scratch

The Institute lists the strengths of glass:

• Little static. Glass picks up little electrostatic charge, so it does not pull loose particles off a picture. The Institute recommends it for pictures made with loose, powdery media such as pastel or charcoal.
• Scratch resistance. Glass resists scratching better than many plastics.
• Rigidity. A 2 mm sheet, which most frames accept, stays rigid even on large works.

Picture framing glass is usually 2.0 mm or 2.5 mm thick, and the Institute says the 2.5 mm sheet may be better because it is more resilient, less likely to chip and easier to handle safely.

It also lists two weaknesses. Glass is heavier than plastic, so large sheets need sturdy frames and hanging systems, and the hooks and wire must carry the combined weight of frame, mat and glass. Glass is also brittle: it is sharp, breaks easily and is a hazard when it is cut, fitted or shipped.

Acrylic: light, tough and easy to scratch

The Institute lists three strengths of plastic glazing. It is lightweight, which makes it ideal for large works. It is less breakable than glass, and when it does break, the pieces are less dangerous to the artwork and to people. And it is available with UV filters.

The Conservation Center for Art and Historic Artifacts goes further. It recommends acrylic glazing instead of glass for framed paper items, to avoid any risk of shattering and to make the whole framed package lighter.

The weaknesses of plastic are just as clear in the Institute's guide:

• Static. Plastic picks up and holds an electrostatic charge. Dry air and rubbing the sheet during cleaning make it worse. A charged sheet holds dust on its outside face, and on the inside it can lift particles from pastel, charcoal, chalk or crumbling paint. The Library of Congress says anti-static acrylic is now available and can be used over powdery media.
• Scratches. Uncoated plastic scratches and scuffs unless it is handled and cleaned with care. Some plastics are made to resist abrasion.
• Bowing. Plastic is less rigid than glass and tends to bow over large works. The Institute says large pieces must be 3 mm thick to avoid bowing, and that this thickness often will not fit an average frame designed for thinner glazing.

If you choose acrylic for a large print, the Institute suggests increasing the distance between the picture and the plastic to reduce the pull of static: use 8-ply mat board instead of 4-ply, or two window mats stacked. The page on mats for framed prints explains both.

Side by side

For the four Purrealism print sizes, the table points one way at each end. On an 8 × 10 inch or 12 × 16 inch portrait, the weight of glass is small and its hard surface is an advantage. On a 24 × 36 inch portrait, the Institute's advice about weight and the Conservation Center's advice about shattering both favour acrylic, as long as the frame is deep enough for a 3 mm sheet. The page on choosing a frame for a royal pet portrait covers what a heavy frame needs.

Ultraviolet filtering: what it does and what it leaves

Ultraviolet light is the band of light just beyond violet that the eye cannot see. The Institute says UV causes paper and other organic materials to weaken and turn yellow, and can make colours fade or change.

Ordinary glass does little about it. The Institute says clear glass transmits over 90% of visible light and cuts out UV only below 300 nanometres (a nanometre is one millionth of a millimetre, the unit used for the wavelength of light). UV-absorbing glazing is designed to cut out the band from 300 to 400 nanometres. The Institute tested several glass and plastic glazings and recommends them as UV filters, but it adds that none of these materials filter out all of the harmful UV. Its advice is to use them together with sensible light levels.

UV filtering also leaves visible light alone, and visible light fades colours. The Institute's guide to light says that for colours sensitive to light, UV usually causes less than half of the fading and often only one tenth. It says many museums, private donors and their framers have assumed that a good UV filter would prevent fading, while its own figures show that visible light causes most of the fading of sensitive colours. The Library of Congress says the same in plain terms: filtering out UV removes the most damaging part of sunlight, but all light is damaging to paper.

So a UV-filtering glazing is worth having, and the wall still matters more. The page on how to light a framed portrait at home covers light levels and lamps.

Glare and reflections

A glazed picture reflects windows and lamps. There are two kinds of glazing that reduce this, and the Institute treats them differently.

Etched non-glare glass. This glass is etched on one side so that the rough surface scatters light. The Institute says it looks hazy, and the haze clears only when the glass touches the artwork. Conservation framing keeps a space between the print and the glazing, so the Institute does not recommend this glass.

Coated anti-reflective glass. A thin coating cuts reflections without blurring, and it works at any distance from the picture. The Institute's guide to light says genuine anti-reflection glass uses the same coating as camera lenses, computer monitors and some eyeglasses, and that it costs a great deal. Its guide to glazing says coated glass may show a faint purple or green cast, and names one conservation glass with a reflection-control coating that can sit up to three mat boards away from the print.

Before paying for a coating, try moving the light. If the reflection comes from a lamp, moving the lamp moves the reflection, and the lighting page explains where to put it so the reflection falls away from the viewer.

Keeping a space between print and glazing

Whatever you choose, the print must not touch it. The Institute's guide says a framed work of art should not touch the glazing, to avoid image transfer, staining or mould growth, and that the inside of glass can collect condensation that passes into the paper. A window mat is the usual way to keep the gap. The Institute also asks for a good fit: glazing cut too tight can snap if it is glass or bow if it is plastic. The page on how to measure a frame gives the fitting rule.

Cleaning without harming the print

The Institute and the Library of Congress give matching advice:

1. Dust first. Remove dust with a soft brush or a lint-free cloth. The Library of Congress stresses this for acrylic.
2. No rough towels on acrylic. The Institute warns against rough paper towels and abrasive detergents on uncoated plastic. It suggests acrylic cleaner and cheesecloth, and says an anti-static cleaner helps prevent scratches and reduces static.
3. Cleaner on the cloth. The Institute prefers taking the glazing out of the frame before any wet cleaning. If that is not practical, spray the cleaner onto the cloth. This keeps liquid from running behind the glazing and staining the mat or the print.
4. Let it air. The Institute says to leave the glazing to air after cleaning, and to check the maker's instructions for any coated glass or plastic.

Dry dusting is enough for most weeks. Keep the wet steps for marks that a dry cloth will not lift. When you choose the portrait itself, the finished examples in the gallery show the colours that will sit behind the glazing.