Choosing glass for picture framing


Clarity

Glass is clear which means that most of the light that strikes it passes through. Some loss is inevitable however the loss should be balanced equally throughout the visible spectrum so that the balance of light received by the eye has the same balance of colors as the picture seen without glass.

Modern glass has a better balance. Often traditional glass will have a green tint. This tint can be removed by reducing the iron content hence the term 'low-iron' referring to particularly clear modern glass. Some brands refer to this as 'Water white' glass.

UV spectrum

The UV spectrum is high energy light that bumps up on the visible spectrum. As high energy radiation it affects materials more than low energy. The effects of a dose of high energy UV radiation is greater that visible light or infra-red radiation. 


artworkdamaginglist


UV damages stuff quickly, which is not to say that the rest of the spectrum of radiation/light passing through glass or clear plastic does not. Of all the damaging radiation that will pass through a regular piece of glass or clear plastic 60% of that damage comes from the UV spectrum. So if a material can block all the UV radiation it will significantly slow down deterioration. However even if you block all the light falling on a piece of artwork it will still deteriorate to some extent due to heat, humidity and the chemistry of the object.

uvdamage

Source: NFRC (National Fenestration Rating Council)


If a material, whether glass or plastic, filters out UV this may well affect the transmission of high energy visible light. In practice it seems to be impossible to block all UV light without tinting the glass and plastic slightly. The highest levels of UV blocking usually come with a very slight green tint.

Magnetron spluttered glass v. organic UV filter glass

The highest levels of UV filtering use an organic UV filtering layer on the glass surface. This is relatively fragile to scratching and cleaning. This is often advertised as 99% absorption of the A and B bands of UV.

The alternative method of increasing the UV absorption of glass is called magnetron spluttering which uses a voltage to spray ions onto the surface of the glass. This give a slightly lower absorption of the A & B bands of UV - usualy given as 92%. When you take into consideration the effects of all other radiation this 7% difference becomees a 3% difference.

The magnetron spluttered glass with 92% UV absorption also has better handling characteristics, it is easier to clean and less likely to scratch, and most importantly is clearer without the green tint associated with the 99% glass. 

Our preference

We stock TrueVue's Museum Glass that offers 99% UV (A and B) absortion but we often recommend ArtGlass WaterWhite UV AR glass that offers 92% UV (A and B) absortion as it is clearer and more hardy.


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Further information can be found at http://en.wikipedia.org/wiki/Picture_framing_glass

and at www.groglass.com/en/education/light-and-art-framing/facts-about-uv‎