Lightfast Paint and Lightfastness Ratings

This Ask the Experts Q&A by Bradley Lance Moore first appeared in the May 2013 issue of The Artist’s Magazine.

Q. Please explain lightfast paint. To what degree are finished works in the various media (oil, watercolor, acrylic, pastel and so forth) affected by direct or indirect sunlight? Why does protective glass seem to be recommended more for some media than others?

—Louis Archambault, Helena, Mont.

 

Bradley Lance Moore’s Answer

The stability of paint when exposed to ultraviolet (UV) light is generally discussed with a focus on the pigments in the paints. Lightfast paint ratings conforming to standards set by ASTM International (originally ASTM, or American Society for Testing and Materials) are often printed on paint tubes. Alternatively, these ratings may also be available from the manufacturer’s material safety data sheets (MSDS) and on that company’s website. ASTM lightfast paint ratings are indicated as I (excellent), II (good) or III (poor). Non-American manufacturers may use different standards or grading systems. (See Lightfast Paint Labels, below).

Lightfast paint ratings

Lightfast Paint Labels: Art media manufacturers print helpful information on their labels, but you must check the manufacturer’s literature or website to make sure you’re interpreting the symbols on paint labels correctly. A tube of Golden Artist Colors quinacridone/nickel azo gold acrylic paint (left) has a lightfastness rating of I or excellent. Holbein uses from one to four asterisks to indicate permanence or lightfastness. A tube of Holbein lamp black oil paint (middle) has a four-asterisk rating, indicating that the color is absolutely permanent. The Roman numeral II indicates that the paint will dry in approximately two days.

 

To complicate matters, the particular mediums in which the pigments are bound make a big difference. The binders, mediums and varnishes used in painting absorb UV light wavelengths, thereby protecting the pigments from damaging effects. Oil, for example, more thoroughly encapsulates its pigments than the less robust binders in watercolor or pastel. Consequently, oil paints are generally able to handle more UV impact and may have more lightfastness.

Varnishes on oil and acrylic paintings have limited life expectancies and become discolored or hazy over time, which also affects the appearance of a work. The procedure of replacing or correcting the varnish on an aged painting is a delicate one and may even be prohibited if the painter used the same varnish resin in the paint layer. Some modern varnish mixtures contain special UV inhibitors/absorbers to help extend the life expectancy of the varnishes.

A layer of UV protective glass, however, can benefit all media. Glazing—placing a protective glass in front of the work—is especially recommended when the medium used in the artwork prohibits the application of protective varnish. Some paintings would be too visually altered if a surface varnish were to be applied, yet their porous surfaces need protection. Watercolors and pastels are obvious candidates. Other underbound paintings (works with very low binder-to-pigment ratios) should not be varnished, but must have glazing.

Note that acrylic (plexiglass) glazing should not be used where dry media, such as pastel or charcoal, are present unless the acrylic glazing is museum grade. Standard acrylic glazing can develop a static charge that attracts the particles to the glazing.

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