By Sarah Sands
“I’m not particularly worried about lightfastness,” the customer said with confidence as we discussed the merits of using a UV protective varnish. “The ink system I’m using is rated as permanent for more than 100 years.” I glance at the time, and take a deep breath. “Here we go,” I thought, and began explaining why these assertions need to be taken with a great deal of caution and seen in context. This whole field has become a tangled mess of competing claims and misunderstood concepts and unraveling all the facts can be dizzying.
In the following pages we begin by looking at how the lifespan of a print is determined, the elements that can affect this, and the limitations of accelerated testing to accurately predict the longevity of digital media. We also provide you with some of the information you need to make better, informed choices when deciding to protect your prints. Finally, we share results from our own testing, conducted over the last three years, showing the effectiveness of our MSA Varnishes to increase the lightfastness of these materials – both in isolation and as compared to the major, competitive brands in the marketplace.
Lightfastness in Paints vs. Prints
It is easy to assume lightfastness ratings for printing inks and artists’ paints must be comparable, or at least the methodology for determining them must be similar. But neither assumption is correct. The fields have actually evolved historically distinct criteria for measuring lightfastness and permanence. Artists’ paints rely on measuring changes to a color’s spectrophotometer reading based on the CIE L*a*b* color space. Depending on the degree of change, known as the Delta E (E)1, each color is then assigned a particular lightfastness rating set by ASTM standards. By contrast, digital prints use densitometry readings to track changes in the density of a particular dye rather than measuring a shift in color space. In this system what is significant are not the changes to a particular swatch of color but how much density any of the CMYK dyes can lose before an image is deemed unacceptable based on ‘psychometrics evaluations’.2 As set by Wilhelm Imaging Research, currently allowed percentages of density loss before failure is reached runs from a low of 25% for Magenta to a high of 35% for Yellow. Clearly this is a very different type of measurement than painters are accustomed to. Nor will most of them realize that when a particular ink system claims a specific duration for lightfastness, it is referring to how long it might take to reach this level of density loss. In addition, lightfastness ratings for digital prints almost exclusively assume the prints are mounted behind glass, at 70° F and 60% Relative Humidity (RH), with display conditions of 450 lux for 12 hours per day. While ultimately not a matter of either system being better or worse, it does show the distinct concerns and traditions each medium brings to its field.
Other Factors Involved in Fading
While concerns over print stability usually center on UV exposure, other factors such as humidity, exposure to air, and heat are just as significant. Although not the focus of our own testing, it is important to be aware of their impact.
For many types of digital media, humidity alone can induce major color changes. For example, some dye-based systems exhibited upwards of 16 Delta E units of change in just seven days when exposed to 80% RH and 75° F.3 These changes would far outweigh anything caused by UV alone over a similar time period, and those conditions are not all that uncommon for a humid summer. While it is true that pigmented systems were shown to have excellent lightfastness, changes were still considerable for dye-based inks even at lower humidity levels of 70% RH.
Simple exposure to air found in normal household and office environments will cause changes. It is largely thought that pollutants, and in particular ozone, are the major cause for this. In one reported test, significant ‘air fade’ was seen for prints done on porous media after 4 months of exposure to air in a dark environment. The majority of these samples had a 15-25% density loss in Cyan, with one sample even registering above the failure criteria of 30%.4
Temperature / Dark Fade
Many materials yellow when exposed to heat alone, and various chemical reactions will continue in the absence of light. In some recent testing we noted clearly visible yellowing for various clear coats after 400 hrs at 140° F.
Limitations of Accelerated Testing
It is critical to recognize the limitations of accelerated testing to predict the longevity of digital prints. Very often we are asked if we can guarantee a certain result, or to certify a specific effectiveness for our products. And nothing would be nicer than if the test results and data allowed for that. Unfortunately, the best we can say is how a particular set of materials performed under very specific conditions. Beyond that, one enters into rougher and rougher approximations to any real life application. The strength of accelerated testing is that it can subject materials to very controlled, repeatable, and standardized settings. The complexity and synergy of actual situations, however, will always be more intricate and challenging to analyze. Nor is it simply a matter of taking into account the environmental elements we already alluded to. For example, we know the combination and choice of paper stock and ink system plays a dominant role in deciding the degree of permanence. At the same time, this is not a static field. New systems and technologies are being constantly unveiled, so test results quickly become dated or irrelevant.
This is one of the most essential concepts for understanding the limits of accelerated testing for digital media. At its most basic, it describes the fact that inks fade more quickly when exposed to lower light levels for longer periods than the short exposure/high intensity used in accelerated tests would predict. In other words, one cannot assume that exposing a print to twice as much UV in half the time is comparable to the same accumulative exposure done in real-time. Even taking into account more conservative estimations, reported rates of reciprocity failure suggest accelerated tests can overstate expected lightfastness results anywhere from 40% to as high as 1000%, or ten times longer, than would be expected in real life results.5 Under these circumstances, a system rated for 100 years by an accelerated test might only last anywhere from 60 years to as few as 10 years in real life.
Lastly, how much light, and what type of light, a print is exposed to will effect results to a degree most people do not realize. Cool fluorescent or Xenon Arc, at various levels of intensity and either filtered through glass or allowed to directly expose, are possibilities that attempt to simulate different environments. Currently there is no industry-wide agreement on which of these configurations should act as a standard. To make matters worse, there are equally wide disagreements on what constitutes a ‘typical’ level of lighting that prints are exposed to, which in turn determines how you correlate total accelerated exposure to an equivalent number of actual years. Wilhelm Imaging uses as typical indoor exposure of 450 lux for 12 hours per day. Kodak uses 250 lux/12 hour. Museums expose sensitive materials to as low as 50 lux, and a high of 250, while a piece hung near a window that receives direct sunlight for several hours a day might receive as much as 50,000 lux.
Predictive vs. Comparative
As you can tell from the arguments presented, there exists some profound obstacles to accurately determining the permanency of prints with current methods of accelerated testing. However, it is also important to understand the difference of conducting comparative rather than predictive tests. Comparative testing simply states that a set of samples, subjected to identical circumstances, produced a specific set of results that can be used as a measure for comparing performance under those particular conditions. The requirements for predictive testing would be quite large by comparison, since a host of variables representing possible conditions would need to be accounted for, and the results used to make predictions for how these materials might perform in real life. This distinction defines many of the parameters and procedures used in the testing and the significance of the results.
Results of Recent GOLDEN Testing
For creating standard test samples we chose fugitive dye-based inks printed on typical inkjet media. We felt this ‘worst case scenario’ presented a good test of the effectiveness of GOLDEN MSA and Archival Varnishes to protect materials from fading due to UV exposure. The samples were created from a digital file using an Epson 700 desktop printer with standard ink-set on ordinary Glossy and Matte Photo Paper, as well as Inkjet Canvas, recommended by the manufacturer. Additional samples, used for some of our controls, were made with a professional Giclee printer and widely used pigmented ink system on watercolor paper. Measurements were always taken from bands or squares of pure Cyan, Magenta, Yellow, and Black inks printed at maximum density. It is critical to note that the focus of these tests was solely the performance of the clearcoats and never meant as an assessment of the inks or substrates.
For accelerated lightfastness testing we utilized Q-Lab QUV type instruments with UV-A 351 bulbs. These provide a very similar UV energy curve as natural daylight (filtered through window glass) in the most important region of short wave energy. Typical test parameters use an irradiance setting of .762W/m2@ 340nm and a temperature of 60° C. Each 400 hour cycle approximately correlates to 33 years of indoor UV exposure. There were no dark or condensation cycles employed for these tests. Humidity is ambient and normally below 50%.
Color measurements were done using a Minolta Spectrophotometer. Initial data points were taken prior to the start of the test and subsequently at set intervals of exposure. Delta E was computed using CIE 1976, as required by ASTM D4303-03 for computing the lightfastness of artist paints.
More than three years ago an earlier round of testing was completed using a large number of our standard inkjet samples treated with varying coats of either brushed or sprayed MSA Varnish. Initial spectrophotometer readings were taken of representative data points on each card to establish a basis for measuring subsequent color shifts. Readings from a broader group of target areas were then taken at intervals of 200, 400, 800, and 1,200 hours. Each particular substrate/coating combination was prepared and tested in triplicate, with one test sample pulled and saved after each 400 hours of exposure. Data used for this article was culled from 66 test strips and more then 900 data points. Values for the Delta E’s used in the graphs were generated by averaging readings collected from similar substrate/coating combinations exposed for identical lengths of time.
Figures 1 & 2 compare the color changes of both unvarnished dye and pigment-based inks versus dye-based samples protected with 6 sprayed or 2 brushed coats of GOLDEN MSA Varnish after 400 and 1,200 hours of exposure.
Figure 1 & Figure 2
The dramatic improvement in lightfastness provided by the varnish layers is clearly evident, while color losses experienced by the uncoated samples were equally striking, with even the pigmented system experiencing considerable color loss after just 400 hours, or 33 years of typical indoor exposure. By contrasts those treated with our MSA Varnish barely budged. Even after 1,200 hours of exposure these samples retained their color near or well below 8 Delta E, which is the threshold for an ASTM Lightfastness II rating and the maximum level of change deemed acceptable for artist paints.
Figure 3 shows the degree of color change recorded for various inkjet media coated with one or two brushed layers of GOLDEN MSA Varnish. The impact of each substrate on the final results seems indisputable. It is also significant to observe the amount of improvement a second coat of varnish provided, often cutting the Delta E by more than 30% and bringing many of the color shifts within or near the Lightfastness II threshold. Lastly, it is worth noting that nearly all of the fading associated with the inkjet canvas was confined to the peaks of the weave, where both the ink and varnish were naturally thinnest. By contrast, little change was seen in the valleys, where the ink and varnish pooled.
While these results clearly demonstrated the significant protection GOLDEN MSA Varnish could provide for even a very fugitive ink system, we felt they needed to be further examined within a broader context of other products available for coating digital prints.
Our first major round of comparative testing included a wide sampling of fine art varnishes, clear protective coatings, and several topcoats marketed specifically for the serious digital artist and Giclee printmaker. In all, we selected 18 products by 13 manufacturers including: Bulldog Ultra Gloss, Clearjet Gloss, Clearshield Gloss, GAC Archival Varnish Gloss and Matte, Grumbacher Picture Varnish, Krylon Crystal Clear and Kamar Varnish, Lascaux UV Varnish Gloss and Matte, Lyson Printgaurd, Optima Millenium, Premier Art Printshield, Schmincke Glanzfilm and Mattefilm, Suregaurd Pro-tecta-cote #911 Gloss and #941 Matte, and Winsor Newton Artists’ Picture Varnish Gloss. Samples were prepared in duplicate for each product, one coated with two and the other with six spray applications respectively. Data points were taken at both the outset and conclusion of the testing, which ran for only 400 hours due to evident, widespread loss of color as well as signs of considerable physical deterioration in some of the samples.
In Figure 4, due to space limitations, we only included results for the samples with six sprayed coats, along with one uncoated control. The results are arranged according to the averaged Delta E calculated from the CMYK of each sample. Starting from the left, these averages ran from a low of 1.15 to a high of 45.67. Please note that letter designations on Figures 4 & 5 do not refer to specific products, but merely denote relative position within the results.
Beyond our own Archival Varnish, a handful of other brands also provided significant protection and kept the averaged, overall color shifts to below 8 Delta E. These products included: Lascaux UV Varnish Gloss, Suregaurd Pro-tecta-cote #911 Gloss, Optima Millenium, Schmincke Glanzfilm, and Clearjet Gloss. Also keep in mind these results are based only on one sample of each product, and only for 400 hours of exposure, so cannot be reliably used to predict actual long-term results. The sole purpose is to allow you to make a rough comparison of how these coatings performed under similar, controlled conditions.
Figure 5 focuses on a current, ongoing test that exposes a slightly smaller subset of the same products to Xenon-Arc irradiance as specified by ASTM D4303-03. As we go to press the first 200 hours of exposure will have been completed, which represents a total irradiance roughly equivalent to the previous round’s 400 hours of QUV, or about 33 years of typical indoor conditions. Spectrophotometer readings for each CMYK data point are taken at set intervals and Delta E calculated from initial measurements. Overall performance was then assessed by averaging these individual CMYK calculations. As before, for the sake of clarity, the graph only shows results for samples with six sprayed coatings.
Beyond our Archival Aerosol Varnish and our MSA Varnish, which have continued to do the best, Lascaux’s UV Varnish Gloss, Optima Millenium, and Schmincke’s Glanzfilm are also performing at or near acceptable levels. It is also worth noting how vital it is for artists to test their materials as one product in this group, which claimed to provide substantial UV protection for inkjet prints, is already performing worse than the uncoated control.
The test results we have shared represent only a small amount of the work we have done in this direction over the years, and of course the work is ever ongoing. When interpreting them, it is important to note that many products we tested were not made specifically for the digital print market, and several make absolutely no claim of providing UV protection. However, as a group, we feel they represent a good cross section of what artists currently use or might find on the shelves of their art store. A full bibliography is available at www.goldenpaints.com.
1 The difference between two colors, expressed in units corresponding to the smallest perceptible change that someone with normal color vision could notice.
2 How Long Will They Last? An Overview of the Light-Fading Stability of Inkjet Prints and Traditional Color Photographs, Henry Wilhelm, Wilhelm Imaging Research, published in IS&T’s 12th International Symposium on Photofinishing Technology, Feb. 2002, pp. 32-37, http://Wilhelm_IS&T_Paper_Feb2002.pdf
3 Humidity-Induced Color Changes and Ink Migration Effects in Inkjet Photographs in Real-World Environmental Conditions, Henry Wilhelm & Mark McCormick-Goodhart, Wilhelm Imaging Research, Inc., IS&T’s NIP16 International Conference on Digital Printing Technologies, Oct., 2000, pp.74-77, http://Wilhelm_IS& T_Paper_Oct2000.pdf
4 Inkjet Photo Prints: Here to Stay, Dr. Nils Miller, Hewlett-Packard Company, June 2004
5 See in particular: Reciprocity Behavior in the Light Stability Testing of Inkjet Photographs, Henry Wilhelm & Mark McCormick-Goodhart, Wilhelm Imaging Research, IS&T’s NIP17 International Conference on Digital Printing Technologies, Oct., 2001, pp.197-202; A Review of Accelerated Test Methods for Predicting the Image Life of Digitally-Printed Photographs – Part II, Henry Wilhelm, Wilhelm Imaging Research, IS&T’s NIP20:2004 International Conference on Digital Printing Technologies, Nov. 2004, pp. 664-669; How Long Will Inkjet Prints Last: Estimating Print Life Using Accelerated Test Methods