Ink manufacturers use fadeometers, along with known ink pigment characteristics, to test fade resistance by exposing the print to light radiation produced from a carbon arc or xenon tube. The arc emits an intense actinic light which in a matter of hours approximates the destructive effect of a much longer period of ordinary daylight. Although it does not exactly duplicate the effect of prolonged exposure to natural light, it is still an effective indicator of the degree of light stability and of the comparative resistance to fading. The results are interpreted with the aid of a chart that correlates the number of hours a printed sample lasts in the fadeometer to the equivalent exposure to direct sunlight taking into account the amount of UV light that different regions receive based on their latitude.
If resistance to fading is an important criteria for a print project, the best source of information is the vendor supplying the inks. They will know the characteristics of the pigments in their ink formulations and can suggest alternatives that may provide better fade resistance - though often at the expense of some other attribute like rub resistance, color vibrancy, or cost.
You can also do a simple test yourself. Simply take a presswork sample and cover half (front and back) with heavy black card. Then attach the sheet to a window.The covered section of the press sheet is the control against which you can compare the fading of the exposed part.
Some typical fade resistance numbers for sheetfed 4/C process inks based on outdoor exposure near the equator:
• Black (pigment black 7)
- Tint - Max tolerance: greater than 12 months.
- Fadeometer, Max tolerance: greater than 240 hrs.
• Process Cyan (pigment blue 15:3)
- Tint - Max tolerance: 24 months with fading
- Fadeometer, Max tolerance: 120-160 hours shows fading and loss of gloss
• Process Magenta (pigment red 57)
- Tint - Max tolerance: less than 1 month with fading & loss of gloss
- Fadeometer, Max Tolerance: 5-25 hours shows fading and loss of gloss
• Process Yellow (pigment yellow 12)
- Tint - Max Tolerance: less than 1 month with fading
- Fadeometer, Max Tolerance: 2-30 hrs shows fading and loss of gloss
How lightfastness is measured
The "Blue Wool Scale" is the internationally recognized method of quantifying lightfastness, defined under the British Standard BS1006. The scale consists of 8 different strips of wool, each dyed with a blue dye of differing lightfastness. The scale ranges from 8 (excellent – very low rate of fading) to 1 (very poor – extremely fast fading). The blue wool scale is not a linear scale but is rather logarithmic, so that each increase in level is greater than the previous.
When exposed in the same manner as the print for the same length of time, the level of fading of the printed solid is compared to that of the equivalent strip of wool to provide a value indicating the lightfastness of the print.
The "Wool Scale" lightfastness classifications are:
WS 1 - very poor
WS 2 - poor
WS 3 - moderate
WS 4 - fairly good
WS 5 - good
WS 6 - very good
WS 7 - excellent
WS 8 - maximum lightfastness
Tips for the printer:
• When interpreting degrees of lightfastness into production requirements - real-world conditions need to be taken into account. For example a paper that contains a high degree of wood fiber will soon yellow and therefore effect the color of ink that overprints it - even if the ink has a high WS rating.
• A higher ink film thickness than indicated in the standard specifications will result in an increase of the lightfastness of the print because there will be more pigment particles in a given area to withstand the destructive influence of light. The same applies to a higher pigment load ink. On the other hand a thinner ink film thickness will reduce lightfastness. Also, lightfastness in halftone screened areas is generally lower than in the solids.
• Varnishing and lamination of presswork will generally improve its lightfastness. Specific over varnishes exist which contain UV inhibitors and high-grade resins. These can increase the lightfastness of presswork, however, they will not prevent ‘weak’ pigments from fading, merely slightly increase the time they take to fade.
• If two or more printing inks of different lightfastness properties are mixed, the low WS rated one is not improved by the high WS rated one. Instead the low WS rated one lowers lightfastness of the high WS rated ink. Effectively the ink of the lowest lightfastness determines the lightfastness of the mixed Ink.
• Lightening of an ink with transparent white will, in most cases, diminish its lightfastness. Since white pigment does not fade easily, pastel colors with a small concentration of colored pigment and a large concentration of white will fade rapidly to white. For similar reasons, large quantities of varnish should be avoided in colour matches for presswork exposed to sunlight.
As a general guide, lightening with transparent white at a ratio of 1:1 will reduce lightfastness by 1 grade. Lightening with transparent white at a ratio of 1:3 will reduce lightfastness by 2 grades.
• Lightfastness of presswork exposed to sunlight will be lower near the Equator and higher as one move towards the Poles.
• In the northern hemisphere lightfastness of presswork exposed to sunlight will be lower in the summer and higher in the winter.