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textile  printing - inkjet 
Since returning from holidays, Rod Urquhart spoke to two of the absolute technical pillars of the inkmaking fraternity, Fred Vale of DIC Colortron and Otto Stumbras of Siegwerk, about the lightfastness of printed material. This inspired him to update one of his early articles on ink fading So here goes – I trust that the following answers any questions that you may have on the subject, but if not, the i-grafix website is a good place to ask some more!

What is “Lightfastness”
Light fastness is defined as the resistance to fade or hue change of a coloured object when exposed to a light source, most notably sunlight.
Fading is caused by the action of ultra violet energy from the light source on the chemical composition of the coloured compounds in the object. The effect of long-term ultra violet attack is to break the coloured compound down into by-products of a different shade, either to cause a hue shift such as darkening, or to remove the colour altogether (bleaching).

Light Resistance
Light resistance is a term often used to avoid Trade Practice regulation and implies a level of light fastness without making any particular specification. For example, a “Light Resistant Blue” may in fact have a light fastness rating of anywhere between months and years when exposed outdoors, as there is no reference point.

Fade Testing
There are two principal methods of determining the light fastness of a material such as a printed item.
These are:
• direct exposure under glass to daylight (at a 45º inclination to the north*)
• accelerated exposure testing.
(*only in the Southern Hemisphere, above the equator it would be facing south).

Direct Exposure
Exposure to daylight is the most specific test as it is how the print will behave under actual conditions. However, in order to conduct a test to ascertain the light fastness of a material for a particular length of time, for example six months, logically the test will take six months.

Accelerated testing
The most common method for testing light fastness under accelerated conditions is by exposing prints to a high intensity carbon-arc or to a powerful gas discharge lamp to simulate sunlight.
Carbon arc lamps contain very high levels of ultra violet when compared with sunlight and although very quick answers may be obtained by their use, the results are often ambiguous or misleading due to the influence of the excess UV.
Gas discharge lamps can be designed to give very similar spectral output to the Sun. The closest output is from a Xenon filled cold cathode quartz lamp.
Using such an accelerated method, a result equating to a year exposure may be obtained within a month.

Reference standards
It is meaningless to state light fastness in terms of “good”, “medium” or “fair”, as such descriptions are arbitrary and relate to no reproducible standard. Similarly, descriptions such as “10 best – 1 worst” are equally useless unless carefully related to an accurate standard.
The international standard for light fastness testing is the Blue Wood Scale (BWS).
(A former standard called the Madder Scale is now completely out of date as many modern colourants are more lightfast than the original “standard” Crimson Madder).

Blue Wool Scale
The Blue Wool Scale (BWS) for light fastness testing was developed by the British Society of Dyers and Colourists www.sdc.org.uk and although originally intended as a textile standard, the BWS has found international acceptance as a reference point for testing of all coloured materials including inks, paints, plastics, and textiles. The BWS standards may be purchased from www.sdcenterprises.co.
uk/products/lightfastness.asp
The scale consists of pieces of woven wool cloth dyed with different blue dyes, which are rated from one to eight such that each increasing number is twice as lightfast as the former, ie four is twice as lightfast as three and four times as lightfast as two and so on.
The only irregularity in the scale is between six and seven. At this point, there is a factor of 2.5 as no dyestuff is known that will exactly maintain the doubling ratio.
Also, a rating of eight means that the colour will last in effect for an indefinite period, as there is no upper limit.

Test procedures
To carry out a test related to the BWS, a print sample is exposed alongside strips of the blue standard wools, and light fastness is assessed visually as the number of blue wool which has degraded to the same extent as the sample.
BWS equivalence should be listed as the first noticeable sign of fading. It is not appropriate to wait until the colour is completely degraded, as this procedure would give an overly optimistic reading.
In order to relate a BWS rating to time, a broad approximation may be applied.
If one is taken as a few days exposure to sunlight, then two equals a week, three a fortnight, four a month, five two months, six four to five months, seven a year, eight two years plus.
It must be stressed that such an approximation is not always reliable, depending on such things as colour strength, exposure conditions and substrate.

Exposure conditions
Colour degradation can be hastened by external factors such as chemical attack from sea-spray or smog, algae or fungus growth, breakdown of the substrate, and high temperatures that may affect the carrier of the colourant. It should be remembered that most paper surfaces usually go yellow or brown when exposed to sunlight.

Colour concentration
As colour degradation is caused by the chemical breakdown of each particle of coloured compound (pigment or dyestuff), it logically follows that the more coloured particles there are in the film of ink, paint etc, the longer it takes to completely degrade the colour.
Therefore, a concentrated colour or thick film will be more lightfast than a tint or a very thin film of the same coloured product.
In a printed item, tone screens degrade more quickly than solids of the same colour. This is due to a larger ratio of coloured pigment in the dot being exposed than in a solid, plus the effect of the substrate surrounding each dot also suffering change.

Colour change
Fading is caused by the colour in the print being broken down to a point where the substrate (usually white) shows through, giving the effect of a tint or weak colour.
“Darkening” is either a shade change in the colourant (eg Pantone Rhodamine slowly goes brown in sunlight) or in the substrate itself.
Breakdown of the substrate can also have a drastic effect on the visual result of the print.
(In the case of darkening, rather than fade, a “Gray Scale” is referred to instead. This scale is principally used in photography and consists of a piece of cardboard printed with strips of grey in numbered steps from white to black).

Dyes and Pigments
Dyes are defined as colourants that dissolve in the medium into which they are put, while pigments are coloured materials that may only be suspended in their carrier. A simple dye example is beetroot juice - a simple pigment could be rust.
The ability of dyes to become solvated is related to the fact that dyes usually have many times smaller particles than pigments, (often down to molecular size), which also accounts for dyes having a pure, transparent appearance, but also explains why they are generally much less lightfast than pigments.
There are light resistant dyes available, but they have limited solubility in most ink-making materials, and are very expensive.
Fluorescent colours also have poor light fastness, as these materials are not pigments, but dyed resin chips, and so behave as do typical dyestuffs.

Colours
With modern colourant technology, virtually all shades are now available in BWS values of six or better. However, with few exceptions, as the level of light fastness increases, so does the cost of the colourant. Further, the colour strength and purity of hue of the light fast colour is almost always poorer than the non-light fast colour that it replaces.
The Pantone standard colour series ranges from colours with BWS 7+ (Process Blue), down to three (Warm Red), with most in the four to five area, consequently many standard intermixes, particularly tints, have a very low fastness rating.
Light fast alternative colours are available from all ink manufacturers, although these are generally less clean in hue and thus may not give exact matches to the standard Pantone intermix colours.
In a standard four colour process set of inks, the Cyan (phthalocyanine) has excellent light fastness (7+) compared with both the magenta, which is usually a pigment called Red 57.1 or Rubine Red, and has a light fastness rating of five, and the yellow, which, depending on the type of (Diarylide) pigment used can range from three to five. Thus it is the yellow closely followed by the red that fades first. Who has not seen old pictures of the Queen, or posters of vegetables in a shop window with only the blue and black shades surviving?
Similarly to the PMS range, lightfast process colours are also available, magentas based on a class of pigment called quinacridone, and yellows on various azo complexes can up the levels to six or seven, but these materials are less clean in hue, considerably more expensive, and usually lower in colour strength than standard colours.
So there it is, not all that difficult when you think about it, and remember that thought is believed to travel at the speed of light, (299,792,458 metres per second!)