Pigment Lightfastness Checklist: What to Check Before You Buy a New Tube

Buying paint is easy. Buying paint that will still look like itself in thirty years takes a little more thought. Lightfastness — a pigment's ability to resist fading or shifting colour when exposed to light — is the most useful piece of information printed on a tube, and the one most often ignored at the till. The label is small, the shop is busy, and the swatch on the shelf looks lovely. Here is what to check before a new tube comes home with you.

What lightfastness actually measures

Lightfastness is not a measure of how bright a colour is, how well it covers, or how long the tube lasts. It describes what happens to a dried film of paint when light hits it, particularly the ultraviolet end of the spectrum. Some pigments are close to inert — think ultramarine, burnt sienna, most iron oxides, titanium white. Others are organic dyes that break down under UV, and no amount of careful handling changes that.

Two details matter more than most people realise. First, pigments often fade faster in tint than at full strength, so a red that looks solid straight from the tube may drift noticeably once it has white mixed into it. Second, a thin wash is more vulnerable than a thick, opaque passage, because there is simply less pigment on the paper for the light to work through. Ratings describe the paint as applied, not the paint as imagined.

The five-minute label check

In the shop, turn the tube over and look for these things, roughly in this order.

• Colour Index names. Codes such as PB29, PR122, PY154 or PW6 identify the actual pigment, whatever name the manufacturer has given it. Two tubes called "cobalt blue" by different brands should carry the same code; if one does not, they are not the same colour.
• How many pigments are in there. A single-pigment tube is easier to predict and easier to remix later. If a "hue" contains three or four pigments, the weakest one sets the lightfastness of the whole mixture.
• The lightfastness rating itself — and its system. A roman numeral and a star rating are not the same thing. Check what the manufacturer is using before comparing across brands.
• The word "hue" in the name. It usually signals a modern substitution for a traditional pigment, which is often good news for permanence, but it means the colour will handle differently.
• Which range it belongs to. Artist, professional and student lines can share a colour name while using entirely different pigments. A cheaper tube is not automatically fugitive, but the formulation is more likely to have been changed.

Fugitive pigments worth recognising by name

A fugitive pigment is one with a known tendency to fade, darken or shift. Alizarin crimson (PR83) is the classic example, along with genuine rose madder, carmine and some of the older hansa yellows such as PY1 and PY3. Aureolin (PY40) is frequently flagged for darkening rather than fading, which can be just as disruptive to a painting. Certain naphthol reds hold their masstone well but drift in tints.

None of this means you must avoid them. It means you should know what you are buying. Keep fugitive colours for sketchbooks, studies, prints of scanned work, or pieces that will live in a portfolio rather than on a sunny wall. If you want the look of alizarin with far better behaviour, look for a "permanent alizarin" built on PR177 or a quinacridone — and check the codes rather than trusting the name.

Decoding the ratings you will see

Most reputable manufacturers use one of two systems. The ASTM scale runs from I to V, where I is the best and V indicates poor lightfastness. The Blue Wool scale, borrowed from the textile industry, runs from 1 to 8, with 8 the most resistant; a rating of 7 or 8 is excellent, 6 is usually fine for work that will not sit in constant bright light, and anything lower deserves a second thought. Some brands use their own stars, dots or words such as "excellent" or "fair".

Two warnings. A "permanent" or "lightfast" claim on a tube aimed at students or hobbyists may describe a range rather than a tested pigment. And the non-toxic AP seal concerns safety, not fading — it tells you nothing about how the colour will age.

Where the painting will hang matters as much as the tube

Lightfastness is a property of the pigment, but fading is a property of the situation. A watercolour behind UV-filtering glass, on a wall away from direct sun, will outlast the same painting taped to a south-facing window. If you know a piece is destined for a bright room, or for someone who loves sunlight, lean towards ASTM I pigments for the colours that carry the composition, and be strictest about reds, violets and yellows, which tend to go first.

You can also test for yourself. Paint a stripe of each colour, half at full strength and half diluted, cover one end with card, and tape the strip to a sunny window. Check it after a few weeks and again after a few months. It is a slow experiment, but it teaches you more about your own palette than any chart.

Storage: protecting the tube, not the pigment

Storing paint in the dark will not make a fugitive pigment permanent. What sensible storage does is stop the tube drying out, separating or going mouldy before you get to use it.

• Keep caps firmly closed and tubes upright, so pigment does not settle into the neck.
• Store in a cool, dark, dry place. A cupboard beats a windowsill, a hot car or a damp shed.
• Do not let acrylics or watercolours freeze; freezing can spoil the binder even when the pigment survives.
• Check pans and half-pans for mould if they have been shut away damp, and let them dry fully before closing the box.
• Write the Colour Index code on the tube or in a swatch book as soon as you buy it, while it is still legible.

A shop-floor routine worth keeping

Before you buy: read the pigment codes, count how many there are, find the lightfastness rating and check which system it uses. Then ask where the work will live. If the answer is "a bright room, for years", stick to pigments at the top of the scale for the colours that matter most, and save the fugitive ones for pieces that will not be exposed.

After that, look after the paint properly. A tube kept cool and closed is a small thing, but it means the colour you chose today is the colour you can still mix next spring — and that the painting it goes into has a fair chance of looking the way you intended.

Photo: TheOtherKev / Pixabay