What Does Lightfast Mean?
It’s a way of gauging how long a dyed fibre will retain its colour when exposed to UV light for a standard length of time.
There are several different systems for assessing relative lightfastness, but the most widely used one in the textile industry is probably the Blue Wool Scale.
Most industry scales use a numeric system (e.g. 5 = excellent fastness to 1= poor fastness), or a descriptive system (e.g. “excellent”, “good”, “poor”, “very poor”). But most natural dyers use their own visual comparison to other dye samples they have tested.
ALL dyes - whether natural or synthetic - will fade, given enough exposure to light. Lightfastness helps us understand the relative lightfastness of one dye, or dye method, compared to others, or compared to a standard scale, such as the Blue Wool Scale.
Why Do A Lightfast Test?
Knowing the relative lightfastness of a dye is vitally important information to museum conservators, who are responsible for protecting important archaeological textile samples for posterity. It’s also important information for textile manufacturers, who want customers’ dyed purchases to last.
But it’s also important information for natural dyers - whether professional or hobbyist - as understanding the relative lightfastness of our dyes signifies that we value this as a skilled craft, and want to promote evidence-based practices. We also may not want to waste precious resources - our fibres, time, water, and energy - on a colourant that doesn’t have lasting power.
Lightfastness is not solely a property of the dye used, but can also be influenced by how the fibre is prepared before dyeing, and also by the actual method used for dyeing. So, it can be worth doing lightfastness tests of samples dyed with the same dye, but prepared or dyed in different ways.
How To Do A Lightfast Test?
The basic principle is that you want to be able to fully protect half of your dyed sample from any UV exposure, while fully exposing the other half of the same dyed sample.
You can do this by covering half of a sample with a couple layers of cardboard, or any UV impermeable material, while leaving the other half exposed. You can tape, stitch, or staple a sample so it is half covered by your impermeable material (see image below), or if you’re testing dyed thread/yarn, many natural dyers use something like a popsicle/ice lolly stick to wrap the material around before covering half with a UV impermeable material.
If you are in the northern hemisphere, place your sample in a south-facing window for one month. If you are in the southern hemisphere, place your sample in a north-facing window for one month.
Some colourants may fade so quickly, that you can end the trial in less time.
At the end of the exposure period, you can remove the covered half to reveal the side-by-side difference between the portion of the sample that was protected - i.e. that shows the colour when your sample first emerged from the dye pot - and the portion that was exposed to UV light.
Bertie provides quality control!
If testing multiple samples at once, label to keep track of what is what. Put your labels on the back, away from the light, so that the pencil or ink won’t fade or soak through to your samples.