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More about Lightfastness Tests (part 2)


For the industry, which has a considerable interest in knowing if their products will stand the test of time (especially if stuff is intended for outdoor use), there are a variety of tests and testing apparatus available, from placing things into the Arizona desert (yes really!) to simulating direct sunlight with filtered special lamps. Even there, though, a lot of different parameters are not closely monitored or regulated (there’s a nice German pdf here about the challenges, from a symposium on the topic of colour fastness).

So if even the industry has some trouble getting comparable results… well. Let’s face it: Our household methods will always be squishy. It’s like dyeing tests, or so many other experimental archaeology things using natural resources: a huge amount of variables, many of them not easily measurable or not measurable at all.

I had a chat with a conservator about just the topic of how reliable these tests are, or how helpful, because of anti-UV-coatings on windows, a while ago. Basically, what I remember her saying was that yes, some of the UV is filtered out – but as the coatings also degrade with time, it is hard to say what comes through, and how much of it.

However, these household method lightfastness tests usually serve one of two purposes – an absolute indication or an indication of relative lightfastness, compared between different dyes or procedures.

One, trying to get an “absolute”, is of course difficult – but if we accept that the absolute is not necessarily comparable to other tests, or will not give a definite number, it can still be useful. A case for this could be: a dyer wants to know if she can use a specific dye method (type and amount of mordant, type and amount of dyestuff, and method of mordanting and dyeing) to get lightfast results. Or someone has fabric or yarn with unknown dye used on it and wants to figure out if the colour will last when used for a garment before investing knitting or sewing time. In that case, as most of the textiles will be used mostly indoors, having a fastness test inside a window will simulate real life nicely enough – and hanging stuff into a window for 3 or 4 months in summer should show if the fade is strong, or within tolerable limits for everyday use. This will, of course, have different results depending on how sunny the exposure time is, what place the test is taking place at, the humidity, the type of glass and so on, but it should still give an indication for the useability of the dye run.

The relative lightfastness, use number two, is our reason for the lightfastness tests. It’s intended to give a direct comparison between different dyes or different procedures, in our case if there is any difference in fading between the samples that have a very similar colour to start with. I fully expect there to be fading, even significant fading, as birch leaf is not the most lightfast yellow to start with, and we used weak end-of-year birch to boot, but no matter how strong the fade will be – as long as there is noticeable fade, we will have the possibility to directly compare the fade between similar colours.

Another use for this relative test would be testing the relative lightfastness of different dyestuffs resulting in a similar colour, such as birch, weld, onion skins (widely famed for their rapid fading) and friends (much of what grows green dyes yellow, so there’s no dearth of choice here), or testing the relative lightfastness when using different mordants, or different dyeing temperatures, or different lenghts of immersion, and so on. In these cases, as all the samples are stuck into a given place at the same time, and thus have the same conditions over the course of their test, it is again of little matter whether these conditions are normed or not.

So, to sun, er, sum it all up: Yes, lightfastness tests done by sticking stuff inside windows are not really comparable or give absolute numbers, and it’s unknown how much bleaching actually takes place through the glass as compared to unfiltered light, but this very simple test still serves its purpose.

If there is a real necessity to make the results a little more comparable, and get something more in the direction of absolute values, there is a possibility – which is using a comparison scale with known lightfastness values as a benchmark. One of the things used for this is called the Blue Wool Standard, a card with textile strips on it that are dyed blue in progressing depths. These bleach out at a known number of megalux hours, so there is an absolute indication of lightfastness of your candidates. If you are a dyer or an experimental archaeologist who needs something like this, you can buy the scales, for instance here. (No, I’m not affiliated in any way.)

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