Meredith,
The reference to a south-facing window is specific to the Northern Hemisphere, where a south-facing window would receive direct sunlight. In the Southern Hemisphere, of course, a north-facing widow would be the one to use. Direct sun is an accelerated-aging test, which gives us an idea which colors would be most likely to change in normal indoor light over a longer period of time, and which colors would not.
The Lightfastness Test System is a kit sold by Golden Artist Colors, Inc., and contains three Blue Wool Control Cards, along with other paraphernalia and complete instructions for conducting lightfastness testing. For pastels, I've found it is better to make my samples on a paper more suitable for pastels than the forms supplied in the kit, and to make somewhat larger samples and masks, but otherwise to follow the procedures as recommended in the instructions. Golden may be reached by telephone at: (606) 847-6154. The kit is not terribly expensive.
When artists make their own pastels, they can use the most lightfast pigments if they so choose, as some do, and have done in the past, but commercial manufacturers of today avoid using the more toxic pigments in pastels out of concern for the safety of the consumers and of the employees at the manufacturing plants, as well as to protect themselves from product liability lawsuits. However, that's only a part of the picture. Because we cannot do much in the way of color mixing with pastels the way we can with oils, many more colors are needed. Manufacturers, in order to make available a full selection of colors, sometimes choose pigments of less than optimal lightfastness to achieve this full range of colors, tints and shades. This has always been a factor with pastels, and is not unique to modern times. Most museums keep their pastels in darker rooms than the oil paintings, to limit the amount of light exposure, out of concern for possible fading of some of the colors. Today there is a much wider range of pigments from which to choose, including many of higher lightfastness than was previously the case, so manufacturers can (and the best of them do) produce more lightfast pastels if the consumers (artists) insist on them. However, if pastellists continue in blissful ignorance to issues of permanence, or seem to be unconcerned with it, the manufacturers have no reason to improve the archival qualities of the products they sell, and they will not spend money to change the way they've been doing things.
The belief that pastels are permanent is a popular one with pastel artists, but it is not based on much more than wishful thinking, and is harmful because it does not compel people to push for better materials. If we demand it, the manufacturers will respond.
Virgil Elliott
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