One method of testing the lightfastness of pigments is based on the use of natural light, while the other is based on the use of artificial light. When testing the lightfastness of pigments under natural light, the tested pigments are generally ground together with an oil-based adhesive or glue solution. But for this purpose, it is recommended to use pure gelatin. The lacquer is applied to the iron board and wood board, and the painted board is covered in half with a piece of paper or laminate. After a certain period of exposure, compare the colors of the two halves of the test plate (quickly covered and uncovered).
Of course, this measurement is very subjective and inaccurate, because the effect of sunlight is not taken into account when performing this test in the first place. Secondly, in addition to light, there are also some auxiliary factors: such as water, gray growth, etc., also affect the color of the paint, so it is difficult to determine what role light plays in the change of pigment. In order to eliminate the influence of some auxiliary factors, the test plate is sometimes spread out indoors so that the painted side faces the sun.
The solar hours can be measured by means of a "daylight scale", which is made of blue paper coated with an organic dye. The blue paper is illuminated with light, and the paint layer is irradiated at the same time to measure the number of solar hours that the paint layer is subjected to light.
This method yields good results when all the detailed rules of the method are strictly followed.
To prepare blue paper, 200 grams of kaolin are mixed with 200 ml of water, and then 400 ml of solution is added to this mixture, which is made by dissolving 2 grams of alkaline J blue dye in 1000 ml of water. Subsequently, the precipitate is filtered out and crushed in a dry frequency well in a dark room at room temperature. Thereafter, 50 grams of the resulting pigment was stirred into 35 ml of water and 45 ml of a clear gel solution (1; 5) After the mixture is carefully taxed, it is brushed on slightly glued paper, and the well is dry in the dark. When the paper is exposed to the sun, it gradually fades. Every half hour, a piece of paper is cut out of the paper to create a "daylight scale" composed of a series of pieces of paper of different colors.
In recent years, textile lightfastness standards have been adopted for measuring the lightfastness of pigments.
Small strips of wool fabrics coated with organic dyes with different lightfastness are used as the standard for lightfastness: these strips are the standard for lightfastness (light fastness). It should be noted that although fabric standards only specify criteria for fading and not for color darkening and color light changes, they can often indicate the lightfastness of pigments and common pigment films, especially those with a mixture of organic and inorganic pigments.
In order to test the lightfastness of the pigments, the corresponding color paint film can be placed in the sun (under natural light) with a comparison standard well row, and then the color changes over time.
In the Soviet Union, in order to determine the stability of organic dyes to light, a standard of five stability classes was used.55 This standard is made from "Record" brand wool fabric, which is dyed with various dyes according to a special formulation.
