A method for the determination of heat and light fastness of insoluble organic pigments

The European association of pigment and dye manufacturers established a method for the estimation of heat and light fastness of organic pigments. In such method, pigments were dispersed in partially dried linseed oil whereby prints were made on standard writing paper. Degraded prints were visually estimated after comparing with a freshly prepared standard scale of 8 grades. A sample similar to scale 5 or more was considered to be of good or excellent fastness. Practically, it became familiar with controversial personnel estimations, especially in regard to the critical level of 4–5. Furthermore, the specific side effect interference of the linseed oil resin could not be avoided. It becomes reasonable to put forward a method depending on instrumental measurements. In the light of this method, heat and light fastness of organic pigments can be colourimetrically determined. The pigment is pasted in water containing 2–3% weight of sodium silicate as binding agent. Thin film prints are then made on a transparent non‐porous standard paper 80 g/m2. Prints are dried over P2O5 in a desicator placed in a cool and dark place. The dried prints are striped in 2cm width. Some of these stripes are taken for the colourimetric determination of the colour density with the help of a colour densitometer. Other stripes are subjected for 72 hours to daylight (not to direct sun rays) or placed in a drying oven maintained at the required temperature for the same duration. For colour density measurements, prints are underlined with 100 g/m2 white standard paper. Peak heights obtained with certain light wave‐length are computed with reference to the peak of the undergraded sample. It has been shown that a change in heat or light fastness as low as 2% can be determined with an accuracy range of 0.5% compared to 12.5% and 3.5% for the European method respectively.