*2.4.1. Lightfastness*

The lightfast properties of printing inks can be defined as the amount of resistance to fade or color change of a printed surface when exposed to daylight (or an artificial light source) over a set period of time.

For determination of lightfastness of prints, Blue Wool Scale is utilized. According to this method, samples are exposed to a standard xenon light in appropriate equipment.

**Figure 21.** STEM image of TiO<sup>2</sup> with aluminum phosphate coating.

The lightfastness is indicated by the grades on the Blue Wool Scale: BWS 1 = poor, BWS 2 = low, BWS 3 = average, BWS 4 = rather good, BWS 5 = good, BWS 6 = very good, BWS 7 = extremely good, BWS 8 = excellent.

TiO<sup>2</sup> with aluminum phosphate coating successfully underwent testing by the end-user with grades 4/5 on the Gray Scale (grades 1–5) and with grade > 6 on the Blue Wool Scale; we met the requirements of the user.
