I have always been curious about why things happen, and the solar fading vs lightfastness phenomena was one of my knowledge “black holes” until I decided to do a bit of research about it.
Many materials are particularly sensitive to light, specially those which are organic like paper, cloths or organic pigments, they are inherently prone to fading. Light and ultraviolet radiation (UV) provide the necessary energy to fuel the chemical reactions that lead to fading.
This photochemical solar fading process is cumulative and irreversible.
Solar fading, but why this happens?
Molecules, the basic chemical building block of all materials, are in constant motion. When energy is introduced in the molecules, they vibrate more quickly (they get “excited”) and begin to spread out. Short wavelengths (those from the ultraviolet portion of the spectrum) will increase the vibration of the molecules and encourage them to expand and change their chains lengths.
All physical properties of the organic materials are strongly dependent on the size or length of their molecular chain. For example, as chain length is increased, melting and boiling temperatures increase quickly. Impact resistance also tends to increase with chain length, as does the viscosity, or resistance to flow in its melt state.
As a result, the re-conformation of the molecules changes the way that visible light interacts with the material, causing a change in the wavelength of the light and, as a result, we see a different color.
UV radiation is the most energetic yes, but visible light and heat are also significant factors in the fading process.
How is then lightfast artists pigments rated?
The German company Hoechst developed 100 years ago a method for rating lightfastness by comparing the dyes with a indigo dyed standard (Blue Wool scale) . This method is international practice today. This wool lightfastness standard shows values from 1 (fugitive) to 8 (very lightfast= indigo). Level 2 takes twice as long to fade as 1. Level 3 is double the time of level 2, and so on.
Generally, a wax enrolled pigment will be more protected against UV light than a ‘free’ pigment particle
e la resistencia a la luz mediante la comparación de los colorantes con un estándar de teñido azul índigo (Escala de Lana Azul). Este método es práctica internacional en la actualidad. Esta norma sobre lana indica los valores de 1 (fugitivo) a 8 (muy resistente a la luz = índigo). El nivel 2, por ejemplo, tarda el doble de tiempo en decolorarse que 1. El nivel 3 es el doble del tiempo que el nivel 2, y así sucesivamente.
En general, cualquier pigmento inmerso en cera estará más protegido contra la luz ultravioleta que una partícula de pigmento “libre”.
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