However, sometimes it can also result in distinct changes from one colour to another. This usually occurs when just one dye in the dye mixture used fades, eg black leather often fades to green as the blue and red dyes in the mixture fade. In addition, colour changes in other process chemicals, eg vegetable tanning agents, can cause colour shifts.
The cause
For the colour of an object to be perceived by the eye, it has to be illuminated by visible light (400 to 700nm wavelength). Assuming the object is illuminated with white light (an homogeneous mixture of all wavelengths between 400 and 700nm (Figure 2)), if it reflects all of the light then the object will appear white. If it absorbs all of the light and reflects none then it will appear black. If the object absorbs some wavelengths of light and reflects others it will appear coloured depending on which wavelengths are reflected. For example, if an object reflects only light with a wavelength of 650nm it will appear red. The use of dyestuffs enables the tanner to determine which wavelengths of light will be absorbed and which will be reflected, thus producing the required colour.
When a dyestuff absorbs light energy it raises the dye molecule to an electronically excited state. This excited state is very short lived and the dye molecule rapidly returns to its original state. The excess absorbed energy can be lost in several ways:
* The evolution of heat
* The emission of radiation (fluorescence or phosphorescence)
* Photochemical reactions which can cause degradation of the dye molecule resulting in fading of the colour
It is thought that there are several different photochemical reactions that can occur to produce fading of the colour which can be influenced by:
1. Dye structure; the rate of fading of azo dyes tends to follow the order chemical reactivity: blues>reds>yellows. Also particle size can be influential.
2. Dye concentration; it is thought that high concentrations of dye or pigments has the effect of protecting them from oxidation.
3. Substrate composition; direct contact of the dye molecule with the functional groups of the substrate can degrade the dye by photo-redox reactions, ie oxidation or reduction. Photo-reduction is more common on protein substrates such as leather1.
4. Atmospheric conditions; moisture allows the diffusion of reactants such as oxygen. Atmospheric pollutants such as sulfur dioxide and oxides of nitrogen can also be influential.
5. Other chemicals within the substrate; some metal ions can affect the length of time that the dye molecule stays in its excited state. Photo-degradation of tannages and retannages can also affect the light fading of leather.
6. The wavelength of light; light at the ultra-violet end of the spectrum is of higher intensity and likely to accelerate fading.
It can be seen that the problem of poor light fastness is a highly complex issue and is not solely due to dyestuff selection, although dye selection obviously plays a vital role; premetallised and mordant dyes generally having good lightfastness and basic dyes having poorer lightfastness on leather. The photo-stability of other components in the leather needs to be taken into consideration, eg retannages and fatliquors etc. The presence of metal ions such as copper or nickel should be avoided with iron complex dyes. However, copper can improve the lightfastness of some anionic dyes. The use of water softening agents and sequestering agents such as EDTA should be avoided with all metal complex dyes.
Preventing light reaching the dyestuff in the leather is a useful approach; the use of pigments in the finish or ‘pigment dyeing’ techniques2 provides a significant improvement in lightfastness and UV absorbers can also be used to filter out the more damaging UV light.