No interaction between dye molecules is required to explain the shift. If you increase the concentration of a dye, then it will start to absorb significant amounts of light even at those wavelengths for which the absorption spectrum is for weaker concentrations close to zero (almost no absorbance). As the absorption spectra are typically smooth functions of wavelength, the result of increasing the dye concentration is that the “cutoff” wavelength (where one starts to have significant absorbance) is effectively shifted. If you try comparing Procion Yellow MX-8G and Yellow MX-3R, you see that there is a shift towards red when increasing Yellow MX-3R and not much shift with Yellow MX-8G. This is because the spectrum of Yellow MX-3R is smoother (there is some absorbance for a longer range of wavelengths). The transition from no absorbance to significant absorbance is very short with Yellow MX-8G, so there can’t be much shift with that dye.

In my experience, Procion MX dyes are not very good for bright reds like scarlet. It will look a bit dull or a bit orange, unless, perhaps, if you use huge amounts of Red MX-5B or Red MX-8B in the recipe, which is not convenient.

Your thanks are much appreciated! Happy dyeing!

I found your applet through Paula Burch’s wonderful site. Thank you for creating this nice application. My question, why does increasing the concentration of a single dye shift the spectrum – just the dye molecules interacting with themselves due to the concentration, modifying the absorption spectrum? (I’d call it quenching if this was fluorescence; in absorption I’ve forgotten what it is.)

PS – I’m trying to get an idea of creating a nice “scarlet” with Procion Red MX-5B and Orange MX-2R.

Thanks again.