The effect of the particles' colour on the intensity of the scattered light they create is generally overestimated. For example, people are often sceptical of the idea that even black particles will scatter light. It is an undisputed fact, however, that elementary carbon in various forms (soot, graphite, coal) produces scattered light of the same order of magnitude as white diatomaceous earth (kieselguhr, SiO2). Dust measurement
The following Figure compares the scatter intensities of soot and white products of various particle size in air and water:
Fig. 32: Scatter intensity of various materials. Concentration = 1000 ppm. n=refractive index
Laboratory tests conducted by SIGRIST have demonstrated that the following quantities of carbon of various particle size in water produce the same amount of scattered light as 10 mg/l of Kieselguhr (SiO2) :
| Particle size | mg/l | |
|---|---|---|
| Activated carbon | $> 10 \mu m$ | $20 \, mg/l$ |
| Coal dust | $ \sim 1 \mu m$ | $0.7 \, mg/l$ |
| Graphite | $< 0,1 \mu m$ | $5 \, mg/l$ |
These figures, which are approximate, reflect the heavy dependence of scatter on particle size. Refractive index; Dust measurement