The rings were cleaned in an ultrasonic bath with isopropanol and then placed in an oven according to EN 14775-1[14] at 550°C to remove all combustible particles. The rings were weighted before and after the measurement to the nearest 0.01 mg to measure the amount of deposits. As the rings were placed in a horizontal tube, fly ash that was not fixated to the surface was removed by carefully blowing the substrates.