Slag is foamed in electric arc furnace during steelmaking to reduce the amount of energy emitted to the furnace walls. The behavior of the electric arc plasma during slag foaming is not very well understood, since it is very hard to obtain empirical data about arc plasma from industrial furnaces. In this work the effect of foaming on the arc plasma is studied by generating electric arc plasma over a foaming slag in laboratory scale. The arc was generated on top of the slag with a system modified from a welding power supply and the foaming of slag was induced by blowing oxygen gas into the slag. The electric arcs were monitored with two cameras filtered to measure different wavelengths and three spectrometers measuring different spectral regions with varying spectral resolution. The first camera was able to detect only the light emitted directly from the arc plasma while the second can detect the high temperature slag surrounding the submerged arc. By comparing the two images, it was possible to differentiate between arc burning inside the foaming slag and free burning arc. The use of plasma diagnostics with optical emission spectrometry provided information about electron temperature and density of the arc plasma. The plasma diagnostics and camera pixel intensities indicate that the arc burning inside foaming slag is constricted and has high plasma temperature. Comparing the free-burning arc on top of the foaming slag and non-foaming slag show that the CO bubbles cause the plasma temperature to fluctuate significantly.