Ignition and combustion of pyrotechnical mixtures in a closed volume are studied with an inflammatory device. In a previous study, both, measurements and numerical simulations showed, that the burning phenomena are very fast, with a typical duration of about 200 mus. Temperature measurements by a two-color pyrometer made it possible to evaluate the temperature as a function of time. The temperature seems to reach a constant level during the first 50 mus of combustion. Then the radiance continues to increase, obtaining a decrease of the temperature which reaches approximately 3000 K. These temperature measurements indicate that the radiative medium (the flame) is a gaseous phase with a high density of particles. In order to check this assumption, spectroscopic measurements have been made and are presented in this study. The results are commented by visualization of fast camera recordings confirming that the combustion phenomena can be separated into two stages: before 50 mus and after 50 mus. The first part of the process corresponds to the optical emission of a very hot gas which can be a potassium plasma, assumption justified by the presence of an intense emission line of neutral potassium identified at 770 nm. In this first stage, a few particles contribute to the optical emission. The second stage of the process reveals a typical grey body spectrum, being a medium charged with particles. The experimental spectrum reveals gradually the absorption lines at the wavelengths of the atomic lines observed in the first stage. The assumption of a cold gas surrounding a hotter reacting area seems to be confirmed for the second part of the combustion.