Precipitation from the gas phase
This image (secondary electron image taken with an electron microscope) depicts materials which precipitated from the gas phase. During high-temperature processes conducted under an atmosphere, a small quantity of the process materials will evaporate and form a vapor pressure of their compounds in the surrounding atmosphere. When the atmosphere saturated with evaporated compounds undergoes cooling or otherwise transitions to different conditions, the evaporated material carried in the vapor phase can exsolve out of the vapor and precipitate as condensation.
This process mirrors the condensation of water in air, which forms clouds and leads to precipitation of rain. In this image, such condensation products are seen from a high-temperature oven used for the investigation of refractory materials. The condensation from the high temperature vapor led to the formation of both solid and liquid condensates. The predominant crystallite observed in the image is MgO, which precipitates as solid due to its high melting point (> 1800°C). Other compounds, such as CaO, Al2O3 or SiO2 also condensated from the atmosphere. However, due to the high temperature during condensation, the condensation of these elements formed a liquid, which ‘rained' as droplets of liquid slag onto the MgO crystal. The grainy detailed surface of the slag droplets is a result of the final crystallization of the slag as the furnace gradually cooled down.