There are several different decorating techniques, but the most common is enamel using coloured mixtures obtained from finely ground powders of opaque and transparent low melting glass.
This enamel is applied by brush on the glass surface to achieve the desired decoration.
The decorated object is then placed in an oven, and what has been applied by brush, softens and adheres to the surface of the glass.
Usually the oven does not exceed 500°C for this type of processing.
Once this melting cycle is over, the glass is cooled down and the finished work can be admired.