In the present investigation process modelling techniques have been applied to describe reaction layer growth during active brazing of ceramics. As a starting point, the classical solution for parabolic growth of transformation products is considered. Specific computational features are then explicitly built into the model to allow for transient effects during heating and cooling as well as changes in the growth kinetics due to depletion of the active element during brazing. This approach gives considerable scope for optimization of both process and joint properties through adjustment of the filler metal composition and the temperature-time programme under which brazing takes place. The aptness of the process model is illustrated in an accompanying paper (Part II).