This work is part of the Joint Action Theory in Didactics. It is focused on the transaction dimension of communication in educational environment in which knowledge is the object of the transaction. This research aims to understand the structuring nature of knowledge within interactions and in the context of science education. The descriptive model of the didactic transactions we have previously developed is enriched and refined by linking two concepts, one being derived from the comparative didactic: the didactic equilibration and the other being derived from the language didactic: the double semiotization. This enrichment is a theoretical contribution to the characterization of linguistic action and instructional strategies of the teacher. In a more enlarged manner, this work allows us to better understand how and for which purposes the teacher produces his speech. In this study, we look at the teacher‘s work as a process of double semiotization of the learning objects. According to this view, the testing of this model has given us access to some of the strategies used by this beginner teacher and has allowed us to reconstruct the genesis of the observed phenomenon: the «fading away knowledge».