The main purpose of the present paper was to study the elastic properties of silicone impression material containing an overdose or underdose of hardener as a function of the time elapsed from the start of the mix. Additional measurements of the quasi-viscosity as a function of dosage and mixing temperature were performed to cover the plasto-elastic transitional phase after mixing. The experimental conditions have been established according to the results from previous processing tests. Overdosage as well as underdosage of the hardener caused (according to its extent) marked or in part considerable deterioration of the elastic properties. The differences from normodosed material decreased with the time elapsed from the start of the mix. Having discussed his observations from the viewpoint of materials science, the author draws conclusions as to the processing of silicone impression material and to the differential indication of alginate and silicon materials for one-stage impression.