A composition with no water consisting essentially of at least one fluoride-ion-generating agent and at least one viscosity modifier, characterized in that its viscosity, measured at 25.degree. C. using a Brookfield.TM. LVT viscometer fitted with a No. 1 spindle rotating at a speed of 30 revolutions per minute, is between 50 and 5000 mPas Method for preparing the composition, dry composition and method of frosting flat glass.