masking a graphics

Hello, In my application, I need to mask a graphic. Mycode : Graphics gDetails = e.Graphics ; gDetails.DrawString("Détails carte", new Font("Microsoft Sans Serif", 9), new SolidBrush(Color.Black), new Point(480, 250)) ; gDetails.DrawRectangle(Pens.Gray, 480, 270, 300, 300) ; When I use : gDetails.Clear(Color.White) ; All the graphics disappear. What the solution ? Thanks a lot Marc