Puttying the Transom Window and historical use

Today I finished puttying the panel for the transom window, it now has to set up for a week or two before cleaning.  Historically this was done for stained glass windows to weather proof the window as it was the only thing blocking the outside weather from the building interior.  Today stained glass windows are either mounted on the inside against a thermopane window or suspended or sandwiched between tempered glass.  This process is still done because it fills the space that is in the channel where the glass fits in.  It also makes the panel more solid.