Putty glazing is the traditional way glass is held into a timber window. The pane sits on a bed of material in the rebate of the sash, is secured with small metal glazing sprigs, and the outside face is finished with a weathering fillet dressed to a clean bevel. Done properly the fillet sheds water away from the joint between glass and timber, and reads as a crisp line from the outside.
It is the reason a period window looks like a period window. The sightline is slim, the bevel catches the light along its length, and the glass sits almost flush with the face of the sash. Bead-glazed and factory-sealed units cannot reproduce that, which is why the method survives on listed buildings and in conservation areas long after it disappeared from ordinary joinery.
How window putty fails
The material hardens with age. Once it has lost its flexibility it can no longer accommodate the seasonal movement of the timber beneath it, so it cracks, parts from the glass and falls out in sections, usually starting at the bottom of the pane where water stands longest. Paint failure accelerates it: the fillet relies on a continuous paint film lapped a little way onto the glass, and once that film splits, water gets behind the material and into the rebate.
The consequences run past the glazing itself. Water sitting in an open rebate goes into the bottom rail and the joints, which is where sash timber decays first, so a failed fillet on a cheap-looking window is often the reason a much larger repair is needed a few years later.
What re-glazing involves
Old material is raked out of the rebate and the sprigs removed, and the rebate is cleaned back to sound timber and primed — bare wood draws the oils out of anything bedded against it and the repair fails early. The pane is then bedded, secured, and the perimeter dressed off to a bevel that matches the other windows on the elevation. The work is left to firm up before it is painted, and the paint is carried a whisker onto the glass to seal the edge.
Where a single pane has gone in an otherwise sound window, this is a contained repair. Where the fillets have failed across a whole elevation it is usually done alongside the rest of the restoration, because the sashes have to come out anyway.