I have been taking a set of sash windows to bits to replace some of the rotten wood. After removing the glass and dowels I find there was not glue in the mortise and tenons just the dowels, is this or was the practice in the old days. Should I glue the mortise and tenons together to make sure they are whetherproof, or stick to the old method. What was the reason, if any, for not using glue? AndyS

I agree, it would have almost certainly been glued and the glue has just disintegrated with age. I'd use a water resistant glue like Resin W when you reassemble the frame.