Context 162 - November 2019

C O N T E X T 1 6 2 : N O V E M B E R 2 0 1 9 45 Deterioration Poor maintenance or poor design can quickly lead to moisture collecting at the bottom rails, under cills, or in joints or hinges, leading to rusting. Ungalvanised steel windows are espe- cially susceptible to corrosion, and those with faulty galvanising or mechanical damage may be affected too. As rust builds up, windows may become difficult to open or close, and thus subjected to rough treatment that causes further damage, including breakage of the glass, more likely. Rust jacking (that is, pressure exerted on surround- ing fabric as the corroding metal expands in volume) may eventually distort the steel frame, interfering with the operation of the window and cracking the glass (and also the surrounding fabric). Water running off corroding steel will stain the surrounding building fabric, adding to the general poor perception of steel windows. Maintenance and repair Regular painting is essential to prevent cor- rosion of steel. It is also important to make sure that joints between the frames and the surrounding building fabric are watertight. Maintaining hinges, handles, locks and stays in good working order will help prevent damage from mishandling. Repairing steel windows may involve address- ing distortion, corrosion, losses or missing glass panes.To realign very distorted frames, the glass panes have first to be removed, and sometimes the window has to be taken off site to be straight- ened using heat. The distorted corroded section may sometimes need to be cut out to relieve the pressure, and the frame re-welded back together. Superficial corrosion of steel can be sim- ply rubbed down, before applying a zinc-rich metal primer and repainting. Deeper, but non- structural, losses should be raked out as thor- oughly as possible, then primed and filled with a suitable metal filler before repainting. In many cases, windows are capable of refurbishment and repair, but where the corrosion loss is so great that the strength of the frame is compromised, the window will need to be replaced. In such cases, finding sections with matching mouldings from current manufacturers can be a problem. In complex cases, where moisture problems are endemic to the building and removal of the window frame for repair would be difficult and damaging, cathodic protection (which uses electrical potential to transfer corrosion from the steel window to a sacrificial metal anode) may be considered to prevent corrosion of historically significant steel frames. Replacement glass should be cut about 3mm smaller than the frame, to allow for thermal movement. All old putty and mastic should be removed, and any corrosion treated by cleaning and priming, before the new glass is fitted. The best method of setting in the replacement glass depends on how the pane was fixed in the first place. Putty should be profiled in such a way that water will not be trapped in the frame. Stow Maries Great War Aerodrome Stow Maries Great War Aerodrome in Essex has the largest surviving group of Royal Flying Distortion and loss where there has been water ponding