Conservation of Cast & Wrought Iron

7 Repair Techniques - Cold

Cold repairs avoid the thermal stresses and fire/fume-risk of hot processes, and so are particularly suitable for repairing cast iron, and for use in historic buildings. Cold processes also generally employ simpler/cheaper equipment, and should be adopted in preference to hot techniques where possible.

Traditional ‘grey’ cast iron contains flakes of graphite which lubricate cutting operations such as drilling and tapping (threading) holes. All the techniques described in this section are commonly used for repairing cast iron

Plating - A strong and discreet repair can often be achieved by bolting a steel plate across a cracked component on a hidden face. Plates must be bedded on red-lead or two-pack epoxy putty to exclude water, and can be secured with counter-sunk screws for a neater appearance. If stainless steel plates or fastenings are used they must be physically/electrically isolated from the casting to prevent dissimilar-metals corrosion. Stainless steel bolts can be insulated by painting with two-pack epoxy primer and assembled before the paint cures.

Structures are increasingly being repaired or strengthened by bonding on plates of carbon or glass-fibre. (see below).

Studding - Broken rod-shaped components can be repaired by drilling and tapping both parts and screwing together onto a threaded bar (studding), bedded on red-lead or two-pack epoxy putty. If components cannot be rotated, the studding can be screwed into one part and secured by epoxy putty into the other.

Carbon/Glass Fibre repair/strengthening - Filaments of carbon or glass are bonded with two-pack epoxy resin onto a blast-cleaned surface to strengthen a component or repair a crack. Preformed and tensioned plates may also be used to upgrade structural strength.

Stitching - A modernised traditional technique for repairing castings in which holes are drilled across a crack, slotted, and a ferrous lock or ‘stitch’ is driven in to tie the sections together. Advantages: Cold process requiring only portable hand tools. Safe on site. Hermetically tight, no sealants used. Invisible on completion. Disadvantage: Not suitable for thin sections.