Conservation of Cast & Wrought Iron

6 Repair techniques - Hot

Hot processes are used in forming, joining and cutting iron, but are often prohibited in historic buildings due to fire risk, or are allowed only on the basis of a daily ‘hot works permit’ where no other process can be substituted. Coke-fired hearths were the traditional means of heating. On sites small portable hearths were employed for heating rivets, etc. and hearths are still widely used in workshops for general forgework. Nowadays three additional methods of heating are commonly in use:
Fuel gas and air - Butane or propane stored in portable bottles mixes with air at a torch to provide a flame at several hundred degrees centigrade. Used for low temperature operations such as heating components to separate them, softening paint, and soft soldering. see below)
Fuel gas and oxygen - Propane or acetylene and oxygen stored in heavy bottles mix at the nozzle of a blowpipe to produce a flame around 3,000 degrees centigrade. Used for forming metals, separating components, flame cleaning, silver-soldering, brazing and fusion welding. (see below) These gases can also be used for cutting thick sections of wrought iron and steel where excess oxygen is used to blow away white-hot metal (flame cutting). Use of gases on site is dangerous due to their flammability and naked flame, and restricted by the size and weight of bottles.
Electric arc - The low voltage/high current output from a transformer provides a continuous spark or arc between the workpiece and an electrode at around 3,000 degrees centigrade producing an instant localised molten pool. Used mainly for welding processes, principally MMA & MIG (see below). Electric arc welding is not recommended for repairs to cast iron due to the risk of thermal-shock damage to the brittle material.
	Soldering & welding processes - Iron oxidises when heated which prevents joining, or reduces the strength of the joint. A flux or inert gas shield must therefore be introduced to prevent oxidation by providing reducing conditions. Soft Soldering - Soft solder is an alloy of tin and lead (sometimes with traces of antimony) which melts around 200 degrees centigrade. Surfaces to be joined are cleaned, protected with a liquid or paste flux, heated by gas-air flame or electric soldering iron, and wire or stick solder melted into the joint. Widely used in joining copper, brass and lead for non-structural applications. Brazing or hard soldering - Surfaces to be joined are cleaned, fluxed, and heated by oxy-propane flame to around 600 degrees centigrade. A bronze filler wire (spelter) is melted into the joint, drawn in by capillary action. Spelter containing silver (silver solder) melts at a lower temperature providing a weaker joint. Suitable for non-structural applications on thin cast and wrought iron sections, but cast iron must be heated and cooled slowly to minimise the risk of cracking. Gas fusion welding - Components are heated by oxy-acetylene flame to their melting temperature creating a localised weld-pool into which a filler rod of the same metal is melted. Suitable for the structural repair of cast iron which must be pre and post heated to minimise thermal shock. Manual Metal Arc welding (MMA or ‘stick’ welding) - An electric arc between a consumable electrode and the work-piece forms a small weld pool in which the components fuse. Oxidation is prevented by the vaporisation of a flux coating around the electrode. MIG welding (Metal Inert Gas) - As MMA welding but the weld-pool is protected by a shield of argon, and the filler-wire electrode is automatically fed into it by a current-sensed servo-system. Suitable for welding wrought iron, but neither MMA nor MIG welding are recommended for repairing cast iron due to the risk of thermal shock causing cracks around the weld. Fire or forge welding - Two pieces of wrought iron will readily fuse together if heated in a hearth to white heat and hammered together. A well-executed forge weld should achieve about 80% of the original metal strength, and is recommended as an appropriate traditional process for repairing wrought ironwork.
	Hot set riveting - This is the commonest traditional method of joining wrought iron components in structural and decorative applications, and should NOT be replaced by welding. Rivets are entered red-hot into prepared holes, the head retained by a hand-held gun or jack, (‘a holder-up’) and the tail forged down to fill the hole, and grip the components. Grip is further tightened by the rivet shrinking, resulting in a strong, watertight joint. Rivet heads are commonly spherical (round-head), conical (snap-head), or flush with the surface (counter-sunk). Small rivets are set by hand, larger ones by pneumatic or hydraulic tools which allow large numbers of rivets to be set quickly. Removal of rivets must be undertaken with care to avoid damaging plates and holes. Heads may be ground off, shanks drilled and the rivet punched out. A quicker (traditional) method is to shear off the head with a rivet-buster, a long pneumatic gun operated by two men. Its chisel is placed under the rivet-head to shear it off, and a punch in the same gun is used to drive out the shank. Appropriate safety precautions must be taken.