Terminology used in the restoration of ironwork
The following glossary has been compiled by Chris Topp to help in understanding the terminology used in the restoration of historic ironwork.
Brazing or bronze welding uses an oxy-acetylene flame and a brass filler rod. It is necessary to thoroughly clean the components before brazing and a flux is necessary to prevent premature oxidation of the surfaces to be joined. A good bond can be achieved with a lesser heat input than welding methods, but care must be taken that the surfaces to be brazed are tinned prior to filling the joint. Excess brass can be dressed by grinding or filing. Brazing is appropriate to a range of metals: wrought iron, mild steel, cast iron, brass, copper and bronze. With care brazing can be used to fill gaps and is often used to fill perforations in water leaves etc. This has the advantage of conserving the original component while the different colour of the filler metal gives evidence of the repair.
REMOVAL OF LEADED IN JOINTS
Where ironwork has been fixed into sockets cut in stonework – a common method re historic ironwork –great difficulty is associated with any attempt to dismantle the joint. Options are as follows:
- Melt out lead. This is not normally possible unless the orientation of the socket will allow the molten lead to flow clear of the socket. This can sometimes be arranged by tipping the copings with railings attached through 90 degrees. Even so the greatest care must be taken to avoid heating the stone.
- Smash copings to release ironwork.
- Cut ironwork just above coping, remove railings to a workshop and extend to original length. Stubs in stone can be removed by core-drilling.
- Attempt to remove the lead mechanically by drilling and chiseling. This can take up to two hours per hole and invariably results in damage to the stone. Often the sockets are not sufficiently accessible for this method.
TIG and MIG
As variants of electric welding, similar comments will apply. They are both applicable in certain situations but the use of mild steel as a filler wire should be avoided in favour of non-corrodible wires of nickel or bronze.
must be made to re-fix components using similar methods – collars, rivets, screws, tenons etc., taking care to coat components with primer paint where possible before re-assembly. Where new parts have to be fitted by cutting out original material, in order to avoid too much dismantling of original sound joints – e.g. sections of branched scrollwork – new components may be attached by brazing or electric welding as appropriate provided that the joints thus formed are dressed back to a fair surface prior to painting.
When the bar can be removed (in workshop), if slight bend use hydraulic press, if severe use heat and straighten at the anvil.
Bars that cannot be removed (in situ repair needed) very often necessary to make up a special jig to suit the dimensions of the bar in question (one jig will serve for any number of similar bars). As above if slight bend use a portable hydraulic power pack, if severe then use heat and leverage as appropriate. In extreme cases it may be necessary to cut out the bar and replace with a new one in the appropriate material.
REPLACING PARTS WITHOUT DISMANTLING
When it will be necessary to create a component or a part of a component use techniques and materials identical to the originals. This new component can then be assembled into the whole by bonding, riveting or welding as appropriate. Welds etc. addressed back to a fair surface.
Repoussé is a metalworking technique in which a malleable metal is ornamented or shaped by hammering from the reverse side to create a design in low relief.
WROUGHT IRON BILLET
Part of the process of forming the material, today this is normally 2 or 3 inch diameter lengths of bars, which can be used under a power hammer or heated and rolled down to bars.
Planishing is a technique that involves finishing the surface by finely shaping and smoothing the wrought iron.
Lost-wax casting is the process by which a duplicate metal sculpture can be cast from an original sculpture. This was widespread in Europe until the 18th century, when a piece-moulding process came to predominate, but not widely used today.
The process used for repairs to cast iron work.
Pinning is a useful method for repairing fractures or attaching two castings to each other. This method results in a barely-visible join, with none of the raised material left by welding. Pinning is achieved by drilling a hole into each of the castings that are to be joined together. A stainless steel threaded pin, bedded in epoxy, is used to join the two halves together. Once the two sections have been joined by the pin, no brazing or welding is required to secure or seal the joint. The joint should be thoroughly painted and kept well maintained. This type of repair is ideal for decorative castings and is typically used for repairs such as attaching finials to bars, reattaching missing sections of cast panels or the broken arms of finials. If the cast iron is too thin in section it is not possible to carry out this type of repair, brazing is an alternative repair method in this instance. The disadvantage of this method is that it requires holes to be drilled, but this is likely to be outweighed by the benefit of being able to retain the original casting
PLATING of CAST IRON
Plating is useful for joining together larger sections of cast iron that have fractured, enabling the original casting to be retained. Plate repairs can also be a useful method of structural reinforcement.
Plating is carried out by bolting a flat section of stainless steel to the fractured cast iron plate. The cast iron should be clean and painted (including the fractured surfaces). The steel plate should also be well painted and should also be isolated from the cast iron by an insulating material such as nylon or PTFE (Polytetrafluoroethylene) to separate the steel from the cast iron and prevent galvanic corrosion. Stainless steel bolts should be used to fix the plate in place.
Another version of this repair uses a steel tube rather than flat plate. This is particularly useful for fractured columns. The steel tube is inserted into the central hollow of the column and bolted in place. Again, it is important to isolate the steel tube from the cast iron using an insulating material.
Again, the disadvantage here is that drilling into original cast ironwork is necessary. However, as with pinning, this repair method enables the original, fractured casting to be retained.
STITCHING of CAST IRON
Stitching is another method of joining two fractured castings together. The broken surfaces of the fracture should be well painted before joining together. A series of holes are drilled along the length of the fracture. Next, a line of holes is drilled at right angles to the fracture, on each side of the fracture. A special “key” is inserted into this line of holes to bridge the gap made by the fracture and is hammered into place. This is done at regularly spaced intervals along the length of the fracture. The disadvantage of this method is that it requires a considerable number of small holes to be drilled into the cast iron; if not properly maintained and painted, water can seep into the fracture and / or holes and begin the process of corrosion.
WELDING OF WELDABLE CAST IRON
Cast ironwork is often composed of a number of individual castings joined together. Traditionally, this was not done by welding. Castings were most commonly joined by interlocking via lugs or bolts. These jointing techniques are part of the character of cast ironwork and it is important to preserve and replicate these during repair works.
Generally, welding is not a good solution for fine or delicate cast iron, although it can in some circumstances work well – usually in heavier castings. Welding is a difficult and specialised repair technique that should only be carried out by a professional with experience of working with historic cast ironwork. If a weld repair is not avoidable: preheat the castings slowly and uniformly. This helps to reduce the risk of thermal ‘shock’ on cooling. The join should be finished by a specialist contractor so that it is as smooth and unobtrusive as possible.
The choice of cleaning method should depend on:
• Degree of corrosion
• Location and accessibility of ironwork
• Significance of existing coatings
• Age and significance of the ironwork
• Thickness and strength of the ironwork
• Paint manufacturer guidelines
Cleaning Methods (from least to most aggressive)
– By hand – bronze wire brush, chisel & hammer, emery paper
– Power tools (not recommended)
– Needle gunning
– Flame cleaning (use with caution, experienced professional required)
– Chemical cleaning: topical preparations / dipping in chemical bath
– Dry ice
– High pressure water blasting
– Blast cleaning (wet and dry, various blast mediums available)
Cleaning by hand
If ironwork is generally in good condition it may be sufficient to clean ironwork by hand, as more aggressive methods, such as power tools, can cause damage by scoring the surface of the iron.
Begin with water and a cloth to remove general dirt and grime or moss growth. Light areas of corrosion can be removed using emery paper, taking care to remove any residue before applying paint.
Areas of chipped paint should be sanded down using emery paper, feathering the edges into good surrounding paint in preparation for paint application.
Vary cleaning techniques on different areas of the ironwork according to need. It may be sufficient to remove upper layers of paint until a stable layer is reached, then apply fresh paint. Avoid painting over corroded areas as corrosion will continue to develop under paint and will compromise the efficacy of fresh paint. The weather is of critical importance in cleaning and coating iron – low temperature and high moisture content in the atmosphere will often lead to failure.
Hand cleaning using an oxy-propane heating gun to soften the paint, which is the removed by wire brush and scraper. Does not work if bitumen paint, tar etc are present. A slow method which requires the use of breathing protection. Suitable only for outdoor use.
This is a non-invasive method which involves chipping the paint off with a set of vibrating needles. To avoid marking the work, the needles should not be sharp. It is a hand process and relatively slow, but it does leave the original oxide layer intact. As the paint chips are relatively large there is little danger from lead dust. As with all vibrating tools there are limits on operator exposure times.
immersion of the whole piece of ironwork in caustic soda. This requires the use of a tank sufficiently large to be able to submerge the whole piece for as long as it takes to dissolve the paint layers. This can be up to three weeks, although the reaction can be speeded up by heating the fluid by means of gas burners beneath the tank. When the paint is dissolved the work is taken from the tank and steam cleaned to remove any trace of the caustic. It may still be necessary to scrape or chip off in places where the paint was particularly thick. However, great care should be taken to wash or steam clean ironwork thoroughly after treatment to prevent chemicals from lodging in the microstructure of the iron and causing longer term damage. Note: if the paint has been dissolved out of joints etc, particular care should be taken to load the joints with paint prior to applying primer.
Hydro-blasting / high pressure water blasting
Water under high pressure (pressures greater than 30,000 psi) is blasted onto the surface of the ironwork to remove paint and corrosion. The advantage of this method is that it is relatively effective at removing surface salts, and there are no blast particles to become lodged in the surface of the ironwork. However, the moisture can sometimes cause rapid gingering (light, flash corrosion that occurs in damp or humid conditions) to the ironwork surface.
regarded as a last resort. Blast cleaning becomes the only feasible method of pint stripping when the work must be done on site and the budget precludes the use of other methods. Unless soft blasting media such as ice or granulated plastic – which render the process rather slow – are used, the usual media such as steel shot or aluminium oxide are very effective but difficult to control. Thus the original oxide surface is usually lost with any evidence this might carry. This process is non-reversible and so not recommended by the principals of conservation. Blasted surfaces should be coated within four hours on a dry day, or with wrought or cast iron, left for a period to develop a stable oxide coating prior to painting. Care must be taken to prevent dust migration in the case of lead based paints.
Overnight application of a thick layer of paint stripping gel, wrapped in cling film. Preserves the original surface. Works well on site, but care must be taken to remove all chemical residues prior to painting. Slow and expensive.