Working with Puddled Wrought Iron
Wrought Iron - Techniques of Renovation
Cast Iron - Techniques of Renovation
Protection and Finishing
Maintenance of Wrought Ironwork
WORKING WITH PUDDLED WROUGHT IRON
Puddled iron is a mixture of nearly pure iron with up to 5% siliceous (glassy) slags, which take the form of linear fibres - giving the metal its characteristic grain. Puddled iron is for the more advanced forger, more so than steel or homogeneous pure irons. Care must be taken to respect the properties of the material. It is necessary, when forging puddled iron always to do heavy forging at a high temperature - around 1350 to 1450 degrees Centigrade (bright to sparkling white heat). At these temperatures, the iron will move very quickly, whilst doing no damage to the grain structure.
Finishing work, bending etc., can be done at red heat. Should heavy working at lower temperatures result in splitting along the grain boundaries, it is necessary only to heat the iron to a welding temperature to close the split under the hammer. At a white heat it will be found that wrought irons are far softer to forge than even pure iron. This is due to the internal slags melting and providing an internal lubricant that reduces friction during distortion under the hammer.
There is nothing to beat the forge-welding ability of puddled iron, as the enclosed slags form a natural flux, allowing the iron to be heated rather more than can pure irons or steel, this extends considerably the heat range over which the iron can be welded.
Wrought iron is the traditional material of the blacksmith. Due to the siliceous slags combined with its fibrous structure, it resists corrosion far better than modern steels or pure irons, as is amply shown by the survival of much of our heritage of wrought ironwork, in many cases centuries old. It is neither necessary nor recommended to galvanise or zinc spray wrought iron.
In the event of any further queries on working techniques advice is readily available - please call or email.
WROUGHT IRON - TECHNIQUES OF RENOVATION
There are two types of wrought iron. The irons of antiquity, now known collectively as "charcoal iron", and a mass-produced iron, produced in the 19th century and early 20th century, known as "puddled iron". Although pre-18th century wrought ironwork is, of course composed of charcoal iron, it is normal to make repairs and replacements in Puddled iron, owing to its similar properties. On no account should mild steel be used on external work without zinc coating by galvanising or hot metal spraying. As neither of these treatments is permissible nor effective with ancient work, the use of mild steel is effectively ruled out.
Removal From Site:
Most work is ideally carried out in workshop conditions, and it is frequently the case that iron components can be removed easily from site. In the case, however, of railings, gate Piers etc that may be fixed into stonework, usually in lead filled sockets, removal may not be possible without sacrificing expensive stonework. Lead may be removed from sockets by mechanical means, but this is very laborious and any attempt to melt the lead will inevitably result in failure and damaged stonework unless the socket can be held horizontally to enable the lead to run out.
Ironwork is generally covered in paint and frequently a build-up of rust in water traps etc. Commonly, paint and some of the rust are removed by grit blasting. There is, however good arguments against grit blasting, as follows, so that it should be regarded as a last resort.
Grit blasting will remove the outer surface of the iron, known as mill scale. This mill scale, which is typically 90% intact on work 300 years old or more, is the original surface to which paint was applied, and as such is as worthy of conservation as the rest of the iron. Further, the mill scale, in such a case has a proven record of keeping corrosion at bay. It is a protective surface in its own right, and hence of value. Further still, grit blasting will render all of the iron surfaces the same, thus removing permanently any evidence, which may be present on the surface of the iron. For example, a component, which has been renewed, and is thus not original, will exhibit a different colour of mill scale to the original. It is often the case that successive generations of repair can be detected, on the basis of colour alone. A surface which was originally polished for, say, indoor use, may still retain its bright appearance under the paint, giving us evidence, perhaps of a former use. Likewise, file marks etc, giving evidence of techniques of manufacture, will be removed by grit blasting.
Where possible, we will always recommend paint stripping by chemical means, with a thorough removal of the chemical agents, usually by steam cleaning. This will result, for the most part in the restoration of the piece to its original appearance as it was immediately prior to painting. Rust deposits are normally dealt with by the application of heat. Rust scale does not expand when heated to the same extent as does the iron. The differential in expansion causes the rust to lose its grip, when it may be shaken or brushed off. Heating the area to a red heat also results in the reduction of the surface layer of the metal to an oxide layer similar to mill scale. Often, where there has been a considerable accumulation of rust, the application of heat is needed anyway as part of the remedial process.
SAFETY NOTE: Wrought iron is frequently coated with lead based paints; often with a 75% lead content. Care must therefore be taken, particularly with grit blasting, to ensure that both operatives and the public are protected and that the lead working regulations are adhered to.
Ironwork is often fastened together with riveted, or tenoned joints. It is not possible to part such joints without at least some damage, or weakening becoming evident on re-assembly. It is worth avoiding the parting of frame joints etc, merely to gain access to corroded components, as the frame will never be as strong again. Where tenoned joints must be parted, it is nearly always necessary to replace the tenon with a screw or screwed tenon, in order to regain adequate strength.
Repairs and Replacements:
As a matter of course, the replication of components should be carried out in a manner similar to that which was used for the original creation of the piece, and in similar materials. Ideally, all work to an ancient piece should use the old techniques of forge welding, tenoning, riveting and collaring etc. so that a high degree of blacksmithing skill is generally required.
However, it is often the case that components cannot be completely removed from the job, or that only small work is need to a large component. In this case, recourse must be taken to more modern techniques.
For structural purposes, where part replacement is required, as, for example in the case of a gate back stile, which may be rusted away at the bottom, arc, mig or tig welding is used to join on the new part. No special equipment is required for the electric welding of wrought iron, only that normally used for the welding of mild steel; however, mild steel electrodes or MIG wire are not acceptable, a ferrous non-corrodible alloy must be used. Care must be taken in preparation however, as wrought iron is a laminar material, and welding must be carried out through the full depth of the section. Attaching components to the surface of wrought iron sections is not very strong. Alternatively brazing may be used, and is often useful for the attachment of components such as waterleaves, where the original method of forge welding or riveting cannot be done.
Sections which are heavily pitted, or wasted, but which are still structurally sound, may be repaired by the puddling in of new wrought iron, in the form of thin rods by the gas welding process. Iron thus deposited has no laminar structure, and hence little tensile strength, but otherwise appears to exhibit the properties of the parent metal. Alternatively, these sections can be built up by electric welding, but again use must be made of a suitable alloy. Care should be taken to avoid distortion of any section so treated.
Sheet work, such as leaves, being often impossible to access for the painting of both sides, is the usual candidate for replacement. For many years, there was no commercially available supply of iron suitable for the often-deep distortion necessary in repousee work. Copper was often used, but it is soft enough to be easily bent, and will not hold paint well, while mild steel and pure iron, particularly in thin sheet form will soon rust away. A few years ago we addressed this need, and by recycling the scrap iron resulting from the restoration of pre-19th century wrought ironwork, now produce a sheet charcoal iron of superior quality, for repousee work etc.
It must be said that often, the repoussé leaf work found on ancient work, is of such a high standard of craftsmanship, that one cannot hope to accurately replicate it. In this case, we often make a point of preserving, at all costs, at least one of the originals, in order to give future students at least a clue. When a piece of sheet work is reduced virtually to lace, it may still be conserved, by scrupulous cleaning and the application of a layer of epoxy-resin to the rear surface. The detail can then often be restored by careful carving into any resin protruding on the front surface, with files etc.
The most common reason for the rusting of wrought ironwork is the gathering of water in places that will not dry. Wrought iron will last indefinitely, with reasonable maintenance if rainwater is kept at bay. Such bad places are the joints between members which lie alongside one another, for example, between a shadow bar and its mate, touching points of scrolls, particularly on a horizontal surface, water leaf sockets which are upward facing, and any area which is constantly submerged in vegetation.
When work is assembled, care should be taken to ensure that mating surfaces are protected by paint, as well as are visible surfaces, and that suitable mastic filler is applied before the work is assembled. Accepted practice is to use modern silicone mastic, which sticks very well, and is totally waterproof. Red lead putty is the traditional one, and if well sealed with paint, at regular intervals, will serve well. Waterleaf sockets may be filled with epoxy resin, poured in until it overflows, or pitch can be used, on the basis that in the summer, it will melt and renew its seal with the iron. Lead poured hot was often used, but as this does not stick to the iron, and water will be able to penetrate, it is no longer advised.
CAST IRON - TECHNIQUES OF RENOVATION
The cast irons used for highly detailed work in the nineteenth century were frequently rather high in phosphorous, which lowers the viscosity of the molten metal to enable a high degree of detail to be reproduced. The result is; however, extremely brittle and the greatest care should always be taken to avoid breakage, especially at the dismantling stage.
Fixings are usually of wrought iron, and if flush with the finished surface, will normally be impossible to release. Heating of the component to release fixings is usually not an option, as local heating of a thin casting can also result in breakage. Resort must be taken to mechanical means, i.e. drilling out of the wrought iron fixing. Care must be taken to drill centrally to the bolt or stud, as the wrought iron will often be harder than the cast and the drill will tend to drift off away from centre, also drills will be prone to breakage as they cross the even harder zone of rust between the cast and the wrought iron. The usual technique is to drill an initial hole of a small diameter to locate the centre, and to open this out by degrees until the remains of the fixing can be removed and the hole re-tapped etc.
Repair of broken castings:
Experience has indicated that welding repairs of cast iron decorative components are seldom satisfactory. The welds are not of the same strength as the casting and can crack, while heat stress can cause cracking elsewhere. Resulting distortion or misalignment cannot be rectified.
Alternative methods are;
1. Brazing, using oxy-acetylene and a brass filler rod. 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.
2. Bonding with an epoxy adhesive. Modern adhesives can give a very sound repair, with no need for heating of the components. Added security can be given to a joint, in some cases, by the use of iron or stainless pins located in holes drilled into the mating surfaces. If difficulty is experienced in getting alignment, one or both of the holes may be drilled oversize, and the gaps filled with the epoxy adhesive.
3. Plating with cast iron or non-ferrous plates or straps on the back of the broken casting. This is applicable to thin castings, but only where the backside is not going to be visible. Plates may be attached with screws or copper rivets, and a bonding agent should be used between casting and plate. Mild steel screws are not acceptable.
New castings and components:
Where castings are to be replaced, new components can often be made by moulding from an original, however, shrinkage will occur to render the copy slightly smaller than the original, which may be unacceptable. Also there is often, in common with all copies, a loss of detail between pattern and casting, which, on work of high quality may be unfortunate; however intermediate patterns can often be made from an original, in resin or aluminium, to improve the finish. Ultimately, the quality of the final product will depend upon the skill of the moulder, and examples of their work should always be inspected before engaging a new moulder. Where a casting is re-useable, except for the absence of a small part, a rubber mould may frequently be taken from a similar part and just the missing part cast, as a patch, which must then be joined into the casting by one of the above methods. Old work was sometimes done, in iron, by lost wax methods; a technique not often employed nowadays in ferrous materials. This was done to overcome problems of drawing from the mould of complex, undercut patterns. The more able foundries will still be able to undertake this work, but beware at the pricing stage as the cost is of a different order to that of conventional moulding.
Where an old component exhibits a brittle failure through ordinary use, the temptation may be to replace it with a new one in malleable or sg. iron, which are much stronger than the original grey irons. There is no track record with s.g. due to it being a relatively new material, and its long-term performance is unknown however, there is some evidence that the old malleable irons corrode rather more readily than grey iron. Also difficulty may be achieved in obtaining an acceptable quality in s.g. iron, particularly in thin castings, owing to its greater molten viscosity and hence reluctance to run into a complex mould.
Cast iron was often joined by wrought iron fixings, which are frequently seen to have done good service. However, lack of maintenance can lead to water ingress, and eventually failure of the casting due to rust heave. The same comments would apply to mild steel fixings, where the effect would be seen sooner. Stainless steel is often used nowadays, and will give good results, but water must also be excluded here, as stainless steels are just as prone to anoxic corrosion as their more prosaic brothers. The best fixings are made with non-ferrous materials, bronze, brass or copper. In the case of copper, being soft, the fixing is likely to fail before the casting, resulting in less breakage of iron. Precautions should be borne in mind against electrolytic action, but in practice, this does not seem to occur in situations remote from an electrolyte, i.e. where the joint is more often dry than wet.
Joint surfaces should be painted prior to mating, and filling with non-setting bituminous mastic is a good plan if possible. Attention should be taken, in heritage work, of the shapes of the heads of the fixing screws, as these were frequently used for ornamental effect. Any shape of screw head can be obtained as a special from the fixing manufacturer, and attention to detail such as this is often justified.
Sockets, and tenon joints etc, were frequently fixed by running hot lead between the components. This is a good and neglected method of fixing castings together, but the joint must be caulked cold for full strength to be achieved, and to exclude water from the joint.
PROTECTION & FINISHING
Galvanising and zinc spraying have been mentioned and there are good reasons why these will not do. Galvanising depends upon dipping the work after cleaning in acid, in a bath of molten zinc, which leaves a rather thick layer of zinc on the surface. Drips frequently form which must be ground off. If you add to this that the small joints will remain full of acid after the treatment, it is easy to see why this process is not appropriate to delicate and complex wrought ironwork. Additionally, in certain circumstances, the galvanising process can deeply etch the surface of wrought iron causing irreversible damage to the piece.
Zinc spraying is a far less brutal process. It is a hand method, which consists of removal of all mill scale by grit blasting, and the immediate application of a zinc coating with a type of flame gun. The objections to grit blasting have been enumerated above. Further, it is not possible to clean very small joints by grit blasting, from the physical restrictions imposed by the size of a grain of grit, neither is it possible to clean nor spray material which is not accessible to line of sight. The water traps in wrought ironwork are just such small joints and out-of-the-way places.
Owing to the natural ability of wrought irons to resist corrosion, it is sufficient to protect ironwork by a good coating of paint. However, I cannot stress too strongly that, in common with other items placed out of doors, such as woodwork, wrought ironwork needs regular maintenance. Chips and developing problems should be dealt with at the earliest dry opportunity, and the work should be painted at least every five years.
Wrought Iron minimal paint system product specification for ‘Single Pack 4 Coat'
One coat of Sigmafast 20 primer
Two coats of Sigmarine 35 MIO (Micaceous iron oxide)
One coat Sigmarine 40 Semi-gloss undercoat/finish
If you cannot find a suitable product from a local supplier then this paint system is available from;
Industrial Paint Supplies (Hull) Ltd, 51 Charles Street, Hull, HU2 8EH
T; 01482 321 154 E; firstname.lastname@example.org
NOTE; All ironwork should be adequately protected during transport and installation with any grazes or chips to paintwork made good on site, using primer on any exposed steel prior to topcoat.
MAINTENANCE OF WROUGHT IRONWORK
Ironwork is commonly supposed to be nearly free of maintenance and as such is frequently left to rust undisturbed for long periods resulting in periodic major overhauls, at great expense. This could be avoided by insistence on annual inspection with immediate and usually trivial remedial work to arrest any developing problems.
Suggested establishment of a rolling programme of maintenance, of all items of ironwork, based upon the following schedule.
1. Initial attention in the form of repair or restoration.
2. Thorough inspection of ironwork - Identify any areas of foliage, debris or moss build up and in particular any areas showing loose and damaged paintwork or signs of rust seeping from, or water lodging in, joints. Any chipping of paint in well-ventilated areas is not too significant.
3. Attend to identified problem areas at the earliest opportunity - Clean ironwork and immediate surrounding area of any build ups and ensure all ironwork is kept well ventilated. When the paintwork is dry remove dust, contaminants or loose coatings by sanding or wire brushing and then locally reseal all areas identified in the inspection by touch-up paintwork. Bare metal should be primed with suitable primer before applying topcoat. Run paint into any joints where water is known to lodge to make sure these areas are completely sealed. Note that this work can only be carried out in periods of warm and dry weather when the problem joints are thoroughly dried out.
4. Lubricate to ensure freedom of movement - Hinges; fill all grease nipples on bottom hinge sockets & lightly oil pin inside top strap hinges. Shoot Bolts; lubricate as necessary. Lock; lightly oil the top of the protruding locking bolt and then slide in and out a couple of times to disperse the oil.
EVERY FIVE YEARS
5. Thoroughly re-paint ironwork with original (or equivalent) high performance paint system as proposed under ‘finishing'. Prior to painting the ironwork should be thoroughly cleaned with hot soapy water and a scrubbing brush then degreased. When the paintwork is dry remove dust, contaminants or loose coatings by sanding. Bare metal should be primed with suitable primer. Apply full topcoat as required.
EVERY 15 TO 20 YEARS
6. Inspect gate locks & determine if needs replacing.
7. Decorative ironwork made of genuine wrought iron should be free of long-term damage if the maintenance regime outlined above is adhered to. However, should it be necessary to remove thick layers of paint the ironwork should be grit blasted only as a last resort as this removes the iron's own original protective layer of oxide. Instead all wrought ironwork should be striped by chemical means. Wrought iron should under no circumstances be galvanised or hot zinc sprayed as both cause irreversible damage to the natural corrosion resistant properties of wrought iron.
THEN BACK TO
1. Again et cetera ad infinitum.