|
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 which 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.
Cleaning
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 are, 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 a consistency 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)
Dismantling
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 gain 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 backstile, which may be rusted away at the bottom, arc/mig 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-corrodable 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, gas welding or brazing may be used, and are 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, particularly in thin sheet form will soon rust away. Chris Topp & Co, a few years ago 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 repousee leafwork 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 sheetwork 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.
Reassembly
The most common reason for the rusting of wrought ironwork, is the gathering of water in places which 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 a suitable filler is applied before the work is assembled. We use a 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 seems to me to be worse than useless.
Protection.
Galvanising and zinc spraying have been mentioned, 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. Wrought iron is often etched very deeply by the action of the process. 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, by reason of their in-built barriers of slags, 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. See elsewhere for our preferred paint system.
NOTES ON MAINTENANCE OF WROUGHT IRONWORK
Suggest the 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. Annual inspection, both rigorous and minute, of every detail of ironwork. Look particularly for signs of rust seeping from, or water lodging in joints. Any chipping of well-ventilated areas is not too significant.
3. Attend to any problems at the earliest opportunity, but only in periods of warm and dry weather when joints can thoroughly dry out. Any rust scale should be removed, preferably by local heating.
4. Try injection of low viscosity, oil based rust inhibitor into joints, followed by resealing of damaged areas by touch-up paintwork.
5. Thoroughly re-paint ironwork periodically, but at most five-yearly.
6. Long term plan for stripping of accumulated layers of old paint, accompanied by any repairs. Stripping preferably by purely mechanical and chemical means, rather than by grit blasting, which removes the iron's own original protective layer of oxide.
THEN BACK TO
1. Again et cetera ad infinitum.
SINGLE PACK 4 COAT PAINT SYSTEM
1ST COAT
CRODA 4112 RUSKILLA ZINC PHOSPHATE ANTI - CORROSIVE PRIMER MEDIUM GREY/RED OXIDE
2ND COAT
CRODA 2401 RUSKILLA ALKYD MIO MICACEOUS IRON OXIDE INTERMEDIATE NATURAL/SILVER
3RD COAT
CRODA CMO1 SYNTHETIC ENAMEL BRUSH, SPRAY OR DIP EQUIPMENT ENAMEL
4TH COAT
CRODA CMO1 SYNTHETIC ENAMEL BRUSH, SPRAY OR DIP EQUIPMENT ENAMEL
|
