This article originally ran in the Traditional Building November 1989 edition. Original author was Robert A. Baird, V.P. Operations at Historical Arts & Casting, Inc.
In our preservation projects, we have spent countless hours discussing restoration practices for cast iron ornament with architects and contractors. I thought it might be helpful to answer (in brief) some of the most frequently discussed topics.
Baltimore City Life Museum - Baltimore, MD
Cast iron is a brittle, porous, high-carbon metal. From an engineering standpoint, it has tremendous compression-loading capacity, but very poor tensile strength. Its high carbon content makes it extremely difficult to weld. Consequently, most cast iron ornament is mechanically fastened.
Reviewing the restoration of a typical cast iron façade is a good way to discuss various ways to stabilize and repair cast iron.
A Typical Façade Restoration
The first step is to remove deteriorated, loose or broken elements, and clean them. Most cast iron façades are composed of repetitive elements connected by a series of clips, angles, and mechanical fasteners. Disassembly is handled by using a cutting torch to burn the heads off fasteners. Ornament is attached to the façade with countersunk flat-head machine bolts. It can be tricky to locate these fasteners under several layers of paint, but they are there. After removing the fasteners, iron elements are lifted or hoisted from the façade.
Duane Street - Lower Manhattan (Tribeca), NY
Cast iron can be cleaned on site or at a separate location. For the most part, we have had best results with sandblasting. Although the job is messy, the result is superior to chemical stripping. The high-tech coatings we specify require that the metal be blast-cleaned to an SSPC-SP6 finish.
A cast iron façade can be scaffolded and enclosed with a plastic or canvas barrier to contain the blasting media and debris. Manufacturers specify immediate application of primers after blasting to prevent any buildup of surface rust. For an initial primer, we’ve had good success with TNEMEC-Zinc – a zinc-rich primer manufactured by TNEMEC Company.
Be aware that the paint you are removing most likely contains lead, so disposal of the blasting debris must be done according to Federal EPA regulations. Attention to this detail requires a lot of paperwork, but we’ve found local Environmental Protection Agencies very helpful with lists of certified disposal sites and transporters of hazardous waste.
Cleaning large facades can be sequenced so that blasting and priming are accomplished daily until the cleaning phase is complete. Regular inspections are vital during this phase so that problems that are uncovered can be treated and/or scheduled for repair.
Chemical strippers have their place; they are the only answer for interior and exterior spaces where blasting is not feasible. There are several good products that do an adequate job with a little elbow grease. A good scrubbing with a power wire wheel and degreasing are recommended following chemical stripping.
The major cause of deterioration in cast iron facades is the lack – or breakdown – of the waterproofing system. When putting a façade back together, this is a critical issue. The amount of time spent applying correct weatherproofing principles has a direct effect on the life expectancy of the restored building. We recommend caulking all horizontal and vertical joints. We use a paintable elastomeric caulk (Sikaflex) made by Sika Corp., Lyndhurst, N. J. Sheet metal flashing is also an integral part of a good waterproofing system.
Over the past several years we have used fluid-applied polyurethane undercoating systems on the back and unexposed sides of iron. More recently, we’re using expanding foams to fill the voids in hollow cast elements to eliminate condensation.
ZCMI Building - Salt Lake City, UT *This project is currently being restored again by Historical Arts & Casting to be placed in the City Creek Development.
Repairing Cast Iron
There are several options when repairing cast iron. Replacement is the highest level of treatment – and the most expensive. Other options are to remove the material and repair it in the shop, or make repairs in place. We recommend that the latter be limited to minor repairs only. Generally, shop repairs can be much more extensive because of the controlled environment and the more sophisticated equipment available.
Some architects have asked if they can fill hollow castings with concrete or mortar. We don’t recommend it. It was done historically on some projects that we have restored and has created all sorts of problems. We have seen splits in 1-in.-thick columns caused by the pressure of interior rust created by iron in contact with concrete that’s been subject to moisture penetration.
Frequently, imperfections are found in cast iron after it is blasted. Cold runs and shrinkage can be filled with Bondo and sanded smooth. Those imperfections that could possibly hold water from a driving rain are the ones that should be filled. Restorers should be aware that although good castings have a smooth uniform finish, they do not look like extrusions. We have always believed that the texture of cast iron adds to its character and that too much filling is unnecessary and distracting.
Please check back for part 2 of 2 which will be posted Wednesday June 9th, 2010.
