Working With Bronze
Cap Rail

By Alex Klahm
Alex Klam Architecture Metal Design Inc.

At the 1993 NOMMA convention in Lexington one of our shop demonstrations focused on the specialized skill of forming bronze cap rail. Nearly 100 craftsmen attended the two-hour session at Bluegrass Ironwork to watch David Ponsler form the material hot onto the lower half of a spiral staircase, while Jerry Grice showed how to form the material cold onto the upper half of the same spiral. While the demonstration was going on, informal "shop talk" conversations took place among the fabricators in attendance, with many sharing their own techniques and trade secrets for successfully bending bronze cap. In an effort to share our learning experience with the rest of you, this article is a collection of the techniques considered at that presentation.


Hot Forming Techniques

Bronze cap can be bent hot using a rosebud torch as a heat source. Many craftsmen prefer to do this bending in the evening when they can reduce the light in their shop. The bronze is heated to a dull red color, which is easier to perceive in the reduced light, and the material is then formed onto pre-formed bars or around forms. An ability to determine the optimal bending temperature is an important part of the process. Some craftsmen judge this by rubbing a piece of pine wood over the heated metal. The optimal bending temperature is reached when the pitch in the wood leaves a brown stain on the bronze. Other fabricators gauge the bending temperature by making felt-tip marker lines on the bronze bars before heating them. The amount of felt-tip marker residue is then used as a "temp-stick" to judge when the heat is right for bending. Either method can work well in different situations, depending on your own experience and your feel for the technique.


Cold Forming Techniques

     
  • Hossfeld Bender
    Bronze cap rail can be bent cold in a Hossfeld bender using the flat bar or cap rail bending dies. The material is generally covered with two layers of duct tape to protect the bronze from being scratched by the steel dies. Also, a felt-tip marker is used to mark the duct tape every 2 inches so that each small bend will be consistent and form a graceful, flowing curve. While this forming can be done on a Hossfeld using a hand lever, most craftsmen agree that the hydraulic cylinder attachment on the Hossfeld makes the work much easier.

     

  • Greenlee Bender
    A Greenlee bender can also be used with a hydraulic cylinder and foot-operated air pump to accomplish this same cold forming. Many of us make our own custom dies or adapt the Hossfeld dies to fit the Greenlee.

     

  • Strait-O-Flex
    A Strait-O-Flex is a lightweight and portable bending tool that can also be used to form cap rail. Designed for straightening railings, stair stringers, bar stock and pipe, it is available through Julius Blum & Co. Inc.

     

  • Rolling Machines
    Many craftsmen achieve consistent curves by rolling the cap rail through a three-headed rolling machine. A 1/2 round groove is machined into the rolls to fit the side profile of the material, and as many passes as required are then made to curve the cap to the pattern.


Combining Hot & Cold

Some craftsmen effectively use a combination of these techniques. For example: On a compound curve spiral staircase, a pattern of the compound curve can be made on a piece of flexible cardboard. The cardboard is bent into a compound curve when the pattern is made, and can be laid flat on a table to show what the two-dimensional bend instead of a three-dimensional or compound bend. The material is then rolled to a two-dimensional profile and placed on the channel where it is heated. This allows it to seat itself onto the channel to conform to the compound curve. Blocks of wood are typically used to clamp the hot bronze onto its form. While the wood may smoke and occasionally catch fire, it leaves the bronze undamaged and the charred residue is easily cleaned up in the finishing stages. These wood forms are best made from 3 inch diameter tree limbs which are cut into 3 inch long slices, drilled in the center with a 1-inch wood bit and band-sawed in half. This leaves a green piece of wood (which burns less easily than dry wood) with a wood grain structure that tends not to crack when clamped to the hot metal. [Figure 1]


Attaching Cap Rail

The generally accepted technique for attaching bronze cap rail to the bar it rests on is to drill and bottom tap into the bronze, using a counter sunk 1/4 x 20 flat headed machine screw. These screws are placed approximately 12 inches apart or where they can be accessed with a screwdriver. In areas where the bars must be spliced onto other bars or fittings, they are generally secured 1 inch back from the area to be spliced to hold the joint securely [Figure 2].


Welding Methods

     
  • T.I.G. Welding
    T.I.G. welding is frequently used to join the bronze cap rail sections together. Brass brazing rod or cut pieces of the cap rail are generally used as the filler metal; using these materials ensure the best chance of a good color match in the weld. The laws of physics make it difficult to impossible to achieve an invisible weld using alloy #385, the material used to extrude the various cap rail profiles. This is because alloy #385 is made from 60 percent copper, which melts at 1,900 degrees F and 40 percent low temperature metals like zinc, which melt at 800 degrees F. During the welding process the low temperature alloy metals tend to vaporize out of the weld by the time the copper part of the material can be fused. This means that the metal alloy in the welded area becomes different from the parent metal and consequently turns a different color. This color difference is minimized in the polished state or if the cap rail is given a dark patina. However, in the semi-tarnished state the cap rail and the welded area do discolor differently. Because a perfect weld is essentially not possible, some craftsmen join the cap rail sections using silicon bronze, at 95 percent plus copper, as the filler metal. This produces strong welds that do not look significantly different from welds made with the parent metal as the filler material.

     

  • M.I.G. Welding
    This is one of the easier techniques used to weld cap rail. A standard M.I.G. machine and silicon bronze welding wire are used with argon gas. This produces fast welds which polish out well and eliminates the need for the more specialized and expensive T.I.G. machine which some shops may not have.

     

  • Silver Soldering
    This commonly used technique leaves a very fine silver line between the spliced sections, which is least visible when the cap rail is polished. However, if the cap rail is finished with a patina the silver tarnishes and can leave objectionable stains on the bronze.

     

  • Brazing
    Brazing is a time honored technique for joining bronze cap rail. It is accomplished using a torch, flux, and filler rod, and is certainly an acceptable way to form a joint.

     

  • No Welding
    I have seen some older projects done for historic properties where the bronze bars and components are not welded at all. Generally, these types of joints occurred at direction changes where a natural intersection existed. With the modern welding technologies readily available in today's industry, this approach is not a contemporary industry standard but it has been done and accepted in some situations.


Finishing

Polishing of the cap rail material is best done after the forming process is completed. Some craftsmen have the cap rail pre-polished by a local polishing shop before it is formed. This works best when the cap rail is formed cold since the hot forming process tends to damage the surface and requires re-polishing of the cap. Other than the standard polishing rouges, one good polishing technique involves using a heavy duty rubbing compound, available at most automotive paint suppliers, and buffing the bronze with cotton buffing wheels on a hand-held high speed electric drill.


Statuary Bronze Patina Finish

This special finish is occasionally specified on architectural plans for significant metalwork projects, and is frequently requested on historic and antique pieces, and on high-end designer items. It requires some practice and experience, but is a worthwhile finish to be skilled in as it achieves an elegant and timeless look of high quality. A statuary bronze patina occurs naturally over a period of time, depending on atmospheric conditions affecting the metalwork. An applied patina finish which resembles the natural oxidation process can be achieved using selenious acid. This is the active ingredient in JAX Brass/Bronze chemical darkener which is commercially available. Once the bronze is thoroughly cleaned and all waxes and oils removed, this chemical is applied to the bronze to accelerate the patina process and quickly achieve a traditional statuary bronze appearance.


Conclusion

The many techniques discussed in this article may help you when considering how to approach the varied and unusual custom railings often requested. I have seen some craftsmen achieve "impossible" bends by forming bronze in highly creative ways. Perhaps the most unusual technique I have seen was while I was in France two years ago. The railing was done by L.M.C. Corp. using Julius Blum cap rail #4538 [Figure 3].

Their artist/craftsmen created compound curve twists and their own lateral scrolls by ripping the stock material down the center, bending the left half and the right half individually to the inside and outside radiuses, and then re-welding the two pieces down the center.

Their exceptional creativity and craftsmanship produced a magnificent bronze railing system for a Paris hotel that appeared virtually impossible to have formed.

[IMAGE OF BRONZE CAP RAIL]

Good luck as you develop your own techniques with bronze cap rail.


Mr. Klahm is chairperson of NOMMA's education committee. The Education Committee plans to hold a "follow up" on the physical demonstration of bending cap rail that took place at the Lexington convention by presenting a two-hour round table classroom discussion at the Chicago convention in March.

Reprinted from Ornamental & Miscellaneous Metal Fabricator, July/August 1993, p. 24


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Last Updated: Tue., Oct 21, 1996