PRESERVATION TECH NOTES     HISTORIC GLASS NUMBER 2

Repair Work

	Figure 5. Workers removed any remaining corrosion, grout, and other contaminants with hand chisels and power-operated wire brushes.
	Figure 6. Cracked panels were repaired by welding in new material from a stock of salvaged panels. The welded seams were then ground down before repainting.
	Figure 7. Repaired panels were painted with two coats of oil-based metal primer and three alkyd gloss enamel top coats.
	Figure 8. Before the glass lenses were set, a ring of silicone was placed along each flange on the lower edge of the opening. The silicone kept the lens centered in the cavity while a waterproof grout was used as the main sealer.
	Figure 9. A caulk gun helped workers apply the waterproof epoxy grout around the lenses.
	Figure 10. The grout remained water-soluble until it hardened, enabling workers to remove excess material and smooth the joint between the lens and cast-iron panel with a sponge.

The first step in repairing the vault lights at 552-554 Broadway was to dismantle and remove the panels from the sidewalk. Over two days, a crew of five experienced workers drilled the heads off of the panel bolts and gently pushed them through the holes in the cast-iron framing. After the bolts were removed, one corner was loosened by inserting a chisel between the saddle and the panel. Other chisels were worked around the other sides of the panel. Then, two workers raised the panels, each weighing approximately three-hundred and fifty pounds. Each panel had to be lifted straight up out of its frame because any uneven pressure could have cracked the cast-iron. A tag with an identification number keyed to a drawing was secured to each panel, to ensure reinstallation into the same opening after repair. The twenty-one panels were packed upright into the back of a truck and delivered to the repair shop.

The sidewalk at 552-554 Broadway had to remain accessible to pedestrians while the vault lights were restored offsite, so a 3/4” thick plywood sheet topped with 3/16” steel diamond plate was placed over the openings. The temporary covers were tack welded into the corners of each hole and the seams were sealed with a flexible polyurethane-based caulk.

When the vault lights arrived at the repair shop they were placed upside down and the surviving glass lenses were knocked out by hand. A variety of materials, used in the past to fill holes where lenses were broken or missing, were also removed. Next, each panel was sandblasted. This cleared off the paint, chewing gum, tar, asphalt, concrete, corrosion, grout and other grime that accumulates on a New York City sidewalk over the course of a century, without pitting or otherwise damaging the cast-iron. Then, workers hand cleaned each of the panels with small chisels and drills fitted with wire brush attachments to remove any material missed during sandblasting (see figure 5).

At the outset of the restoration program, six of the twenty-one vault light panels were found to be cracked. Rather than replace them with newly cast panels, the contractor chose to repair the damaged units using two different techniques: welding cracked sections and splicing in replacement pieces. Due to inconsistent heating and the presence of impurities in the blast furnace mixture, historic cast-iron is a notoriously brittle material. This brittleness makes historic cast-iron susceptible to cracking and a challenge to properly weld.

The contractor had developed a system for repairing cracked or damaged vault covers. Primary considerations were to ensure that the pieces to be welded were correctly positioned and level and that the cast-iron was properly preheated. Then, using a wire-feed MIG (metal inert gas) welder, the two pieces were joined together (see figure 6). The welded seam was ground down to the level of the adjacent panel surface so that the repair would not be visible after painting. The result was a strong, durable and complete cast-iron panel. When replacement pieces were required, the contractor drew from a small stockpile of salvaged panels identical to those at 552-554 Broadway.

When repairs to the cast-iron were completed, two coats of primer were applied, followed by three coats of gloss black paint to match the existing paint color (see figure 7). The contractor used a Benjamin Moore system including an “ironclad” oil-based metal primer and alkyd gloss enamel (Impervo) top coat that offered high abrasion resistance. A rust conversion coating system would also be suitable. Painting was followed by the resetting of new glass lenses.

After ensuring that the panels were dry and free of oils, dirt, dust and other contaminants, a bead of silicone was applied at the bottom of each of the round openings in the cast-iron panel (see figure 8). This preliminary seal would keep the glass centered during the grout application. Replacement lenses were obtained from a specialty glass manufacturer that has supplied new lenses for a number of vault light rehabilitation projects. The glass lenses were set into place and a two-part epoxy grout was used to seal them in the panel and waterproof the joint (see figure 9). The gray, chemically-resistant material was the same used for grouting floor brick, quarry tile pavers and ceramic mosaics. Workers mixed the compound, placed it in a caulk gun and squeezed it into the areas adjacent to the lenses. Before it hardened, excess grout was removed using a dampened sponge. After twenty-four hours, the grout cured sufficiently to complete the reglazing process (see figure 10). The panels were turned over and workers used a razor blade to cut away excess silicone that had dripped out of the space between the lens and the cast-iron along the underside of the panel. Then, the panels were shipped back to New York City.

Replacement Panels
Because vault lights are no longer manufactured, new custom panels had to be fabricated to replace the two missing when the project began. The contractor chose a surviving panel that was in good condition and sent it to a foundry in Alabama for use as a pattern. The foundry, which had worked on a number of vault light projects in the past, cast two new panels using techniques that had changed little in the past century. The original sample was packed in a bed of casting sand to make a three-dimensional mold. To account for shrinkage in the casting process (1/8” per foot), the mold was made slightly larger than the pattern ensuring that all panels, original and new, would be the same size. Vents and gates were created in the mold to allow the flow of molten iron and gas.

Although the method of casting new panels was nearly identical to the historical process, the composition of the cast-iron was altered to improve the material’s tensile strength. To produce what is referred to as ductile iron, the iron was combined with a slightly higher percentage of carbon (+/- 3.75% in the new cast-iron versus +/- 3.5% in the historical). Very small amounts of magnesium and graphite were also added to produce a material two to four times stronger than that of historical cast-iron. Following fabrication, the new panels were shipped to the contractor who set the new glass lenses and transported them to New York City for installation alongside the repaired panels.

Reinstallation

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