Restoring the Minton Tile Ceiling

Bethesda Terrace Arcade,
Central Park, New York City

By Mark Rabinowitz and Peter Champe
(Originally published in the APT Bulletin, Volume XXX-2/3, 1999)

Introduction
Until it was taken down due to structural instability in 1984, Central Park’s Bethesda Terrace at 72nd Street housed an artistic treasure – a unique encaustic ceramic tile ceiling. The tile ceiling was designed by Jacob Wrey Mould in the mid-1860s and manufactured by the Minton Company of Stoke-on-Trent, England. An examination of the condition of the panels during the restoration of the Terrace in the early 1980s revealed that after 120 years of useful service, the iron suspension structure had severely corroded due to water and salt infiltration, resulting in a hazardous condition due to the weakened structure. The 49 panels that made up the ceiling were removed in 1984 and put into storage where they still remain. In 1996, the Central Park Conservancy (the private not-for-profit organization which administers Central Park) received grants from the Getty Foundation and the National Endowment for the Arts to develop a restoration plan for the tile ceiling and to restore two panels as prototypes. We successfully restored and reinstalled two panels and are now able to develop a scope and a budget for the restoration of the remaining 47 panels. At the time of this writing, the Central Park Conservancy is raising funds to complete this major architectural restoration project. This article will focus on the technical issues involved in the restoration of the Minton Tile Ceiling.

The Minton Tile Ceiling: History and Technology
Encaustic ceramic tile was developed in the 12th Century by Cistercian monks for the floors of churches and cathedrals and consists of inlaying colored clays into the body of the tile to create the design layer. The skill was lost in the 16th Century with the Reformation but revived in England in the 1840s by Herbert Minton. It is unusual for encaustic tile to be found on a ceiling as its traditional purpose is for walking surfaces. The rationale for the use of this more expensive tile on the ceiling might be found in the fact that the floor of the arcade was originally covered with encaustic tile and the designers evidently found it convenient to the use the same type of tile on both surfaces. Unfortunately, the encaustic floor tiles were taken up and discarded sometime in the early 20th Century, presumably due to their poor condition and the lack of available resources or will to maintain them.

The tile ceiling is made up of 15,876 tiles divided among 49 panels, each panel measuring eight and one-half-feet square and weighing approximately one ton. The design, which borrows from Islamic tile work in its use of repeated abstract floral motifs, is composed of 7 colors: buff, brown, dark rust-red, chrome green, ochre, white and cobalt blue. There are two panel designs that differ only in their central medallion – 25 panels bear a small central medallion and 24 panels bear a large one. Each panel consists of 324 tiles divided into three sizes: (4) 3” x 3” tiles define the corners, (64) 3” x 6” tiles make up the edges and (256) 6” x 6” tiles form the center. Each 1” thick tile is mechanically attached by a bronze dovetail bolt which slots in the back of the tile and secures to the iron frame with a nut. The dovetail slot in the tile was probably made by inserting a piece of wood in the wet clay, which would have then burned out in the firing process.

Condition
After a century of water infiltration through the Arcade ceiling, the iron structural elements of the panels were severely corroded. The corrosion/expansion of the iron frame exerted enormous pressure, effectively pulling the bolts out of the backs of the tiles. A random sampling of the condition of the tiles revealed that the structural integrity of the point of attachment of the bronze bolt to the ceramic body was compromised. This damage precludes the reuse of the bronze bolts as an attachment system. Despite this condition, the front of the tiles are generally in good to excellent condition.

The Current Restoration
In 1996, the Central Park Conservancy received grants from the National Endowment for the Arts and the Getty Foundation to develop a method for restoring the tiles. In the late 1980s, the Central Park Conservancy commissioned a feasibility study from the architectural consulting firm Ehrenkrantz, Ekstut and Whitelaw to develop a method for restoring the panels. The resulting recommendations included dismantling the 49 panels into their constituent 15,876 tiles, cleaning the residual mortar from each tile and reassembling them into panels. The budget that developed out of this scope far exceeded the financial resources of the Conservancy due to the labor intensity of the approach, and as a result, the project faltered. As we currently approach the project, we have decided to challenge previous assumptions, such as the necessity of dismantling the panels in order to restore them. What has allowed us to consider this approach is that the tiles are generally in good condition – it is only the iron support elements that require removal and replacement with a more durable material. We also noted that the process of dismantling the panels increases damage to the tiles. Therefore, maintaining the tiles relationally intact throughout the restoration not only reduces the overall cost of the project, but reduces potential damage to the tiles as well. Our restoration philosophy is summed up as follows:

Restoration Criteria
1) Satisfy long-term public safety requirements with the application of sound engineering principles.
2) Employ corrosion resistant materials.
3) Retain as many of the original tiles as possible.
4) Retain the tiles relationally intact throughout the restoration.

Restoration Method
Key to the approach of maintaining the tiles relationally intact is a platform on which to work and which prevents the tiles from shifting during the restoration. The platform which we custom-designed for this project incorporates the following features: Transparent Lexan surface to allow us to see the tile faces during restoration. Extruded aluminum tracks on which to mount a moveable tile saw. Set screws to prevent the tiles from shifting after the iron structural frame has been removed.

To begin the restoration, the panel is placed face down on the platform and the iron frame is removed from around the tiles. After the tiles are laid bare, they are secured in place by lateral pressure applied by set screws to four temporary side bars – one on each side. The original mortar setting bed is removed using pneumatic chisels and the bronze bolts are cut flush to the tile surface using a carbide cutting wheel. Any tiles that have been previously identified as unsalvageable are removed and replaced with new tiles at this point. This process is aided by the transparent Lexan surface of the platform, which allows us to view the front of the panel. As the tiles bear asymmetrical designs, it is crucial to have a visual confirmation of proper orientation in placing the new tiles.

Stainless Steel Tile Clips
In keeping with our first restoration criterion, to assure long-term public safety, we required a positive mechanical attachment method for the tiles. We considered many methods of mechanical attachment, but one overriding obstacle was that due to the pressure exerted by the corrosion/expansion of the iron panels against the bronze bolts, the center portion of the tiles were structurally unsound. Our solution to this situation was to span the area of damage using a clip. The clip, which we custom-designed for the application was made of 14-gauge stainless steel (type 316). The feet of the clips are bent at a 15° angle to insert into plunge-cut slots also of that angle (see diagram). The two sides of the clip are secured with a nut to hold the tile by a ‘wedging’ action. Pullout tests were carried out to determine the effectiveness of this system. Our consulting structural engineer, Robert Silman Associates recommended that the pullout values should give a safety factor of 20: since each tile weighs 2 ½ pounds, the clip system should be able to withstand 50 psi of downward pressure before failure. In fact, pullout values were in the range of 500 psi and one tile withstood 950 psi before failure, exceeding the maximum recommendation by factor of almost 20. In all cases, the cause of failure was the fracture of the ceramic body. For these tests, we used original Minton tiles but only those tiles which bore no design and so were considered relatively expendable.

Slots in the tiles were cut using a tile saw with a 4” diamond blade mounted on an extruded aluminum track. The saw mount consisted of a screw spindle, which cranked down to plunge the saw to a pre-determined depth in the tile. By moving the saw along the track and using pre-measured calibrations, we were able to cut precisely spaced slots in the tiles for insertin of the clips.

After the 324 tile clips were installed, we filled the slots with a non-viscous epoxy – Sikadur 52. The function of the epoxy served less of a structural function (as the mechanical effectiveness of the clip system had been clearly demonstrated without the use of epoxy) Rather, its purpose was to fill any void in the slot to prevent water from getting in, freezing and breaking the tile. We then installed a mortar “setting bed”, more accurately, a reverse setting bed as it was poured over the tile backs. The material used was Sika 611 self-leveling mortar with the addition of Sikament 86, a super-plasticizer to aid in leveling.

The 7-gauge stainless steel back plate was fabricated off-site at Welding Works in Connecticut. Due to precisely placed plunge cuts and clip placement, we were able to assure a typical distance of 6” between threaded studs. However, the edge-stud distance is unique for every panel. We communicated the measurements of the edge-stud distance to Welding Works and they punched the 324 holes in the back plate. We were able to lower the back plate onto the panel and each of the 324 threaded studs inserted properly in each of the punched holes. Each threaded stud was then secured with a washer and a nut.

Tile Repair
In keeping with our third restoration criterion, we attempted to retain as many of the original tiles as possible by restoring their decorative surfaces. Those tiles which had suffered significant damage to their body or more than 25% damage to the design surface were considered non-salvageable and were replaced with new tile (to be discussed below). Approximately 20% or 3,000 of the original tiles will have to be replaced or repaired due to damage. We commissioned a study from Integrated Conservation Resources of an assessment of various repair materials for restoration. The two classes of materials we tested were vapor permeable materials employing cementicious mortars and non-vapor permeable epoxies. The accelerated weathering testing consisted of exposure to intense conditions of ultra violet light, condensation, heat, water spray and freeze-thaw conditions carried out over many cycles. While the testing program cannot simulate actual outdoor exposure conditions, it can allow a comparative evaluation of weathering characteristics between different materials.

The conclusion of our extensive testing was that the vapor permeable materials as represented by Jahn M-100 cementicious mortar with Keim potassium silicate paint performed equally well as the more durable and non-vapor permeable materials of epoxy putty Gapoxio with Tnemec two-part aliphatic polyurethane paint. While the basic conservation philosophy calls for the use of vapor permeable materials which are sacrificial to the original substrate, we had an overriding requirement which was public safety. We chose to use the material that possessed greater adhesion – the epoxy putty, in order to avoid the possibility of failure of the material a potential of falling. The material Gapoxio is well suited to working on ceramic tile to achieve the smooth texture – it can be smoothed while wet using water and a spatula to achieve a smooth surface. The Tnemec paint is high build, high gloss which successfully simulates the glossy glaze surface.

Tile Replacement
Those tiles which are lost, structurally damaged, or which have suffered damage to more than 25% of their design layer must be replaced. We are currently working with the Minton Company to develop replacement encaustic tiles. One of the obstacles to fabricating these tiles is that we are requiring that the thickness of the replacement tiles be identical to that of the original which is one inch. Tiles being manufactured today are in the range of ½ inch. The technical problems that they have encountered include shrinkage and cracking. They have also been working to achieve a proper color match.

Installation
The method of attachment of the panels into the ceiling is as simple and elegant as the rest of the original design. The 8 ½ square foot panels rest in the ceiling on a cast iron molding secured with bolts. In order to install the panels, the molding is removed from the cast iron decorative beams, the panels are lifted straight up into the space and the molding is reattached with bolts. In Clarence Cook’s 1869 description of Central Park he describes the process: “ The whole floor is laid with Minton’s tiles, and the ceiling is composed of richly gilded iron beams, enclosing large squares of colored tiles, this being the first time, we believe, that tiles have been used here for ceiling decoration. It was for a long time a problem how to fix them securely beyond the peradventure of a fall, perhaps upon some luckless pate. By a very ingenious, but very simple, device, the desired safety has been secured…”