Large-Size Porcelain Slabs for Building Surfaces

Interiors and exteriors finished with half-inch slabs can create lightweight and beautiful results
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Sponsored by Walker Zanger
By Peter J. Arsenault, FAIA, NCARB, LEED AP

Manufacturing Process

To get a better understanding of the nature of porcelain products, let’s take a look at the basic life cycle of the product, starting with the extraction process. The basic raw materials are mined from the earth and can include clay, silica, kaolin, sand, and feldspar. These are transported to the manufacturing facility, sorted, and stored until they are needed for production. In addition, glazing raw materials can include things like feldspar, clay, frit, and coloring agents that are also sorted and stored. Some of the coloring agents can contain precious metals, which makes them more costly but also less commonly used.

In a typical production process, the raw materials needed for a specific porcelain product are selected and mixed along with 30 percent water. This usually occurs in a ball mill machine, which turns the mixture into a homogenous slip. That slip is dried into prills (spheres) in a spray drier that reduces the moisture content down to 6 percent or so. At this point, the material is stiff but pliable and ready for pressing. Essentially, pressing involves placing the prepared prills into a pressing machine and exerting an immense, uniform load to create thin, flat tiles or slabs. There are several different press types based on different manufacturing processes and brands, but they all rely on pressures on the order of 7,000 pounds of force per square inch or total pressures up to 15,000 tons depending on the size and configuration of the pressing machine. It is the advanced development of these large presses that has made the large-size porcelain slabs possible. Such presses are among the largest in the world and necessary in order to press the prills to be large enough and dense enough to produce the needed characteristics for the large porcelain slabs.

Once pressed, the porcelain mixture is ready for further drying and finishing. The details of these steps will vary based on the type of product being produced and the type of finish sought. For example, smooth surface products will rely on smooth-faced pressing and processes, while molds may be used during or after the pressing to create texture in the surface of the porcelain. Additional drying may also occur to bring the pressed slab to the preferred moisture content for a particular finish. If the slab is to have a glossy colored surface finish, then glazing may be added if appropriate. In other cases, the glaze may be omitted and the color of the material itself be retained or a pattern within the material can be created to replicate stone, wood, etc. Once these details are finalized and carried out, then the slabs are ready for firing in a large kiln that can reach temperatures up to 2,200 degrees Fahrenheit. The time required for firing will vary based on slab thickness, but the goal is to achieve a 4 percent moisture content at the end of the firing. The porcelain is now hardened and, once cooled, achieves its final characteristics of strength, durability, and basic finish. Any final finish enhancements can be made at this point, such as cleaning, polishing, or edge cutting (rectifying) to final precise sizes.

The finished large porcelain slabs need to be transported to the building site and handled properly both in the factory and on-site to protect both the finished slabs and the people working with them. As such, they are commonly handled using carefully placed suction cup lifting devices, much the same way large glass panes are carried and transported. Then they are set into protective crates and made ready for shipment. Once received on-site or in a fabrication ship, the crates need to be carefully opened and the large slabs lifted out again with suction cup based carriers. This means that experienced tradesmen are needed to handle, and prepare the slabs for final installation.

With an understanding of the material, we can now look at some of the applications, both interior and exterior, where large porcelain slabs can become an integral part of a building design.

Photo of porcelain tile slabs are handled and shipped

Large porcelain tile slabs are handled and shipped from the factory to the site within much the same way stone slabs or glass sheets are handled.

Interior Building Applications

Large half-inch-thick porcelain slabs can basically be used in interiors anywhere other ceramic or porcelain products are used. That includes traditional applications, such as new or existing rooms for walls and floors, as well as specialty treatments in kitchens and bathrooms, such as backsplashes and countertops. However, it is also durable enough that it can be used in commercial spaces, such as public circulation corridors, elevator cabs, and stairwells.

From a design standpoint, large-format porcelain slabs offer a variety of looks that are not limited to traditional tile appearances. In particular, they can be patterned to look like white or black marble, other stone, or even wood grain in a choice of patterns. The large size of the slabs allows coverage of floors or walls with minimal joint lines, replicating again the look of natural stone or other large slab materials. Of course, the large slabs can be combined with smaller tiles, planks, or other porcelain products to create a varied appearance that is coordinated in style and uniform in performance.

The edge condition of the slabs can be selected based on the final appearance and performance as well. Common edges are square and quite acceptable for most installations with common joint widths. For installations that seek the appearance of minimal joint lines and tight tolerances, then precision-cut (i.e., rectified) panels can be specified to be created in the factory. This creates very thin grout lines, giving the porcelain the appearance and control of solid stone slabs. For countertops in particular, porcelain slabs can be preferable since they overcome some of the maintenance requirements of some natural stone. For example, recommendations for stone slabs include avoiding the use of certain abrasive cleansers or cleaning pads. In addition, acidic materials, such as lemon juice and vinegar, can etch some stone surfaces, especially limestones with low porosity. And because normal wear and tear can affect even the most durable stone, some scratching and chipping may occur over time. Porcelain slabs can overcome many of these issues if denser and less porous products are selected.

Photo of countertops in kitchen

Large-size porcelain slabs are a preferred material for countertops since they overcome some of the limitations of natural stone for this use.

When reviewing porcelain products for commercial or residential projects, it is important to recognize that different products will be rated for different levels of durability. A common rating breakdown follows:

  • Light duty: These products are intended for use on interior vertical surfaces only. This may include commercial spaces, depending on the intended use of the space. Light-duty tile may have special cleaning or care requirements.
  • Medium duty: These products are intended for use on interior vertical and some horizontal surfaces in most residential and some commercial applications. This includes bathroom walls, showers walls and floors, vanity tops, wainscoting, kitchen backsplashes, as well as light use kitchen counters. Commercial applications may be limited to interior vertical applications.
  • Heavy duty: These products are appropriate for use in all residential applications and many commercial applications, including all medium-duty applications as well as all kitchen counters, residential flooring, pools, and exterior vertical surfaces, but only areas that do not experience freeze/thaw conditions. Commercial uses include most wall and floor applications, with the exception of heavy-traffic flooring.
  • Commercial duty: These products are manufactured to the highest standards of durability, and are recommended for all interior and exterior residential and commercial applications, including heavy pedestrian traffic flooring. Recommended uses include heavy-traffic residential areas, commercial lobbies, and other areas that require a tile with maximum durability.

When considering heavy-duty or commercial-duty porcelain for flooring, there is also a safety concern that needs to be addressed, namely, slip resistance. This is specifically addressed in ANSI A137.1-2012, which defines an updated method for assessing and rating tile surfaces (including large porcelain slabs). Previously, ASTM C-1028 was the standard test method for determining the nonmoving or static coefficient of friction (SCOF) of ceramic tile and other like surfaces. Static coefficient of friction is a term used in physics to describe the amount of force required to cause an object in contact with a surface (e.g., shoe sole on tile floor) to start moving across that surface. A higher coefficient indicates increased resistance of shoe sole material to start moving across a flooring material. While this produces a measured value, no standards have a specifically determined ‘safe’ SCOF value.

Due to the difficulty in measuring SCOF and lack of a specific recommendation, ANSI A137.1-2012 changed the testing requirement to a moving or dynamic coefficient of friction (DCOF). DCOF differs from SCOF in that the shoe material is made to move across the flooring surface and the resistance to movement is constantly recorded and averaged. This test uses an automated device (i.e., the BOT 3000 recommended) instead of the human hand, which reduces the variation in the test method from 30 percent in the SCOF measurement to less than 10 percent for DCOF. As such, ANSI 137.1 now includes a method for measuring (DCOF) with a recommended minimum value of 0.42 for interior, level floors that are likely to be walked upon when wet. Specifically, it states, “Unless otherwise specified, tiles suitable for level1 interior spaces expected to be walked upon when wet shall have a wet DCOF of 0.42 or greater when tested using SLS solution as per the procedure in Section 9.6.1.” With all of this in mind, most manufacturers have their porcelain floor products independently tested using the procedures and guidelines in this standard and report the results in product literature with the goal of achieving the DCOF of 0.42 or greater.

Keep in mind that when selecting a tile for slip resistance, however, the ANSI standard is very clear to state, “Because many variables affect the risk of a slip occurring, the COF shall not be the only factor in determining the appropriateness of a tile for a particular application.” Therefore, it also makes clear that “tiles with a DCOF of 0.42 or greater are not necessarily suitable for all projects. The specifier shall determine tiles appropriate for specific project conditions, considering by way of example, but not in limitation, type of use, traffic, expected contaminants, expected maintenance, expected wear, and manufacturers’ guidelines and recommendations.” This professional judgement and prudent assessment is true for all flooring, and in this case, applies equally to porcelain slabs as well.

Photo of porcelain bathroom

Porcelain tile used for flooring needs to be tested for slip resistance based on the requirements of ANSI A137.1-2012 and is particularly important in wet areas.

Slip resistance is important on flooring but also important in places that are expected to be wet, such as bathrooms and shower areas. Those areas will need some special attention when porcelain is used on the floor. In addition to the surface, all shower floors need to be detailed to include appropriate waterproofing in the event that any seams or joints seep water over time. The general industry recommendation is for shower floor substrates and adjacent walls around the perimeter (12 inches up) to include a waterproof membrane on the top surface of the fully cured mortar bed prior to tiling. No allowances or claims are normally accepted by manufacturers or suppliers of porcelain, or other shower flooring materials for that matter, for damage to products installed in shower flooring that do not have a waterproof membrane system installed.


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Originally published in Architectural Record