Mosaic art has been a part of human culture for eight thousand years. In fact, much of what we know about our ancient history has been gleaned from the remnants of mosaic installations that persist to this day. When viewing an ancient mosaic, with colors and textures still fresh and bright, it is easy to think that the form is eternal, but in reality, producing durable, functional mosaics requires careful attention to detail because the bonding surfaces are so small and the forces acting against the mosaic bits can sometimes be quite large. Size is definitely an issue with mosaics.

The US tile industry considers a mosaic to be any tile 2 inches or less on the side. Most commodity mosaics installed in the United States are sheet-mounted porcelain tiles, but sheet-mounted mosaics are also available as stoneware and non-vitreous ceramics, composite and natural stone, pressed or cast metal, glass, and other materials. Most mosaics are mounted on sheets for simpler, more efficient installations.

Traditionally, mosaic bits were mounted on cloth or paper sheets with water-soluble glues that allowed for removal of the mounting once the mosaic bits were set in adhesive. Today, mosaic compositions are still face-mounted, but for extra convenience, dot or back mountings, which remain in place after the sheets are installed, are also used. Some art and decorative mosaics are face-mounted with clear plastic sheets that do not obscure the mosaic bits from the installer.

Selecting Mosaic Tile

Practically any material is suitable for making decorative, non-functional mosaics installed on dry, interior walls. For mosaics in functional or wet-area applications, though, the selection process is critical. Because point loads are distributed over a bigger area, large tiles have a distinct advantage over small mosaic bits. When selecting a mosaic for a floor, for example, it is important to know what kind and how much traffic the floor will see. In a master bathroom, were mostly soft footwear is worn, and most of the grit and dirt tracked into a home is stopped at entry-door rugs or mats, compressive strength may not be so important, but when they are used to clad the entries of shops, or floors open to the public, the mosaics you select must have a high compressive strength as well as plenty of abrasion and moisture resistance. Get an assurance, in writing, that the tile you are installing is appropriate for the demands of the installation.

Preparing the Surface

Adhesive ridges that pose no problem with large size tiles can cause cleaning headaches for mosaic tiles. Flattening gauged ridges of thinset before installing the mosaic sheets can effectively eliminate this problem.

Mosaic sheets need the same preparation as any other ceramic tile: the floor must be smooth and flat to within a quarter inch in ten feet. As long as the surface of the floor is within spec, large-size tiles do a great job of coving up the usual underlayment ripples, but the small size of mosaic bits—because they cannot bridge over imperfections—can sometimes result in a surface that is quite bumpy and unsmooth. For this reason, whenever mosaics are to be installed, it is a good idea to specify a smoother underlayment surface: an eighth of an inch in ten feet, with no imperfections greater than one-sixteenth inch in twelve inches, is about right for mosaic sheets made up of bits of equal thickness.

Surface preparation is another area where size is very important, and because the mosaics are so small, it is important that every square inch of the setting bed be clean with an open, coarse texture that is free of any contaminant, coating, or curing compound. Mortar beds, self-leveling and featheredge compounds made for tiling, and tile backer boards are ideal for bonding with thinset mortar provided their surfaces have not been contaminated with dirt, dust or other jobsite debris. The importance of keeping the setting bed clean—important for any tile installation—is absolutely critical to the performance of all mosaic floor installations because of the relatively small bonding footprint of the mosaic bits. For best bonding, hold off installing the setting bed until the last minute, and mask off doorways to prevent construction dust from other parts of the building from settling on the mosaic installation site: A setting bed whose surface pores are not allowed to become clogged with dust does not need to be cleaned, and a virgin surface offers the highest bond strength possible. The surface of any setting bed that has been wet- or dry-cleaned only reclaims a fraction of its originally anticipated bond strength.

Installing Mosaic Sheets

In addition to feeding mold when installed in a wet environment, this paper back mounting significantly interferes with the adhesive bond. Face mounted mosaic sheets are recommended for the best possible adhesive bond.

To underscore the point, conditions or voids, that would not even be a consideration under 12-inch tiles, become major issues when tiling with mosaics. Mosaics require a more carefully prepared surface as well as more time spent per square foot to install sheets of mosaic bits; consequently, installers need to charge more per square foot depending on the complexity of the installation and the configuration of the mosaic mount  (face-mounted opaque paper sheets, face-mounted clear plastic film, side-mount dots, etc.). Installers and specifiers also need to account for special work techniques and increased labor and materials to grout and finish mosaic tile compositions carefully and according to spec.

In spite of the increased labor involved, there are ways to speed and improve any mosaic installation. One of the best practices to adopt when working with complicated mosaic installations is to layout all the mosaic sheets required for the install, and dry fit and measure the composition. Sometimes this is best done away from the installation site, but whenever possible, I prefer to lay out the sheets exactly where they will be installed. For wall installations, I plot lines, to indicate wall areas, on open sections of subflooring. Dry-fitting, especially when installing irregularly shaped or sized sheets, is the best way to ensure accuracy of location for each sheet and each mosaic bit, and a few minutes of dry fitting can replace hours of indecision or re-installing.

Spreading Adhesive

To reduce or eliminate mosaic grout joints clogged with thinset, the usual two-step spreading process is turned into three: (1) Hard-trowel the initial coat onto the setting bed, (2) gauge a uniform amount of adhesive with the notched edge of the trowel, and (3) gently flatten the ridges with the smooth edge of the trowel.

One of the most frustrating aspects of installing mosaic sheets is keeping adhesive out of the grout joints. On very large installations, Japanese mosaic manufacturers mount square chips into square-foot sized sheets of thin plastic that are vacuum-formed over individual tiles, that also serve as a barrier to ensure that grout joints would be properly voided, and that are nor removed until the tile adhesive has cured.

Vacuum face-mounting of tiles is not in widespread use in the United States. A more practical approach for an installer dealing with residential or small light commercial projects is to adjust the way thinset mortar is spread. First, select whatever thinset is more appropriate: natural gray thinset for opaque, non-white tiling materials and natural cement colored grouts, or white thinset for transparent or translucent mosaic materials. The thinset should be stored and mixed as any other thinset, allowed to slake, and re-mixed to eliminate any lumps or dry material.

For most mosaic chips, I begin the spreading process with a 1/4-inch V-notch, a 1/4-inch square-notch, or whatever trowel will provide 95% overall coverage for each mosaic tile. With some art & decorative mosaics, a larger notch size, along with some back-buttering, may be required. The first step, to lock the thinset into the setting bed, is to hard-trowel a layer of thinset onto the setting bed using the smooth edges of the trowel. Next, add more thinset, distribute the additional material over the floor, and with the teeth of the trowel held against the setting bed at a consistent angle, and using long, straight strokes, remove all excess thinset. For ordinary tiles the spreading process ends, but with mosaic tiles, there is one more step.

It takes a bit of practice and careful workmanship, but to reduce or eliminate time spent on cleaning out the grout joints, the final spreading step is to flatten the consistent layer of thinset, gauged out by the notches, with the smooth side of the notched trowel. I hold off using a beating block and hammer until I can work on a field containing at least 5 or 6 mosaics sheets. With care, any small amounts of thinset that intrude into the grout joints can usually be cut away after the adhesive has hardened: To maintain the smoothest surface, I avoid touching the tiles once the sheets are positioned and beat in.

Managing how the adhesive is spread is one of the keys to a great finish and frustration-proof workspace.