There is a wide range of glass tiles available for installations: thick, thin, crystal clear, bubbled, translucent, prismatic, colored, slumped, cast, sandwiched, layered, impressed, blown, hand and machine-made, tempered and not. The surface of glass tiles can be altered with luster treatments, overglazes, slumping, and other artistic techniques. Like any other finishing material used on a hard tile installation, the durability of a particular glass tile should, at least, be matched for the demands of the installation (Because the properties of glass tiles and it artistic treatments vary widely, this information should come from the manufacturer).
Color is sometimes applied to the backs of glass tiles, in the form of paint, epoxy, or other applied coating, to (obviously) put some color into the tiles, and (not so obviously) to mask the appearance of un-flattened adhesive ridges, voids in the adhesive, adhesive swirls, and other imperfections that might otherwise be clearly visible through the body of a glass tile. Coated tile backs sometimes call for a specific adhesive but all glass tiles used in wet areas require 95 percent uniform adhesive coverage between the backs of the tiles and the setting bed.
Applications of paint or epoxy, plus the impervious nature of most glass tiles, can even make bonding uncoated tiles difficult. Because of this, the backs of some tiles are made with ridges or grooves designed to increase the back's surface area (See Illustration 1).
Some are impressed or cast with one or more manufacturer's marks or designs, and these features also increase the overall bonding area. The backs of glass tiles are sometimes lightly-and evenly-sandblasted to increase the adhesive bond and to obscure light transmission from the adhesive layer through the body of the tile.
Unlike the opaque bodies of ceramic and stone tiles, a glass tile readily shows up all its internal flaws. The sharp, uneven edges of a snap-cut glass tile are immediately apparent and almost seem illuminated through the glass body of the tile. The appearance of a cut glass edge can be minimized by straightening, righting, and frosting with a wet belt sander, but even with careful finishing, the look of a non-factory edge may be hard to mask. Installers who try burying a cut edge soon discover that methods limitations: the best approach is to design the installation so that no cut pieces are required. An alternative is to use whole glass tile units, only, and use a complementary ceramic tile, of comparable thickness, to frame the rest of the space (Ceramic trim tiles may also have to be used if the design calls for trim not available in glass).
Because the thickness of the glass glazing on the surface of a typical ceramic tile may only be a few thousandths of an inch thick, and the bulk of the tile is comprised of much softer bisque, cutting on a snap cutter or wet saw is usually a simple task.
Snap cutting thick or thin glass tiles, on the other hand, requires a gently touch and the right equipment, but even under the best conditions, the cut edge may not be as smooth as the cut edges of ceramic or stone tiles. When score/snapping sheet glass, keeping the axle of the scoring wheel in plane with the surface of the glass results in edges that are square-shouldered. When the axle tips to one side or the other, the glass edge is likely to flare in or out accordingly. The internal structure of small cast or pressed pieces of glass is highly irregular and quite different from plate or sheet glass whose cross-sections are uniform and continuous. The result is that erratic edges are likely and to be expected (See Illustration 2).
Rough edges can be contoured with a dry belt sander, but the finish of the resulting edge will be coarse and rough to the eye and to the touch. For this reason, glass fabricators use wet grinders to produce a smooth and more finished edge. Snap or wet-saw cutting of glass tiles with painted backs is likely to produce spalling of the paint and erratic cut edges. Such flaws are very visible through the clear glass body of the tile, and are a major source of consumer complaint. If cutting is inevitable, the installer should square up the cut edge (See Illustration 3) using a belt sander or other abrasive tool, and apply repair paint or coating to any cut tiles whose edge is not sufficiently buried to obscure the defect.
If a cut has to be made, square the edge, and then bury it beneath cove base or other overhanging trim (or hide it a neighboring wall tile on a wall installation), so that the sharp reflection from the cut glass is partially covered. When the reflective cut edge is visible on an all-glass tile, roughing the edge with carbide paper will frost the glass and make it non-reflective. If the glass tile back is painted, any spalled edges that result from cutting-and that are visible-should be ground smooth, coated with a touchup paint or coating, and allowed to dry before installing. I recommend that glass tiles not be cut, but rather, used whole.
Glass tiles require an adhesive with a superior grip, and extra-special care to eliminate air pockets or voids beneath the tiles that are visible through the tile body. The color of the adhesive also has a significant bearing on the appearance of the installation: some art & decorative installations are specified with a colored epoxy adhesive and matching grout to impart a muted color treatment. Gray or natural cement-colored thinset or epoxy adhesive will dull the installation while white thinset or epoxy will help brighten the tiles and the installation. To ensure that the appearance of the thinset (as viewed through the body of the tile) is uniform, glass tiles may be back-buttered with a thin film of adhesive, but this practice may mask voids below the tiles.
To ensure maximum adhesion, I select a non-latex grout powder, and mix it with a latex thinset additive, to produce an adhesive I can use for spreading and back-buttering. My preference is to use Laticrete 4237 liquid in place of water, and to mix it with a regular (non-latex or polymer) grout powder (as the thinset base). Since the color of the adhesive is the same as the color of the grout joint filler, bleed-through-a common installation hassle with glass mosaics-is less of a problem.
For installing sheets of glass mosaic, with individual tiles of uniform thickness, no greater than 1-inch, and face-mounted with a water-soluble paper, I use a 3/16 or 1/4-inch V-notch trowel to spread thinset on the setting bed, and I use the smooth edge of the same trowel to hard-trowel a thin layer of adhesive on the backs of each tile in a sheet just before setting. As each mosaic sheet is bedded in thinset mortar, a beating block and hammer is used to gently align neighboring faces (See Illustration 4). It is essential to remove unwanted material before moving on to the next sheet, and to focus on spreading uniform layers of mortar-this is done by holding the trowel at a consistent angle to the floor, and combing away all the excess. I let the sheets firm up in the bed of thinset for about 20 minutes before very gently blotting the face paper with a wet sponge to soften its adhesive, and easily let go of the tiles. I apply moisture with the sponge, as needed, so that the paper stays wet continuously. After about 5 minutes, the sheets should easily peel away, allowing me to do any last minute tile positioning.
Movement joints should replace grout at all panel margins to prevent normal building movement from cracking the tiles or shearing them off the setting bed. All adhesive and grout residues should be removed from these joints. Note: when working with latex grout, the movement joints should be filled after grouting; if an epoxy grout is specified, fill the movement joints with a flexible sealant before grouting. With 24 lineal feet of joint to fill, a 12-inch sheet of 1-inch tiles demands more work than is required for a single 12-inch tile. I let mosaic sheets harden off at least 48-hours before grouting with a latex grout. Although my preference is for latex thinset and grout, a few glass tile manufacturers may specify other materials. Always consult the tile manufacturer for specific recommendations or an installation spec.