A.Michael Byrne responds: On the surface, a tile's suitability for a particular application may not be readily apparent. The strength and durability of a tile are determined by the ingredients making up the bisque, or body, of the tile; the type of glaze used, if any; and how long and at what temperature the tile is fired.

Wall tiles often have decorative or high-gloss glaze applications that are really appropriate only on wall surfaces. Floor tiles, on the other hand, may be used for either floor or wall applications.

How to tell the difference? The best way is to ask the person selling the tile for a written recommendation. Most of the time, this comes in the form of a manufacturer's brochure that states that a certain tile is approved by the manufacturer for use on floors and walls, walls only, or floors only. There may also be a statement regarding a tile's use on a countertop. If no such written recommendation is available, a written statement on the tile seller's company letterhead will suffice.

Keep in mind that some porcelain floor tiles have a somewhat granular surface meant to reduce slipping. Some of these tiles are also manufactured specifically for use in commercial applications where heavy-duty cleaning machinery will be used to maintain the floors. Because such heavy cleaning is impractical in a home or office, the textured surface may get dirtier than if it were installed in a commercial or industrial setting.

While it is important to know the wear qualities, a tile's absorbency should also be considered. Tiles that are bulletproof in one application may not be appropriate for another. For example, Saltillo and other hand-molded paver tiles are used extensively for floors both indoors and outdoors (in nonfreezing climates), but they should not be used in wet interior applications where hygiene is an issue because they are generally too absorbent.

Some tile sellers may refer to the hardness scale for assessing the appropriateness of a tile for floor use. While that might be helpful, with today's new glazes it may not be as reliable as it once was. Hardness is important, but surface texture may be more of a good marker. For example, some porcelain tiles that are extremely hard would be too abrasive for normal home or office use.

Don't rely on anecdotal "advice" if you don't have specific experience with the tile you choose: Ask your dealer and get an assurance in writing if you have any doubts.

Contributing editor Michael Byrne is an expert tilesetter and consultant in Los Olivos, Calif., as well as author of many JLC articles and the book Setting Tile.