The notched trowel is used by installers to apply a uniform amount of adhesive under tiles. A consistent amount of adhesive is necessary for the tiles to sit in a smooth, even plane. A uniform amount of adhesive is especially important under mosaic tiles since their small size tends to accentuate irregularities on the setting bed. As well, because mosaic tiles are usually thinner than regular tiles, even a small amount of excess adhesive can clog up the grout joints and create a nightmarish cleaning job.
Once the adhesive has been spread, it is important to get the mosaic sheets bedded in the adhesive as quickly as possible (Photo 8). I use my hands (Photo 9) to smooth the sheets and feel the joint where two sheet edges meet; the lippage detected by my fingers help me gauge how much beating block to apply.
Before installation, all paper face-mounted mosaic sheets should be inspected for uniformity and regularity to ensure that all the mosaic bits are aligned properly. In spite of this precaution, though, the mosaic bits should be inspected again once the sheets are installed. To do this, dampen the paper mount with a moist sponge, keeping 100 percent of the paper damp for about five or 10 minutes, or until the paper can be gently removed. Start at a corner and peel the paper away (Photo 11). If mosaic bits lift off the setting bed, the paper needs more soaking. With the paper removed, any last-minute adjustments can be made. After that, the mosaic bits should be allowed to rest for at least 48 hours before the paper-mounting adhesive can be scrubbed off the surface of the tiles and the installation can be grouted.
Plastic film is a form of mounting that permits the installer to keep all the tiles and bits in a mosaic sheet in view at all times. The layout for the irregularly shaped mosaic sheets here (Photo 12) is done by positioning the sheets dry and outlining the perimeter with an indelible marker. The mosaic sheets are then stacked in a convenient location to await installation. For spreading an adhesive layer of thinset mortar for these mosaic sheets, I used a 1/4-by-3/8-inch square-notch trowel and, following the same procedure I used on the glass mosaic sheet, finished that process by flattening the adhesive ridges just before installing the mosaic sheets.
Bedding the Sheets in Thinset Mortar
When the adhesive is ready, the first sheet is placed gently over the adhesive bed. When the second sheet is installed (Photo 13), I dip one corner to mate the sheet with its neighbor's edge, lay it gently into the adhesive, and make any adjustments to ensure the remaining sheets align with the outline marks (Photo 14). When all the sheets in the area I am working are installed and properly aligned, I use a beating block and hammer to gently press each mosaic bit into the thinset mortar and ensure that the surface of neighboring sheets is smooth and even (Photo 15).
Removing the Plastic Film
Prior to installation, any loose mosaic bits should be pressed back into the plastic film. Once the sheets have been bedded in mortar, they should remain undisturbed for at least 24 hours before an attempt is made to remove the plastic film - earlier if a rapid-setting thinset is used. Grouting can proceed as soon as the film is removed.
Installing Back-mounted Mosaic Sheets
Back-mounted mosaic tile sheets should be installed over an adhesive bed that is prepared using the same spreading and flattening techniques employed for face-mounted sheets. Unsuitable or unwanted mosaic bits (removed to make way for a design or pattern) should be cut away from the sheet before it is installed, and the beating block and hammer should be used to keep the surface flat.