Materials
Unique, handmade large-format glass tiles are the most commonly susceptible to sanded grout scratching. Modern, mass-produced mosaics — and glass mosaic tile in general — are not prone to scratching from sanded grout. Check the glass tile box or manufacturer installation guidelines to determine whether sanded grout is suitable for your particular material choice. Nonsanded grout does not scratch glass tile but has a significant downside: The grout expands and often cracks in wide grout joints.
Testing
Unfortunately, a scratch test is the only surefire way to determine whether your glass tile is prone to sanded grout scratching. Place an extra glass tile piece flat on a workbench. Mix a small amount of medium-consistency sanded grout. Apply the sanded grout to the face of the glass tile with a rubber grout float. Gently rake the grout across the surface of the glass tile. Allow enough time for the grout to stiffen, and then rinse the piece in a bucket of clean water. Dry the tile and inspect it for scratches. If the piece appears blemish-free, you can use sanded grout. If scratches are visible, use a nonsanded grout, or mask off the tile before grouting with sanded grout.
Masking
It's a lot of work, but the only option for grouting scratch-prone glass tile — especially large-format glass tile — is to mask off the tile's surface area. Use blue tape, as it does not leave a sticky residue. Mask off the body of the tile, leaving the grout joints open. Apply sanded grout to the lines, but make sure you don't smear the grout under the tape. Let the grout stiffen, and then gently wash away any excess material. Smooth out the grout lines with a clean sponge and remove the tape when the grout has stiffened.
Tips
Avoid scratching glass tile by always cutting glass tile face up. Clean grout haze with sulfamic acid, but wait until 10 days after installation to do so. Use liquid soap and warm water for regular cleaning and maintenance. Do not install glass tile with tile mastic; instead, use a latex-modified thin-set mortar. Flatten thin-set mortar notch trowel marks before installing glass tile. Also, do not use gray thin-set mortar with clear glass tile, as gray thin-set mortar darkens or discolors glass tile. Plywood is not an acceptable glass tile substrate. Plywood expands and contracts, raising the likelihood of cracks in glass tile.