If you have already grouted your mosaic, you can still use these instructions, but you will first need to remove the grout using a grout removal tool, which is normally used to scrape grout from the gaps between glazed ceramic tile. It may take a little more care with glass, but this tool can be used to remove grout from mosaics made from small pieces of glass tile.
How To Remove and Replace Glass Mosaic Tile
Glass tile doesn’t usually pry up in one piece. If you are able to get most tiles off in one piece, then there was probably something wrong with your adhesive or mortar. Expect the glass to come up in sharp pieces and slivers and not be reusable. Never chisel the glass off by banging at it forcefully. If your scraping tool slips past the tile being chiseled, your knuckles and wrists will be headed straight toward razor sharp teeth mounted in concrete. Wear leather work gloves. I wear my welding gauntlets.
If the mosaic in question was made with a white PVA adhesive such as Weldbond, then I use a moistened cotton swab to selectively wet the edges of the tile, taking care not to wet the surrounding tiles too much. Note that if the Weldbond has had several months to cure, it may be more water resistant, but you should still apply water. Keep plastic kitchen wrap such as Saran Wrap over the tile and reapply moisture as needed to let the glue soften for an hour or two.
If the mosaic is an outdoor or wet mosaic made with thinset mortar, moisture doesn’t really help. For thinset mortar, it is best to make any changes within a week at most. Thinset is hard and tough and it really grips the glass once it has had a chance to fully cure.
I usually use a medium size standard screwdriver to pull the tile up with a combination of scraping and prying. Notice that when you attempt to pry the tile up, you tend to use the surrounding tile as a fulcrum on which to rest the screwdriver, and this is a problem. It can crack or even shatter the surrounding tile, and it is likely to do so because the glue is usually stronger than the glass. Fortunately, there is a simple solution: Lay a Popsicle stick or ruler over the surrounding tile and use that was your fulcrum. Of course, you will need to use one hand to keep the ruler from sliding back slightly as you pry. If you let the screw driver slide the ruler away, it will make contact with the tile underneath and damage them.
Damage To Backers
Plywood and concrete backer board aren’t as strong as Weldbond and thinset mortar, so they will sometime delaminate when use start prying and scraping and you end up pulling off the top layer of the backer. If this happens, don’t panic. Plywood can be patched with a mixture of sawdust and Weldbond, and concrete backer board can be patched with thinset mortar. However, you can minimize the possibility of this happening in the first place by attacking the tile from multiple angles instead of just working from one side.
When you pry up glass tile that has been glued down, sharp pieces can break off and go flying across the room. (I try to keep my leather work glove over the tile to prevent this possibility.)
You should wear safety glasses with side shields. You may even want to wear the plastic safety shield masks that are made for working with power tools. You also want to make sure anyone else around your work area has proper eye protection.
Another issue is the tiny sharp slivers of broken glass. Keep a vacuum nearby and use it periodically to clean the work area. It is always the tiny invisible slivers that cut you when you wipe the work surface clean with your hand. Use a vacuum instead.
Wear leather work gloves and be cautious of jabbing forcefully at the tiles. Use deliberate motions and think about where your hand will be headed if the tool slips off the tile: The freshly broken glass is sharper than any steel razor and it is mounted. If you punch your hand or wrist into it, you will be going to the emergency room in all probability.