It's possible to create a full mosaic tile table top over a glass table top, but it is not safe to do so. Mosaic tiles and the adhesive materials used to attach the tiles are too heavy if you plan to completely cover the table. You can safely create a mosaic tile border or simply glue a few tiles in a pattern arrangement on top of a glass table. If you desire a full mosaic table top you do have some options.
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Making a Mosaic Table Top
Even though mosaic tiles will adhere to a glass surface, the weight of a full mosaic table top could crack the glass. Your best option is to replace the glass table top with wood before you make the mosaic. If you prefer that your table has a base that you can see through, replace the glass with translucent lexan.
Your wood top should be at least 3/4-inch thick to hold the weight of the tiles, the concrete adhesive and the grout. If you are making a mosaic table top for an outdoor table, you can layer concrete backer board on top of the wood table top, or use treated wood. Indoor mosaic table tops may be created from any type of wood.
Arrange the tiles on your mosaic table top randomly or you can create an image with the different colored tiles. To create a mosaic image, transfer a pattern onto the wood top with a marker. A template or stencils are both good tools for this project.
Fill in between the lines with the appropriate colored tiles to bring the image to life. It's a good idea to work in small sections at a time; spread the thinset or Weldbond adhesive onto the wood and press the tiles into the adhesive. Thinset is best for outdoor mosaic table tops while Weldbond is a good choice for indoor mosaics.
Leave up to 1/4-inch of space between each tile. Don't worry about uniform spacing; mosaic art is not based on perfection. When the adhesive dries, grout the mosaic design to fill in between the tiles. You'll want to clean off any grout that's left on the face of the tiles before it dries, or your tiles may appear cloudy.