The best grout gap for mosaic artwork depends on whether or not the mosaic will be outdoors or in a potentially wet location such as a backsplash.
When Do You Need A Grout Gap?
If the mosaic might get wet or exposed to humid air, you can’t have the tiles touch each other. You need a gap for the grout to fit into to seal out moisture.
Tiles that touch can never touch close enough to seal out moisture.
If the mosaic is a small indoor icon or plaque, you don’t have to worry about sealing out moisture, and so you can fit the tiles tightly together and skip grouting.
TIP: You might want to grout anyway to fill in any small incidental gaps left by imperfectly shaped tesserae, especially if your mosaic is a “dry” tabletop or architectural surface. (If those surfaces are in the kitchen or bathroom, you should consider them “wet” and use a grout gap.)
Smaller Gaps Preferred
Smaller gaps are preferred because they make grouting easier, and they minimize the color impact of grouting.
A grout gap only needs to be large enough to ensure that grout can fill the gap all the way to the bottom and not just a smear across the top and hide a void underneath.
Mr. Pinchapotamus is your new best friend. He holds small glass tile while you cut them with a Mosaic Glass Cutter so that your fingertips aren’t near the blades. He can also hold the tile more firmly in place than your fingertips, and so Mr. Pinchapotamus also improves the precision of your cuts.