Grouting a mosaic gives it a finished look, but some types of mosaics do not require it. Here are a few tips to help you choose whether to grout or not and which grout to use:

Measure the Space Between Pieces

The general rule of thumb is to use sanded grout if the spaces between your tiles are more than 1/8 inch wide. This is because sanded grout contains more, well, sand ― this adds more body to the grout so that it doesn't shrink as much as the unsanded variety. Smaller spaces need the unsanded grout so that it can penetrate the tiny interstices. If your mosaic piece has a combination of large and small grout lines, mix half and half sanded and unsanded grout. This will ensure that the smaller spaces get filled in adequately and that the grout doesn't shrink.

When to Use Epoxy Grout

Epoxy grout is best for applications where there will be a lot of water, such as a bathroom sink or a birdbath. Regular grout will hold up for some time, especially if it is sealed, but epoxy adds another measure of protection from water damage. Epoxy grout is a very sticky and messy product to use, but the extra protection is worth it for some applications.

Experiment With Grout Color

The color of grout is not usually meant to stand out but to provide a subtle background that ties the whole piece together. Sometimes, however, you may wish to provide contrast with the grout color in order to make the artwork more vibrant. Grout is available in a wide range of colors, most of them subtle earth tones. If you're not sure what color to use, an easy way to test is to sprinkle some of the powdered grout on the mosaic and brush away the excess. This will give you an approximate idea of how the grout color will look (though when it's mixed with water it will be much darker). When in doubt, a medium gray is always attractive and provides a subtle background that sets off any design.

It's possible to tint your own grout with acrylic paint or special tints made for dyeing concrete. Follow the directions on the package, and always test before coloring the entire grout batch.

When You Don't Need Grout

When you're creating a mosaic for display only and which will not be subject to the elements, you may choose not to use grout at all; in this case, it's best to lay the pieces as close together as possible. Only experience will tell you when this is appropriate, but sometimes your mosaic materials will dictate; for example, when making a mosaic with smalti, which is chunky glass tile of slightly varying heights, grout would be very difficult to get in all the spaces and might detract from the beauty of the tiles. Also, if you use a lot of found objects in your mosaic work, grout may dirty them or be too difficult to maneuver into all the tiny spaces, such as around pieces of jewelry. Another option in this case is to carefully paint any parts of the substrate that show when your mosaic is done.

How to Apply Grout

Mix up the grout according to package directions. Have a couple of large buckets of water handy, plus grout sponges, a grout float or several pieces of heavy cardboard, and some clean rags. Trowel on the mixed grout and use the float or the cardboard to push it into all the cracks. You can also put on a pair of disposable gloves and use your fingers for this part; sometimes this is the only way to get grout into the spaces around objects like shells or stones. Scrape off as much of the excess as you can, let the grout sit for about 20 minutes, then start sponging. Wring out a grout sponge thoroughly and drag it across the surface. Repeat, wringing out the sponge each time, until you've removed most of the grout on top of the tiles. Be sure you don't dig grout out of the spaces between the tiles. Finish with a few passes with a clean sponge and clean water. After a couple of hours of drying time, buff the tops of the tile pieces with clean rags to remove the grout haze.

This sink grouted with epoxy.

Mosaicked bathroom sink
Credit: Patricia Rockwood

Example of an ungrouted mosaic.

Yellow Tang
Credit: Patricia Rockwood