Mosaic Grout

There is nothing unique about mosaic grouts. The grouts and sealers used for mosaic artwork are the same ones used for ordinary bathroom tiling, and the techniques of applying the grout are the same. If you are ever uncertain and need information in a pinch, you can ask questions at your local building material store or look at their book on how to tile or bathroom repair, which they usually keep on the same aisle where they sell grout and supplies or in the plumbing department or near the registers. The point is that grouting isn't rocket science and most handymen and women have done it before.

Estimating How Much Grout Is Needed

How much grout you need depends on three things: the area of the mosaic = L x W, the thickness of the tiles and the gaps between the tiles. If the tiles are well-spaced (all gaps under 1/4"), then you should be able to cover the entire mosaic of 18" x 18" with 2 lbs of grout. This is assuming your tiles are under 3/8" thick. I always set my mosaic on an old shower curtain or piece of plastic to catch all the wet grout that falls off the side as I'm spreading it. That way I can scoop it up all the dropped grout and re-use it. Otherwise you need a lot more grout because most of it is wasted when it falls on the dirty floor.

If the gaps between your mosaic tiles are greater than 1/8 inch, then you may need grout with sand in it, such as we sell. The sand reinforces the grout to prevent cracking if the gaps are greater than 1/8 inch. Note that standard grout gaps are usually about 1/16 inch.

How to grout a mosaic

  • Grout outdoors if at all possible to minimize dust contamination and clean up of wet grout.
  • Mix the grout according to instructions on package and avoid breathing dust by wearing a dust mask or mixing downwind from your face.
  • Spread the grout into the cracks. Rubber spatulas and gloved hands are best for this work.
  • Wipe off the excess grout. Use a sponge instead of a rag because the rag will catch on any sharp edges.
  • Don't try to wipe it perfectly clean all at once.
  • Rinse out the sponge after each pass. Make sure your sponge isn't dripping water after you rinse it out.
  • Wipe flat across the surface. Don't rub down into the cracks or you will erode the grout between the tiles.
  • Keep the surface damp as the grout cures. Cover with plastic if necessary. It will crack and crumble if it dries out.

Selecting a Grout Color

I always choose a grout color which contrast the colors of the mosaic tiles so that the design is highlighted. Otherwise the individual tiles run together and are lost to the eye, and the mosaic looks more like an ordinary picture and less like a mosaic made from pieces. I avoid situations like gray grout with gray tile. A nice red-brown terra cotta grout works better in that instance. The key concept is color contrast instead of matching colors.

Color is best provided by the tile, not the grout. In terms of a visual element, the grout is supposed to act like a thin gray pencil line whose only purpose is to make each tile distinct. I don't think I've ever seen a mosaic that used colored grout that wouldn't have looked better if all tile had been used instead. For example, let's say you wanted red tile in pink grout. A better way to get the same color effect would be to mix in a few pink tiles randomly throughout the red tiles, or maybe use a few lines of pink tiles. Often this means you have to cut your tile just a little bit smaller. That being said, it is possible to dye grout.

Dyeing Grout

There are mineral concrete dyes sold at building material stores, and these can be used, although the range of colors is limited to black, brown, gray, terracotta, etc. Vegetable dyes such as food colors will not work. You can use artists acrylic paint to color grout by mixing it in the wet grout. I recommend experimenting with tiny batches of grout and letting it harden before grouting a mosaic you have worked many hours on.

The Best Grout Color

Sometimes it is hard to decide which color grout to use because your mosaic already contains so many colors. The best color grout to use for most mosaics is medium-gray. Here's why: The purpose of grout in visual terms is to make each tile distinct in the same way a gray pencil line in a watercolor is used to provide definition but not color. A medium or "natural" gray provides just enough contrast to most colors, unless you have gray tile, and then you might want to consider something else.

Avoid pure white unless you are trying to make something that looks like a little kid's summer camp project. Most building material stores such as Lowes and Home Depot cary about 30+ colors of grout. Bring a few of each color of your tiles with you, and you can pick the color grout that works best with all the tile.

Grouting Stone Tile and Unglazed Ceramic

You should always seal unpolished stone and unglazed ceramic tiles with a tile and grout sealer BEFORE you grout. This is necessary for all porous materials that can be stained by grout. You should seal using a Stone Enhancer instead of tile and grout sealer if you want to darken and enhance the natural color of the stone. Both of these products are available at local building material stores. For a stone mosaic that is already stained, you might want to try some of the muratic acid that contractors use to clean cement from bricks.

Cracking and Crumbling Grout

You shouldn't let the grout dry out as it cures. Cover with plastic if necessary. Lightly mist with a spray bottle, but don't let drops accumulate on the surface of the grout. It also helps if you mix the grout according to instructions. It should be like a thick moist dough when you start with no dry material or lumps, and it should stick to your mixing wand, not drip, slide or crumble off.

Don't use unsanded grout. Unsanded grout is only used to fill hairline cracks. (Or it is mixed with sand for normal use.) If you use unsanded grout, then you can expect cracks in your grout and grout that isn't quite as hard and durable.

Disposal of Grout Waste

Never pour grout down your drains. Grout is concrete, and that isn't good for your plumbing. It will clog the pipes. I make a point of pouring as much of my grout sludge in an old cardboard box and allowing it to harden for disposal as solid waste. I always just dilute the dirty grout water and pour it in on my compost heap. The grout is a limestone product, so it actually helps balance the pH when there is excess decaying organic matter. Of course, this only applies to traditional grouts. I am not familiar with the synthetic epoxy grouts that some craft sites are selling. For easy cleanup, I always do my grouting in the backyard instead of indoors.