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
Step 1: Preparation
Grout outdoors if at all possible to minimize dust contamination and clean up of wet grout. Wherever you work, set up a clean area to work in and make sure you have all the tools you will need. Usually you need a bucket of water, some kind of tray to mix the grout in, a mixing stick, a sponge, rubber gloves, a dust mask, and some paper towel or old rags. You can use latex gloves in a pinch if your mosaic is made from smooth tiles, but rubber gloves are recommended when you’re grouting any kind of tile that has sharp edges. Those edges will rip through latex gloves as you spread the grout around.
The mosaic needs to be fully dried before you begin. Wait at least 24 hours after gluing down the last tile before you grout your mosaic if you used Weldbond.
Step 2: Mix the Grout
Wear a dust mask when you do this step. Estimate how much grout you need for the mosaic and pour it into the tray. In this example we used the cut-off bottom of an old plastic jug. If you don’t mix enough grout you can always mix some more. The grout container should indicate how much water to use. The dust generated in this process is very fine and can spread all over the place, that’s why it’s best to do this outside. If you do grout inside, just be sure to vacuum the area once you finish grouting.
Step 3: Grout the Mosaic
This is the least complicated step. You just need to grab a glob of grout and mush it into the mosaic. Keep doing this until all areas have been covered in grout.
Step 4: Verify Total Grout Coverage
Before you haze the grout, you need to be sure that all of the cracks were completely filled. Look closely and ensure that there are no bubbles between the grout and the wood. Run your fingers along each crack and push the grout down in.
Step 5: Clean with Grout Sponge
For smaller projects you can sometimes omit this step and just use rags and paper towel to wipe off the excess grout, but for anything approaching 1 square foot in area or larger you want to do it with a sponge as demonstrated here.
Step 6: Clean the Haze
The sponge will have cleared the bulk of the extra grout off, but there will probably be a thin grout haze left over. It’s okay to let the haze dry for a few minutes to make it more visible and easier to clean before you try to wipe it off. To clean it use a dry rag or paper towel. You should be able to clean an area of 1 square foot with only 1 or 2 paper towels.
Step 7: Final Touchups
Look around for areas where grout is built up on any tiles. This usually happens where tiles are of different thicknesses. This can be done with a rag or paper towel wrapped around your finger. Make sure the grout is roughly flush with, or slightly lower than the tiles around it.
Once it’s cleaned off you’re finished with the grout. Allow a few days to cure fully, but usually it should be pretty solid after a few hours. The time for full curing can depend on the heat and humidity where you did the grouting. Some pieces of dust or sand may slowly fall off the mosaic over the next few days, don’t be alarmed if that happens.
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.
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 black. Here’s why: The purpose of grout in visual terms is to make each tile distinct in the same way a pencil line in a watercolor is used to provide definition but not color. Black provides just enough contrast to most colors, unless you have black 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.