STEP FOUR – Grout
Grout is very important to your mosaic project. It unifies and strengthens mosaics. I recommend sanded grout. I use Polyblend sanded grout available at Home Depot and other hardware stores in 7 lb., 10 lb., or 25 lb. quantities. If you prefer smaller sizes you can purchase 2 lb or 10 lb containers of sanded grout from Delphi Glass. Sanded grout is used for grout lines 1/8″ (3 mm) and larger. Some people say that sanded grout may scratch glass, but I have never experienced this. Avoid premixed grouts unless you can find one that specifically says it is for exterior use (most are not.)
Grout color choice is very important, and should be carefully considered. The color of your grout can enhance your design or completely muddle things up so that it is impossible to see your design properly. Avoid using grout that is the same color as the tesserae in your project. For example, a design with white, green, and blue tiles should not have white grout. The white tiles will disappear against the white background once you have grouted the project. Neither would you want to use a green or blue grout of exactly the same hue and intensity as the green or blue tiles. With these three colors (white, blue, & green) a neutral gray is a natural choice, but if you want to use something different consider grout that has a green or blue hue, but with a lighter or darker intensity. Considering the white tiles in the example, you would probably want to use a lighter intensity blue or green grout.
Friends don’t let friends use white grout. Please do not use white grout unless you are CERTAIN it will enhance your project. Some mosaic artists say that white grout makes mosaics look like elementary school projects, and I tend to agree with that. If the grout lines are large and the grout is white you will notice the grout more than the tesserae. When in doubt, use a neutral gray, or sandstone, if that would go better with your tesserae or exposed substrate color.
White grout does have one good use, though: you can add acrylic paint to white grout to make any color grout you desire. You can also use concrete color additives and mix them with white dry grout. If you mix your own grout color, make sure you mix up more than you need, as it is almost impossible to match color if you run out and need to make more. Grout color may lighten slightly as it dries.
If you don’t want to mix your own colored grout and you don’t mind buying a large quantity of one color, you will find that grout is available premixed in many colors at Home Depot and some other hardware stores. (This grout should turn out the same color each time you mix it. It may lighten slightly as it dries. ) Check online and you will find that Polyblend brand sanded grout is available in 48 colors (last time I looked.)
Here’s one way to test out a grout color to see if it is right for your project:
- Wear a mask, eye protection, and gloves.
- Sprinkle some dry grout between the affixed tesserae of your project to see how it looks.
- Brush/shake out the dry grout over newspaper and return it to the container.
- Repeat to test another color.
Mix the Grout
Read the directions on the grout. Basically, you mix dry grout with cool water in a disposable container until it reaches a consistency similar to brownie batter, peanut butter, etc. When I am mixing up grout for a small project I do so in a resealable baggie. Place the dry grout in the baggie, add the water, close the baggie, and mush around it until it is all mixed. (Once mixed you can open the baggie. I just close it to contain the dust.)
Read the directions to determine the water to dry grout ratio. It takes a lot less water than you might think.
Let the Grout Slake
After you mix the grout, take a 10 minute break and let the grout “slake.” During this time the water tension breaks down, the water thoroughly mixes with the grout, and the chemicals in the grout work their magic. (Read the directions on the grout container for the slaking time the manufacturer indicates.)
Remix the Grout
After the slaking period, remix the grout before you apply it.
Apply the Grout
You can apply the grout with a flexible squeegee, small plastic spatula, disposable paint brush (foam or other,) or just use your gloved hands. I tend to use gloved hands with smaller projects and a squeegee or trowel for larger projects. Make sure to press the grout down between all of the tesserae.On large projects you might use a grout float.
Allow Grout to Partially Set - Then Smooth the Surface
Allow the grout to firm up for 15 to 20 minutes. After it firms up, smooth the surface of the grout and remove excess grout with a wrung out sponge, your gloved fingers, or a damp paper towel.
If you use a sponge to smooth the grout you will need to rinse off the excess grout you pick up between swipes. Be careful to wring out the sponge very well, as you do not want to add more water to the grout, and you do not want to wash away the grout.
Warning: Do not rinse grout sponges in the sink or pour grout water down the sink! If grout gets into your plumbing it will block up your plumbing. Rinse the sponges in a bucket of water and dump the bucket outside.
I usually wait another 10 minutes or so before I clean the haze off the tesserae with cheesecloth. If there are any stubborn specks of grout on the tesserae you can remove them with a nylon scrubbing pad. Finally, buff the tesesrae with a soft lint free cloth.
Allow your mosaic to cure for 2 to 3 days. I usually mist the mosaic once or twice a day while it cures. If you get too much water on it, gently blot it dry with a paper towel.
You have to be careful working with grout. Dry grout is essentially similar to dry mortar with finer particles. (See the section on thin-set mortar for more information.) It contains Portland cement that is very caustic. Wear rubber gloves, a dust mask, and eye protection while mixing grout, so that you do not inhale particles or get them in your eyes, and to protect your skin from contact with the chemicals in the grout.
This post is part of a series on Mosaics on the Rocks: