Grout is the material which goes in between the tesserae (the pieces which make up  the mosaic). I recommend that you think about what type of grout you want to use early in the mosaic planning process.

Tile Grout for Mosaics - what is it & what does it do?

Grout is a paste made when grouting powder is mixed with water. It is used to seal the gaps between all the tesserae and smoothen the overall image of the mosaic giving it an unbroken surface whilst strengthening the bond between the tiles. While the glue holds the tesserae down from the base, the grout will firmly lock the tesserae in from the sides.

  • Grout keeps out the weather and dirt and will protect the mosaic. It can also enhance or tone down the mosaic depending on color.
  • Grout can be used for practical and aesthetic reasons. It can be washed down to remove any dust, dirt, spills, food or other materials.
  • Grout is available in ready mixed form or powder to be mixed with water when desired.

If you are grouting a floor based mosaic remember to choose a grout that will survive heavy traffic going over it. For this reason I don't advise any light colored grouts or white. Brown, grey and the darker colors are probably more suitable.

You can buy sanded or unsanded grout and latex grout, in a powdered form. Sanded grout handles bigger gaps (up to 10 mm) without cracking. Unsanded grout is finer and only for work with 2-3 mm gaps.

Use bonding liquid in place of water to waterproof your grout completely. This is a liquid latex sealer which does make cleaning the grout off a bit more difficult but worth the effort if your piece is going to be exposed to moisture.

Colored Grout

Grout generally comes in white, grey, beige and brown, and a couple of other colours like dusty pink. If you need a non-standard colour, colour grout powder or liquid grout colourants are available but not easy to come by. If you want to colour your grout, don't add paint to the grout. I know it's suggested often, but it takes a lot of paint, and you'll only get a light pastel shade. The paint will interfere with the integrity of the grout - weakening it. Instead, grout with white grout and let the work dry, then mix acrylic, ceramic or silk paints with water until they are a milky consistency, and dab that over the grout and then buff off the tiles. You'll get a more vivid colour.

Be very careful and aware that color changes as the grout dries and if you are unsure as to the effect a certain amount of color will have then it is always a good idea to do a little trial and mix it up and let it dry before putting it with your tesserae. Be very careful when using coloured grout however it can have a drastically wrong effect if used incorrectly.

Grey grout provides and excellent neutral frame which will enhance most coloured tesserae and will have the most unifying effect out of all the color grouts as it treats black and white equally.

White grout complements lighter tones so is very good for showing off pale mosaics, when used with bright and strong colors like Aztec blue and red you get a Mediterranean feel.

Dark grout obviously unifies darker colored tesserae and segregates lighter ones.

How to Grout:

Grout is generally mixed with water to produce a mud-like consistency - sort of like peanut -butter or toothpaste; it mustn't be too sloppy nor too stiff. If the grout is too stiff you risk pulling off little pieces of tesserae that have not been glued on properly. Don't worry about these pieces, as generally the grout will act as a second adhesive and set them firmly in place. You will generally have one or two loose pieces in each mosaic.

It is important that you have more than enough grout as often it is hard to tell how much you will need, so always make a little more than a little less, especially if you are mixing colour into it. Be careful because if you don't have enough grout you will have to make some more up and if you're unable to get the exact same color as before it can be a problem. As a rule of thumb, 200g of grout will cover a 20cm x 20cm square done in 4mm thick tiles quite amply.

Before you start, clean all the dried glue from the surface of your work. I use an ice-cream stick to scrape it off.

If your mosaic does not have background tiling - such as on a rock, use masking tape around the edges to leave a gap of about 5 mm. The grout edging will cover any design lines or glue along the edge.

If you've used any ceramic tile, run your work under a tap before you mix the grout (remember your base should have been sealed before you started sticking so a little water at this stage won't damage your work). The porous tiles will soak up the water, and not steal it from the grout, which will dry it out too fast. Grout needs to cure slowly in order to be strong. Never put it in front of a heater or in the sun to dry faster.

Use a small amount of water (only 20 or 30 ml for a small project) in a disposable container and mix a heaped spoonful of grout one spoonful at a time, making sure it's smooth and lump-free, until it reaches peanut-butter consistency. Leave the grout to slake (start its chemical reaction) for 10 minutes before applying it.

You should then press the grout into the tiles using your fingers (REMEMBER TO USE GLOVES!) or a tiler's sponge or squeegee. You can also use a spatula to apply the grout, but I prefer to use an old credit card and gloved hands. Rub the grout into the gaps from the middle of the mosaic to the edges. Grout is very drying on the hands and gloves help protect your fingers from any sharp edges on the tesserae.

I don't use a damp sponge to clean off the excess as I find it sucks too much grout out of the groutlines although a lot of mosaic artists like this method. If you are going to be using the "wet-wash" method then make sure that your sponge is just damp and not too wet. I use a dry toothbrush to brush excess grout from the surfaces, then as the grout starts drying (ie, no longer smears over the tiles) I use a dry plastic potscrubber/scourer to polish it up - the rough side of those yellow & green kitchen sponges works well. You will know that the grout is ready for "scrubbing" when a dry haze has formed over the tesserae. Toothpicks or kebab sticks also make great tools for removing grout in awkward places like around inserts.

The final step is a soft polishing cloth - this should remove the last of the grouting dust from the tiles.To really make your project sparkle, use vinegar on a soft cloth - this will dissolve the last of the dust and shine your tiles.

DON'T throw any grout down your drains as this will cause them to block up as the grout sets in the pipes - rather let it get hard in the bowl and then scrape it into the bin if you intend re-using the container or use empty ice-cream or joghurt containers that you can simply throw away with the grout. If you've used the wet-wash method & have dirty water - let the grout settle to the bottom overnight, then pour off the water in an outside drain and scrape the grout slurry into the bin.


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