Grout is the material which goes in between the tiles. I recommend that you think about what type of grout you want to use early in the mosaic planning process.
Grouting powder is mixed with water. It is used to seal the gaps between all the tiles and smooth the overall image of the mosaic giving it an unbroken surface whilst strengthening the bond between the tiles. While the glue holds the tiles down to the base, the grout will firmly lock the tiles in from the sides.
Grout keeps out the weather and dirt and will protect the mosaic.
Grout can be used for practical and aesthetic reasons. It can be washed down to remove any dust, dirt, spills, food or other materials.
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.
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.
Grout generally comes in white, grey, beige and brown, and a couple of other colours like black. 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. 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 tiles.
Grey grout provides an excellent neutral frame which will enhance most coloured tiles 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 blue and red you get a Mediterranean feel.
Dark grout obviously unifies darker colored tiles and segregates lighter ones.
Grout is generally mixed with water to produce a porridge-like consistency - it mustn't be too sloppy nor too stiff. If the grout is too stiff you risk pulling off little pieces of tiles that have not been glued on properly. Don't worry about these pieces, as the grout will act as a second adhesive and set them firmly in place.
It is important that you have more than enough, so always make a little more than a little less, especially if you are mixing colour into it.
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.
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 in a disposable container and mix a heaped spoonful of grout one spoonful at a time, making sure it's smooth and lump-free 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.
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.
DO NOT throw any grout down your drains, this will cause them to block up as the grout sets in the pipes. 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.