Updated: Dec 11, 2019
Aside from its aesthetic appeal, once grout has set it makes a mosaic incredibly strong, combining with the tiles to protect the base underneath from the elements. The colour of grout a mosaic is finished with will either kill the colours of a mosaic design, or make them even more vibrant. This also applies to wall or floor tiling, and is one of the wonderful ways in which artistic and trade skills can be mutually exchanged.
Grout is an incredibly finely-milled cement, with the consistency of talcum powder. Depending on the size of the sand grains it comes mixed with (coarser sand is for gaps 5mm +) grout can actually penetrate a hairline gap. When mixed with water it is soft and malleable- so it is difficult to believe, at first, that it will set rock hard in a few hours time. This is why one must have everything ready and organised to start and finish the grouting process in one go. You want to make sure only a grout film remains at the end of your session and not actual grout still coating your tiles!
Grout is the finishing, sealing touch to any mosaic, or large tiled surface area... unless it has been done in Smalti, or glass on glass (GOG) mosaic which is more aesthetically pleasing when left ungrouted.
Here are some very important colour- related aesthetics to think through when at the grouting stage...
Colour: Choose the grout colour very carefully. Grout will dramatically influence the finished look of your piece…for better, or for worse! If you are a little unsure (particularly with a large mosaic, which has taken months to do) rather make up a ‘sample tile’ and grout that first in the colour you think might work. Sometimes, however, no single grout colour will suit the entire mosaic composition. This is the occasion where you can bring out the paintbrush and retouch the grout colour in a specific area. Retouching works best if the grout already used on the mosaic is dry, and is a lighter colour or tone than the one you intend to apply over it.