Making wonderful glass mosaic tile art is easy! Let me show you how.
The amount of grout for your mosaic project depends on two basic variables: 1) the size of your piece, and 2) the width of your joints. Obviously, smaller joints require less grout. It's important to mix more than you think you'll need to ensure you do not run out. This is especially true when using colored grout to ensure color consistency unless you intend for your piece to have varying shades of grout.
If you have a brand new box of grout, you'll probably use only a small portion of it. Do not dump the whole box into a bucket of water! If this is your first time grouting a mosaic, put what you think is enough into your mixing container and then add at least half that amount (ie, mix about 1.5 times what you think you need). When you're finished grouting, observe how much you throw away so you can better estimate how much you'll need for your next project.
The first rule of mixing grout is to read and follow safety precautions on the bag and then read and follow the label instructions. After reading the bag's label, go somewhere away from your mosaic so you do not splash wet grout on it while mixing (eg, go outside or to the opposite end of the garage). Scoop enough grout powder into your mixing container. Slowly add small amounts of water and mix using a paint stirring stick. Some grout brands are "polymer modified," which means acrylic latex has already been added to the powder. If the grout already contains the additive, the bag says in easy-to-read print, "polymer modified" or similar words. If the grout does not already contain the additive, then you may choose to mix the grout with an admixture instead of water. I never bother adding acrylic latex for my indoor mosaics. Plain hose water works just fine for me.
Add small amounts of water at a time and thoroughly mix it. Do not pour in all the water at once. Do not make the grout too runny by adding too much water. Take your time and mix in a little bit of water at a time. When it's almost at the right consistency, just a teaspoon more water can be too much. So, take your time.
Do not make the grout too runny. This is important so let's say it again together, "Do not make the grout too runny." I use thick, pasty grout because it's easier to wipe off without digging into the joints. The technical reason for not making it runny is that too much water makes the grout weaker and more impossible to staining. So, thicker is better. My favorite consistency is like thick toothpaste or cookie dough (that's thick toothpaste, not runny toothpaste). Some books say to make it like pudding or mayonnaise, but that's too thin for me.
After thoroughly mixing the grout, I use the paint stirring stick to scoop some up and hold it over the container. If it drips off the stick into the container, then it's way too thin. I like it so it stays in place on the stick and does not droop or sag.
If you accidently pour in too much water and make the grout too runny, simply add a little more grout powder to thicken it. Add just a teaspoon of grout powder at a time. A small amount of grout powder can change it from runny to just right. If you add too much grout, it'll be too thick and then you'll have to add a few drops of water to thin it. You'll be taught in an endless loop of adding grout to thicken it, then adding water to thin it. So, add small amounts. After your first grout job, you'll get a feel for how much to add, so it'll be easier the next time you do this.
When you get the right consistency, thoroughly mix the grout to ensure there's no residual powder and no lumps. I like to wipe the sides of the container with the paint stirring stick to get all the powder off the sides. I do not like any unmixed powder anywhere in the container.
Remember, making mosaic art is easy. You can do it. Yes, you can!