This is not a straightforward question because most of the grout does not end up in the gaps between the tiles. Instead, there will be some grout on the sides of the mixing bucket, on the mixing tools, on your gloves and last but certainly not least, on your worktable or floor. This last place is where most grout tends to end up, depending on the skill and experience level of the artist doing the grouting. However, with a little forethought and planning, even a novice can minimize the amount of grout wasted in this way.
Before I explain some practical ways to use grout efficiently, it’s worth the time to talk about what is theoretically possible if all the grout ended up in the gaps.
2 Pounds Per 10 Square Feet THEORETICAL
If you are doing large mosaic walls instead of smaller mosaic plaques, then you could THEORETICALLY gout 10 square feet of 3/4 inch glass mosaic with a 1/16 inch grout gap with under 2 pounds of grout, but again, how much grout you actually consume will depend more on your work methods than anything else. Also, the smaller the project, the more grout you will use per square foot because grout tends to be wasted at the edges.
Practical Numbers for Novice Artists
If I were to propose a rule of thumb for artists grouting small projects, I would say 1/2 pound to 1 pound per square foot, provided your grout gaps are 1/16 inch.
Here’s a better rule of thumb for novices:
A little wasted grout is better than a wasted mosaic.
The last thing you want to happen is run out of grout before you finish grouting the mosaic. You also don’t want the grout to start drying out before it can cure, and this is more likely when the grout is mixed up in small batches under a pound.
Sanitized For Your Protection: Plastics Are The Solution
Grouting requires that you smear grout to the edges of the mosaic and work it in so that inevitably a lot of grout falls off the sides of the mosaic and onto whatever you have beneath the mosaic. If the wet muddy grout falls onto a clean surface, it can be scooped up and reapplied to the surface of the mosaic. If it falls onto the floor or any other surface likely to have traces of dust, lint, hair or other contaminants, then it is best to discard what fell.
At our studio, we keep rolls of construction plastic and use this to cover our worktable before laying the mosaic on it for grouting. You can also use ordinary kitchen plastic wrap such as Saran Wrap to wrap your table. (Note that wrapping may be easier and more reliable than merely trying to tape or tack a layer on top of the table, which tends to get pulled up in all the activity of grouting.)
We also make sure that the worktable we use is large enough so that we have at least 6 inches of surface beyond each edge of the mosaic. This is important for making sure that we can scoop and reuse the clumps of wet grout that inevitably fall of the edge of the mosaic, but also for making sure that we don’t have to stop and clean up a mess on the floor before we step in it.
Misting Spray Bottles & Humidifiers
You should never add water to grout once it is mixed up, and you should not wipe the surface of a freshly grouted mosaic with a rag that is too damp because you can leach the pigment out of the grout.
However, it is important to keep the grout from drying out as it cures. For this reason, we often mist the air around our mosaics as we are grouting to make sure the air isn’t too dry if the heat or AC is running. We also run a humidifier if conditions are particularly dry. These same precautions can help extend the life of clumps of wet grout so that they can be reused.
If you ever pick up a wet-looking clump of grout and find that it has started to form a stiff crust on the outside, then it is best to discard it. Misting spray bottles and humidifiers will help prevent this from happening as quickly.
Putty Knives (And Serving Spoons) Used In Pairs
The main reason so much grout is wasted in the bucket is that it tends to get splattered and streaked up the sides of the bucket where it starts to dry out, and most people don’t notice it until it’s too late to do anything about it. The key is to be disciplined and remember to scrape down the sides of the bucket during mixing and immediately afterward, and after each time you scoop out some grout or do anything that smears it up the sides of the bucket. Try to keep your grout all together in the bottom of the bucket like a lump of dough.
A putty knife with rounded corners or an old serving spoon from the thrift store are good tools for scraping the grout into a lump, but you should always have a pair of these tools instead of a single tool so that you can use them to scrape grout off each other.
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