In the community workshops the participants began by making a smaller piece. Once their skills (and commitment) were established, they helped create a section of the central mosaic that we designed together.
Here is the step-by-step method to create a precast mosaic. This is abridged from the Complete Pebble Mosaic Handbook by Maggy Howarth. The technique will be the same for the larger pieces.
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1. The design
The size of mosaic done by each school child will by 6” x 9” x 2 3/8” deep. They will collect the pebbles on the island beaches (see item Good Stones and Bad Stones) and draw up a design.
2. The pattern
The design will be taped to a piece of plywood and covered with a protective sheet of clear plastic.
3. Building the mold
Pieces of 2” x 4” ripped to 2 3/8” deep, cut to length and predrilled for the nails will be assembled over the pattern. These must be squared, and precisely sized to the exact dimensions of our paving stones.
Each pebble must be selected as appropriate for the design and placed with its top side down on the pattern. This is a bit tricky because you won’t actually be able to see what the finished piece will really look like, since you will be seeing the bottoms of each pebble. Not only that, but the pebbles will be set into sand, which will cover the part of the pattern that you are working on. Also, in a larger mold, the pebbles tend to fall over a lot until there are enough placed to support each other. Larger pebbles can be used as supports to begin with.
Dry, sieved "playbox" sand is worked around each pebble as it is placed in the mold. With a small, soft paintbrush it is brushed to a depth of about 5/16”. When the mosaic is cast and removed from the mold this sand will be washed away, leaving the pebbles protruding above the grout. If you didn’t do this, the grout would flow to the base of the mold, almost covering all the pebbles.
7. Testing the sand depth
To test the sand depth, get a narrow stick, a bamboo barbeque skewer for example, and whittle the end with an Olfa knife to a narrow peg of 5/16”. As you go along, poke the sand to see that it comes to the top of the peg and adjust accordingly.
8. A final check
I rarely get this right when I am laying the pebbles, mainly because I have to move them around so much. So, when I think I’m finished I position a bright light directly overhead and start poking all over. I use a squeezy bottle from a dollar store filled with sand to add small amounts of sand where needed. It takes a bit of practice.
9. Wetting the sand
Wet the sand thoroughly with a fine spritzer. This prevents the sand from sucking the water from the grout and assists in even penetration of the grout between the pebbles.
10. Mixing the grout
Use a special shrink-resistant cementitious (I LOVE that word) grout. Mix it with water according to the instructions on the bag using a paddle attachment on an electric drill. This smooth, sand-free grout allows penetration between all the pebbles, and won’t shrink away from them over time.
Pour the grout evenly over the pebbles. For the small mosaics (6” x 9”) we will fill the mold to the top. With larger mosaics, add the grout until the mold is about half full. A gentle hammering around the periphery helps to release trapped air bubbles. The mosaic must be left for awhile until it becomes the consistency of putty. With the grouts I’ve tried, this takes almost a day. You want the grout to be firm enough to hold the pebbles in position securely for the next step, but not so hard that you get cracks in the surface.
12. Adding the concrete
Fill the mold with concrete. I use a pre-mix, with additional aggregate. Take care to pat the mix down at all the edges.
13. Tamping it down
Using a piece of wood longer than your mold, tamp down on the brimming concrete until a smooth surface is achieved, adding more where needed. Any excess can be removed by sawing the wood back and forth and sliding the extra concrete off the end. Hammer around the outside edges again to help release any air bubbles. Cover the mold with plastic and seal the edges to prevent drying out. The concrete will soon set and will be reasonably hard by the next day.
The grout and concrete will still be “green” the next day so be careful not to knock the edges. Pry off the wood mold to be cleaned up for future use. Stand the mosaic on its edge and brush off the sand. VOILA! This is the exciting bit where the mosaic is revealed. Hose down the piece to remove the last of the sand.
Before the grout sets any more, any high bits can be chiseled away. The sharp edges can be rounded at this time as well.
The mosaic must be completely wrapped in plastic and the edges sealed to prevent it drying out while curing. It achieves 90% of its strength in three days but needs 28 days wrapped and damp to reach maximum strength.