How to make a pebble mosaic

Timeline, tools and supplies

Friday: Sort the stones and lay out the pattern

Saturday: Lay the base, set the stones and let the concrete begin to cure

Sunday: Grout the stones into place

Tools:

• Spade to dig out the base
• 2-by-4s to screed the concrete and hold a tarp in place as the mosaic dries
• Garden sprayer or hose with a fine-mist attachment
• Two large nails and string
• Garden trowel
• 4-by-4 to set and level the stones. It should be about the length of the mosaic's radius so that you can work on one small section at a time.
• Rubber mallet
• Paintbrush to spread the topping mix
• Stiff-bristled brush to shape and dress the topping mix

Supplies:

• Sand and scrap plywood to make a box in which to lay out the mosaic's design. (Or spread the sand on a plastic tarp.)
• Mosaic stones. Choose flat pebbles or cobbles in colors you like.
• Paver base
• Stone dust
• Concrete mix, such as Quikrete.
• Topping mix. Also known as sand mix, it's finer than concrete mix and contains no aggregate. Quikrete and other companies make it. You'll add it like grout between the joints after laying the stones.
• Pavers, bricks or bluestones to surround the mosaic and hold it in place.
• Plastic tarp or sheeting

1. Prep and sort the stones.

Spread out the stones near your project area. Rinse off dirt and grit, then sort stones of similar color and size into piles or buckets.

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2. Lay out the mosaic.

Spread the sand to a depth of 3 inches in a sandbox made from scrap plywood or on a plastic tarp or sheeting. Lay the stones upright in the sand in a pattern and shape you like; the sand will hold the stones in place as you work. The stones should be packed together tightly, with parallel stones touching each other in their middles. Don't lay all the stones parallel, though — varying their direction will keep them from shifting out of place. (We created a double-spiral pattern, with one set of stones laid perpendicular to the other.) When you're happy with your mosaic, take a photo of it and note its size and shape for later reference.

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3. Prep the project site.

The mosaic should sit level with or slightly higher than the surrounding ground so that water won't collect on top of it; take this into account when selecting your project site. Dig out an area at least 1 foot larger than your mosaic on all sides to a depth of 6 to 8 inches. (This will ensure proper drainage of the base and create enough space around the mosaic to add edging.) Tamp 6 inches of paver base and a 1-inch layer of stone dust into the excavated area. Pour dry concrete mix into the center of the installation spot, and use a 2-by-4 to screed the mix to an even 2-inch layer that extends 2 inches beyond the mosaic's footprint on all sides.

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4. Dampen the concrete mix.

Using a garden sprayer or a hose with a fine-mist attachment, lightly mist the concrete mix until its texture changes from a loose powder to a crumbly, dirtlike consistency.

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5. Mark the mosaic's shape in the concrete.

For a circular mosaic, tie a large nail to each end of a piece of string cut to the length of the circle's radius. Push one nail into the center of the installation spot, pull the string taut, and draw a circle in the mix with the second nail. Draw a second circle at half the radius for an additional visual guide.

Tip: One bag of concrete mix will be enough to create a mosaic that's about 10 square feet.

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6. Place the center stone.

To make the mosaic sturdy, first bury the bottom third of the stones in the concrete mix. (Later, when all the stones are in place, you'll add a finer topping mix between the joints; see step 10.) Starting in the center, use a garden trowel to dig a shallow hole in the damp concrete mix. Place the first stone on edge in the hole and pack concrete around it, holding it upright with your fingers.

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7. Lay stones alongside the center stone.

Using the trowel, dig shallow holes in the concrete mix next to the flat sides of the center stone. Set the second and third stones in place next to the first stone. Again, use your fingers to pack the mix tightly around them.

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8. Fill in the pattern.

Working from the center outward, continue laying stones as described in step 6. The joints between the stones should be tight, and the stones should touch each other in their middles. It helps to place stones of similar size next to each other and to introduce smaller or larger stones gradually, as your pattern dictates. Try to keep the height of the stones even as you lay them.

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9. Set the remaining stones.

When you're finished laying a small section, place a 4-by-4 across it and strike it with a rubber mallet to set and level the stones. Don't be tempted to substitute a 2-by-4 here — it may bounce or bow when you strike it. When the mosaic is complete, lay edging or a field of pavers, bricks or bluestones around it to hold the pattern together. Fill any large gaps in the mosaic with a bit of extra concrete mix, mist the mosaic to fully saturate the concrete beneath the stones, and cover it with a plastic tarp. Let cure overnight.

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10. Spread the topping mix.

Uncover the mosaic and make sure the stones are dry. Scatter topping mix onto the stones, working on a small section at a time, and use a paintbrush to spread the mix into the joints between stones. Leave a reveal if you like; it should be no more than one-third of the stones' height.

Tip: The more topping mix you add, the harder the mosaic will set. In high-traffic areas, fill the joints until the mix sits close to or flush with the tops of the stones.

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11. Saturate and shape the mix.

Lightly mist the topping mix until it's fully saturated. You can add more mix at this point if the first layer settled too much after you misted it. Allow the mix to absorb the water, then let it cure for 30 to 60 minutes. Use a stiff-bristled brush to dress and shape the joints to your liking.

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12. Let the mosaic cure.

Cover the mosaic with the plastic tarp to hold in moisture while it cures. For a harder cure, uncover the mosaic and mist it a few times over the course of a day or two as it begins to set. Full curing may take a couple of weeks, depending on the weather. When it's complete, buff the stones with a damp cloth to remove concrete residue.

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