How to Make Concrete Mushrooms for the Garden

1. 1

   Step 1: Gather the Moulds, Gems and Mix

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   The shopping list is short. A few plastic bowls in graduating sizes from a dollar store, a bag of flat glass gems, and sand topping mix.

   Sand topping mix is the right product here rather than a general concrete mix. It has no large aggregate in it, so it picks up the smooth inside face of the bowl and takes a drilled hole without blowing out.

   Two bowls make the cap: the big one holds the concrete, the smaller one presses into it to hollow the underside.

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   Used in this stepQuikrete sand topping mixFlat glass gemsPlastic mixing bowls

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2. 2

   Step 2: Mix the Concrete Stiff

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   Mix it stiffer than feels right. Somewhere around thick oatmeal.

   A wetter mix is easier to pour and it cracks as it dries. That is learned the hard way, and there is no fixing a cracked cap after the fact.

   Tip:

   Mix a full batch for the cap in one go. A second batch mixed to a slightly different consistency shows as a colour difference in the finished piece.

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   Used in this stepConcrete mixing bucketMargin trowelNitrile gloves

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3. 3

   Step 3: Fill the Bowl Mould

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   Trowel the mix into the largest bowl right up to the rim.

   Then press the smaller bowl down into the middle. That displaces concrete up the sides and leaves you a domed cap with a hollow underside, which keeps the weight down.

   Tap the sides as you go to bring the air bubbles up.

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   Used in this stepMargin trowelPlastic mixing bowls

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4. 4

   Step 4: Release the Cap

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   Once it has cured, work the plastic bowl off. Tap around the outside and it should let go - the inside of a plastic bowl is smooth enough that concrete does not grip it hard.

   Wipe the bowls with cooking oil or wax before you cast next time. It takes ten seconds and turns a fight into a clean release.

   The demolded surface comes out surprisingly smooth.

   Tip:

   If a bowl really will not release, it is cheap enough to cut. Do not lever against the concrete edge and chip the cap.

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   Used in this stepCooking oil sprayConcrete mold release

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5. 5

   Step 5: Drill the Gem Holes

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   Mark where you want the gems and drill each spot with a masonry bit.

   A hammer drill and a masonry bit make short work of it. A regular drill and bit will get there too if you take your time, though it is hard on the bit.

   Concrete this young is not fully hard yet, which works in your favour.

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   Used in this stepMasonry drill bit setCordless hammer drillSafety glasses

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6. 6

   Step 6: Set the Glass Gems

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   Press a glass gem into each drilled hole, flat face out.

   Use two sizes if you have them and scatter them rather than laying out a grid. Random reads as organic; a grid reads as a bathroom tile.

   Push each one down so it sits proud of the surface but seated in the hole.

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   Used in this stepFlat glass gemsGlass gem assortment

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7. 7

   Step 7: Skim a Second Coat Over the Cap

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   Mix a small second batch and smear it across the whole cap, working it around every gem.

   This fills the gaps at the edge of each hole and locks the gems in. Without it they sit in loose sockets and will eventually work free.

   Wipe the excess back off the gem faces with a damp sponge before it sets. A sponge is far easier than a towel here.

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   Used in this stepQuikrete sand topping mixGrout sponge

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8. 8

   Step 8: Cast the Stem

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   The stem is a styrofoam cup filled with concrete around a short piece of pipe.

   The pipe makes a channel through the middle, which is how the solar light wires get from the cap down and out of sight. PVC offcut works; so does anything tubular.

   When it is cured, tear the foam cup away. That is the easiest release of the whole project.

   Tip:

   Push the pipe in before the concrete stiffens and check it is vertical from two directions. A leaning channel makes the cap sit crooked.

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   Used in this stepStyrofoam cupsPVC pipe offcutQuikrete sand topping mix

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9. 9

   Step 9: Paint It

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   White base coat first, then the colour. Concrete is grey and drinks pigment, so a colour sprayed straight onto bare concrete comes out muddy.

   Classic toadstool red over white is what makes it read as a mushroom from across the garden. Spray paint is fine outdoors; use an exterior product.

   Paint will get on the gems. That is expected and it comes off in the next step.

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   Used in this stepExterior spray paintWhite primer spray paintConcrete sealer

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10. 10

    Step 10: Clean the Gems and Fit the Lights

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    Wipe the overspray off each gem with lacquer thinner. A sponge with a scouring pad on one side is the fast way - the sponge holds the thinner and the rough side lifts the paint.

    Then push the LED string into the drilled holes, one light per gem, and run the wire down through the stem channel.

    Twenty lights is not many. Thirty or forty looks better if your string will stretch to it.

    Tip:

    Seal the concrete before it goes outside for a winter. Unsealed concrete takes on water, and water that freezes inside a cast piece cracks it.

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    Used in this stepSolar LED string lightsLacquer thinnerScouring spongeConcrete sealer

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Keep going

How to Make Terrazzo

10 steps · about 10 minutes

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