In this Instructable you'll learn a very quick and easy way to make your own concrete fossil (leaf impression). Leaf impressions can be nice details in larger-scale concrete work, or you can follow these instructions as-is to make stepping stones for the backyard or garden. If you have a big project to pour, this is something easy for the kids to play around with and it keeps any leftover concrete from going to waste.

Materials:

Fresh Leaf
Modeling Clay
Smooth, Flat, Waterproof surface (a scrap piece of 3/4" melamine was used, but thick plastic will work too)
Sacked Concrete
CHENG Outdoor Pro-Formula Admixture

Tools / Supplies:

Spray Adhesive
Diamond Sanding Pads / Sandpaper
Plastic Bag to cover while curing

Concrete Mixing:

Bucket
Water
Particle Mask
Rubber Gloves

Step 1: Step 1: Make Clay Wall

Clean up the surface you'll be casting on with denatured alcohol. Any dirt, scratches, imperfections and texture will be visible in the finished piece of concrete.

For a very smooth finish, you might try casting on a piece of glass or acrylic. Any surface that won't absorb water will work. If you want to use something like wood, it has to be sealed first or the wood wick the moisture away from the concrete and the surface will be clouded and rough.

Roll out a few 'snakes' and form up the walls by pinching the clay between your fingers and pressing down. It's important that the clay has a good bond to the casting surface. This technique will only work with relatively short walls - anything too tall and the pressure of the concrete will cause the walls to blow out.

Step 2: Step 2: Glue Leaf to Base

Liberally spray the 'back' of the leaf with spray adhesive, wait a few minutes for it to get tacky, and then press it down somewhere inside the clay wall. Don't spray the adhesive directly on the casting surface - it will leave a rough texture.

Try to pour the concrete as soon as possible after the leaf is stuck down. The longer you wait, the more likely the leaf will be to peel up.

Avoid using dry leaves because they will suck moisture from the concrete. I used one in this example, but would have had better results with a fresh leaf.

Some leaves will give a better impression than others, so just experiment. The leaf used here is from a great Sequoia sempervirens that thrives behind the concrete workshop here in wonderful Berkeley, California. Try using leaves from the backyard, local park, or your favorite forest. Part of what makes this project fun is foraging for leaves and exploring along the way.

Step 3: Step 3: Mix + Pour + Vibrate Concrete

Mix up some concrete! I added CHENG Outdoor Pro-Formula to Sakrete 5000+ bagged concrete. Outdoor Pro-Formula is an ad-mixture that contains pigment (comes in 6 colors), water reducer, and is air-entrained to help it stand up to the freeze / thaw cycle outdoors.

For tips on mixing concrete, check out the How-To-Mix Concrete Instructable.

Press the edges of the leaf down one last time before pouring concrete in the form. If the concrete seeps under the edge of the leaf, you might only get half of a leaf impression.

Take a handful of concrete and drop it into the form. Smear it around the casting surface (this will help minimize air bubbles on that surface) but don't hit the leaf. Be careful not to bump the clay walls in the process, and try not to over-fill the form.

Vibrate the concrete by pounding on the table and by gently dropping one edge of the base down on the tabletop from a few inches (this is called drop compaction). When the air bubbles come to the surface, wipe across them so they pop and smooth out the top surface as much as you can.

Step 4: Step 4: Cover + Cure

Cover the concrete with painter's plastic or a plastic bag.

Place the form on a level surface, NOT in direct sunlight, to cure for 3 or 4 days. You can always add Rapid Set to your concrete mix if you want to de-mold your pieces the next day.

Step 5: Step 6: Remove Clay Walls

Pry the clay walls back with your fingers and peel them away. The clay can be re-used, so try to remove any pieces of concrete.

The concrete piece should come loose from the casting surface at this point.

Step 6: Step 7: Remove Leaf

Now is the tedious step of removing the leaf from the concrete.

The grey area around the leaf is the result of the leaf sucking up moisture. Using a fresh leaf will solve this problem.

Use a knife, razor blade, or push pin to remove the old leaf. You're actually breaking away thin pieces of concrete and digging out the leaf below.

Work deliberately and patiently like any good archaeologist.

Step 7: Step 8: Sand + Finish

Sand any rough edges with diamond sanding pads or sandpaper. Diamond pads will be more effective, but sandpaper can work too.

The finished concrete can be ground, polished, and then protected with CHENG Sealer and Wax.

Using clay to build form walls does have some limitations beyond a small project like this, but it gives a range of freedom that you don't normally find in concrete formwork.

Have a few forms ready to pour at the same time and with just a little bit of work you'll quickly be on the way to making a unique garden pathway.