Mold-Making.com

 


 


HOW TO MAKE LIGHTWEIGHT, HOLLOW,
ARTIFICIAL ROCKS

One way to make fake rock from concrete


Finished artificial rock
The finished artificial rock in the garden

This technique shows one way to make relatively lightweight, freeform artificial rocks and boulders. By "freeform" we mean that the rock is not an exact duplicate of anything. You give the rock the shape you want, as you make it.

If you want an exact duplicate of an existing rock, you can make a mold of the rock and use the mold to cast concrete.

Making a lightweight artificial rock or boulder requires at least several hours of your time, plus materials. The pictures accompanying this technique show a rather small rock because it was faster to make and easier to take pictures of. As a practical matter, you would be better off just buying a rock if you only wanted a small one like this. Except - making small rocks is good practice before you make larger ones!

Making your own artificial rocks or boulders is practical for the following types of situations:
  • If your landscape is already planted, and you want to add rocks for interest. You can make rocks that fit the existing plantings, which will make the plantings look as if they grew around preexisting rock.
  • If you want rocks or "outcroppings" that are big enough so that a real rock would be too expensive to buy and/or move to your location. Remember that for a large-size real rock outcropping, most of the rock is hidden underground, and you probably only see the tip of the iceberg. If you make your own outcropping, you only need to make the tip of the iceberg!
  • If you want rock outcroppings or boulders in your landscape, but want the option to move them at some later time, without requiring a fork lift.
  • If you want rock outcroppings that have planter holes. This can give the natural effect of plants growing in rock crevices.
  • If you want to add interest to your mailbox post or other types of structures. You can either make the rock in place around the mailbox post, or make the rock with a hole that the mailbox post can then go through.
  • If you want to make rock walls, rock waterfalls, or rock liners for a pond. If you live in a city with a zoo, take a look at the natural-looking rock walls you see there. They are all really just concrete, made with techniques very similar to what we show here, but with more sophisticated equipment.

Materials needed:

  • Portland Cement
  • sand (builder's sand, mason's sand, play sand, "all purpose" sand, etc.) Most "play sand" is easier to use for this technique because it usually has fewer pebbles. Other types of sand may have a lot of pebbles that make working with the concrete more difficult.
  • hardware cloth with 1/4 inch mesh
  • leather gloves
  • plastic self adhesive shipping or packaging tape, 2" or 4" wide
  • tin snips or wire cutters
  • cement trowel or mason's trowel (steel trowels are much better than plastic)
  • mixing bucket or mortar tub
  • plastic sheet or plastic garbage bags
  • disposable paintbrushes
  • powdered concrete pigment (brown or black)
  • exterior latex paint or masonry paint

Procedure

The first secret to making an artificial rock is that you don't need to make a bottom for the rock. The rock will really be just a "shell". Unless you expect people to come into your yard and turn over all your rocks to check, nobody will know that the rocks don't have a bottom!

Underside of the artificial rock
The underside of the finished artificial rock

The most effective way to use rocks and boulders for landscaping is not to scatter them around so they are sitting "high and dry", but to bury the edges in soil so that the rock appears to be an outcropping. You can make completely enclosed rocks if you want, but ordinarily you should just make a shell and save time, materials, and weight.

We will make the framework for this shell from hardware cloth. Hardware cloth with 1/4 inch mesh is better than "chicken wire" or "poultry netting" for the following reasons:

  • It has smaller openings, which makes it easier to get the concrete to stay in place on near-vertical surfaces.
  • It is not as "springy" and holds the shape you give it.
  • It makes a more rigid framework, which is important when you start applying concrete.

1) If you will be making a freeform rock (not one built "in place" in your yard), you need a workspace. You will be making a mess, so cover everything you care about with plastic sheet. Your work area should be one where you can make the rock, and NOT need to move the rock for several days thereafter.

It is very helpful to use a workbench or table that you can walk around and get at your rock from all sides. Or you can use a swivel or heavy-duty lazy Susan on a table, then put your rock on a board on top of the swivel. Any movement of the rock while you are making it, such as trying to rotate a rock that is sitting directly on a tabletop, will loosen and damage the concrete you have applied.

Your workspace temperature should be at least 60 degrees F. Do not try to make concrete outdoors if there will be freezing weather in the next several weeks. Concrete has a lot of water in it during the curing process, and freezing will destroy the concrete.

2) This step is only necessary for making large rocks, more than about 18 inches wide or long: build a simple framework out of scrap lumber, so that the hardware cloth can be supported every 12 inches or so. If you are building an outcropping "in place", you can just pound stakes into the ground to form a framework. You could also use cardboard boxes or empty tin cans, etc. When your rock is made, the framework will be completely hidden.

You may want to dig a shallow trench, 2 or 3 inches deep, around the outline of your outcropping. When you make your rock, you can extend the hardware cloth into the trench, so your rock extends below ground level instead of just sitting on top of the ground.

3) Cut a piece of hardware cloth big enough to make your rock, or several pieces if your rock is wider or longer than a roll of hardware cloth. Wear leather gloves whenever working with hardware cloth. You will almost certainly cut yourself if you don't wear gloves!

4) Cover one side of the hardware cloth with plastic shipping tape. When you apply concrete to the framework, you want the concrete to "lock in" to the mesh of the hardware cloth, but you don't want the concrete to fall through to the inside of the shell. The plastic shipping tape on one side lets the concrete lock into the hardware cloth, but not fall through.

Applying the plastic shipping tape
Plastic shipping tape is applied to one side of the hardware cloth

5) Form your hardware cloth freehand, or shape it over your framework, or use an existing rock as a form to help shape the hardware cloth. Bend excess hardware cloth at the corners and fold it back on itself, similar to gift wrapping a package. Crease the bends so they stay put and don't stick out from the shell too much. Use wire ties on these parts if you need to.

A real rock used as a form
A real rock was used to help form this hardware cloth shell

If making a rock on a framework, bring the hardware cloth down into your trench. Fasten the hardware cloth to the framework with wire, staples, or nails. If making the rock freehand, shape the hardware cloth so the shell can sit flat on your workbench or floor.

Remember, the shipping tape should be on the inside of the shell. The hardware cloth should be exposed on the outside of the shell. After forming the shell, check the shipping tape to make sure it is still stuck against the underside of the shell. Apply more if necessary. Cut off any shipping tape that may now be on the outer surface of the shell where you made bends - you want the hardware cloth exposed on the outer surface.

Your shell should have a slope from the base towards the top of the rock. A perfectly vertical slope is acceptable in some spots, but it will be trickier to get the concrete to stay put. You should only make "overhangs" on rocks not built on a framework. The concrete will not stay put on the underside of an overhang, but with a freestanding rock you can apply all the concrete you can to most of the rock, let it harden, and then turn the rock so the underside of the overhang is as near horizontal as you can get. Then apply concrete to that area and let it set before turning the rock "right side up".

Hardware cloth shell
The hardware cloth shell, ready for concrete

6) Mix your concrete. First, add 1 part Portland Cement (by volume) to 3 parts sand. Mix these dry ingredients together very thoroughly. Then SLOWLY add water, a little at a time, and mix the concrete completely before adding more water. You want your concrete mix to be very much like brown sugar. Brown sugar is moist and sticks together. It is not dry and crumbly. It is also not a liquid. No amount of shaking or vibrating can make brown sugar liquefy. Your concrete mix should be the same. You should be able to trowel it against the vertical sides of your mixing bucket, and have it stay put.

If you accidentally add too much water, and the concrete won't hold a shape, but slumps down like mud, just add more cement and sand in the same 1:3 ratio and mix the concrete again.

If this is your first time mixing concrete, start with a mix of 1 cup Portland Cement to 3 cups of sand. This small batch will be relatively easy to mix in a bucket, using the mason's trowel. To mix concrete with a mason's trowel, don't "stir". Instead, "cut" the cement over and over while rotating the bucket. "Cutting" cement is similar to the cooking technique of cutting butter into flour.

If you have experience or are making a large rock, you can mix all the concrete you want in a mixing tub or wheelbarrow, with a hoe.

The right concrete consistency
The right mix of concrete is moist, holds its shape, and does not slump

7) Apply your concrete by pressing it onto the hardware cloth shell with your trowel, STARTING AT THE BOTTOM. It is very important to start at the bottom and work entirely around your shell, making a 2 or 3 inch high row of concrete, about 3/4 inches thick. This first row of concrete will support the next row, and so on until you get to the top.

Mix as many batches of concrete as it takes to cover your shell. Work on one 2 to 3 inch high row at a time, all the way around your shell, before going higher.

Each time you trowel more concrete onto the shell, use the trowel with a pressing and sliding motion to pack the concrete firmly to the shell, and also connect the fresh concrete to the concrete that you have already laid. Even if you have never worked concrete before, you will catch on to this technique quickly.

After the first row
After applying the first row of concrete mix

Concrete mix will constantly fall down the sides of your shell and collect at the base. Don't worry about it while you are working on a row. When you finish a row, use your trowel to bring the concrete at the base up over the side of the shell, similar to spreading icing on a cake.

Mix your concrete several times while applying a batch. With a "dry" concrete mix like this, it is common for some unmixed sand or cement particles to remain hidden at first. If you should happen to apply any concrete that shows dry sand in it, remove it, put it back in your bucket and re-mix. Dry sand is totally worthless in your rock and could create a void or weak spot.

After the second row
After the second row

After you reach the top and the shell is completely covered, it is time to add some character to your rock. At this point it may look like a “football helmet” - too even and smooth to be real.

Add character by applying more concrete in lumps. Do not cut down or otherwise weaken the concrete that makes the basic shell. Only add more concrete in spots, and decorate only the additional concrete. You can form rounded or rough domes, or make plateaus, or form irregular stairstep patterns like slate, or cut fissures into the additional concrete. Study some real rocks and try to see what it is that makes a rock look like a rock. Then recreate it in concrete.

After the shell is covered
After the shell is covered and some "character" applied

8) Once your rock is completely formed, you need to let the concrete cure for at least 24 hours before continuing.

Concrete does not cure by "drying out". Concrete cures by a chemical reaction, and the chemical reaction requires water. Make sure your freshly formed rock is damp. You can use a spray bottle of water to mist the rock, or a fine mist from a garden hose. Don't wet it too much or the concrete may sag. When the concrete is evenly moist, cover the rock with plastic to keep the water from evaporating.

9) After 24 hours, your concrete will be hard, but it is not cured enough to move. Any movement or lifting at this point could easily cause the concrete to crack. Leave the rock right where you made it while you continue.

You will notice that the concrete looks very "grainy", almost like sandstone, and it may have some cracks or fissures. Now you are going to fill in all the pores and protect the rock against water seeping in, where it could damage the concrete during freezing weather. Plus, this step adds a "base coat" of color so you can start making your rock look realistic.

Add 1 tablespoon of dry concrete pigment to 1 cup of dry Portland cement and mix them together thoroughly. Then add enough water and mix so that the cement has a thick liquid consistency, like a thick paint.

Brown or black pigment works well for this. This color will be the base coat of your rock, underneath everything else you may apply. Using brown or black at this point will make it easy to get the appearance of a real rock. Apply this cement mix to your rock with a brush. Let it flow into and fill in all the pores and fissures. Cover the rock completely.

After the top coat
After applying a coat of plain cement and pigment

At this point your rock may be a little too smooth. It is more difficult to make a smooth rock look realistic than one that has some texture.

Add texture by using your brush to stipple the cement mixture you just applied. You can also dust the surface of the rock with sand, or use both techniques.

Now cover the rock with plastic again, and leave it alone for at least 3 days. Check under the plastic every day and mist the rock with water if it looks like it is drying out. After 3 days the concrete should be cured enough so that you can move your rock without damage.

10) After 3 days you can give your rock the final decoration. This can actually be very easy. It is very similar to "dry brushing", so you may want to read about that technique. Your rock should already have a brown or black base coat, from the pigment you applied in the last step.

To make your rock look like granite, simply mix a white exterior latex paint or masonry paint with enough water to make it thin and runny. Pour this mix over your rock, or put it in a spray bottle and spray your rock. The "high side" of the texture you applied in the last step will catch and hold the paint. The "bottom side" of the texture will stay brown or black.

A basic granite look
A basic "granite" look. A little spatter painting will improve it.

If you get "runs" of paint that don't look natural, you can dab at them with some crumpled newspaper or a grocery sack. Don't use a paper towel or cloth - you don't want to smear the paint, just try to stipple it until the runs disappear and you get a mottled look.

If you like the look of your rock, you're done! If not, you can stipple or spray on other colors such as black, pink, green, or yellow, but they should usually be used sparingly. And you can use an old toothbrush to "spatter paint" specks of color wherever you want.

Too much paint on this rock
The paint is too obvious. A sprinkle with water and some dabbing with crumpled newspaper will help.

If your painting effects don't look realistic, get out your spray bottle or garden hose and give the rock a sprinkle. Many times just running the colors together a bit makes all the difference. And if you're just not happy with the effect, use a jet spray nozzle on your garden hose to wash off everything and start over!

Remember that this is just paint, and you can change it. If you continue making rocks and discover how to make a certain "look" that you really like, you can always come back and repaint your previous attempts.

Advanced techniques

  • When you make your rock, you can provide planting holes, or holes for inserting pots or mailbox posts. Do this by cutting the hardware cloth shell where you want a hole, and inserting any sort of tube such as a cardboard tube or section of PVC pipe. The tube should extend to the ground or your work surface. Tape the underside of the tube to the hardware cloth so the tube helps support the hardware cloth. Tape around the part of the tube above the hardware cloth so the cement won't stick to it. Don't remove the tube until your concrete has cured for several days, or you risk cracking the concrete in that area.
  • You may want to make rock shapes that can't be done with just a single piece of hardware cloth. To do this, make a basic shape from hardware cloth, then add smaller shapes on top of the base shape. You can form the smaller shapes by bending hardware cloth around smaller rocks or pieces of lumber. You can create a "rock ledge" by forming hardware cloth around 1 x 6 boards, then stacking the hardware cloth in a stairstep fashion. Fasten the different hardware cloth pieces together with wire or wire ties.
  • If you want a rough duplicate of a real rock face, but don't want to make a huge mold, you can use latex rubber to make copies of the sections of a rock face that are particularly interesting. Make a basic rock outcropping frame from hardware cloth, and apply a shell of concrete. Then firmly tamp more of the same concrete mix into your rock face molds. Turn a mold over on top of the concrete shell, and peel off the latex mold. Connect the concrete from the mold to the shell with your trowel. As long as both batches of concrete are fresh and moist, they will bond together.
  • Another way of making a rough duplicate is to take pieces of hardware cloth to a real rock outcropping. Form the hardware cloth to the rock. Pounding with a rubber mallet can be helpful for this. Then use the pieces of hardware cloth to make the shell to construct your own rock face. Taking photos of the real rock face can help a lot when it comes time for you to recreate it in concrete.

Download a PDF source list

Includes 26 suppliers of all mold making and casting materials mentioned in ALL the demonstrations on this web site, including silicone rubber, polyurethane rubber, latex rubber, polyurethane plastic, epoxy resin, polyester resin, molding clay, foam board, release agents, pottery plaster, Hydrocal, metal powders, other plastic resin fillers, and gelatin molding compound. With company names, addresses, phone numbers, and web sites. Downloads to your computer as a PDF file.



More Techniques
castcraft.com
vacuum-forming.com
moldmaking and casting books and videos
privacy