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HOW
TO MAKE LIGHTWEIGHT, HOLLOW,
ARTIFICIAL ROCKS
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The
finished artificial rock in the garden
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This
technique shows one way to make relatively lightweight, freeform
artificial rocks. 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:
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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!
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The
underside of the finished artificial rock
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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.
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Plastic
shipping tape is applied to one side of the hardware
cloth
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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.
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A
real rock was used to help form this hardware cloth
shell
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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".
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The
hardware cloth shell, ready for concrete
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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.
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The
right mix of concrete is moist, holds its shape, and
does not slump
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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.
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After
applying the first row of concrete mix
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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
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.
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After
the shell is covered and some "character"
applied
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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.
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After
applying a coat of plain cement and pigment
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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.
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A
basic "granite" look. A little spatter painting
will improve it.
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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.
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The
paint is too obvious. A sprinkle with water and some
dabbing with crumpled newspaper will help.
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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.
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