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What kind of cement?
Ordinary portland cement is
perfectly strong enough for this work if properly reinforced . However, it takes days to become dry enough to
handle without crumbling, during which time any slight movement
will weaken the inherently thin structure.
Far better to use a "quick-drying"
cement. This is what builders and construction types call high
aluminum cement (or high alumina cement, depends where you live).
This is much easier to obtain. Any builders merchant will be
able either to sell you some or point you in the right direction.
It hardens sufficiently for the piece to be handled very quickly,
anything from twenty minutes to an hour.
So what else will you need?
- Tools: A small trowel, perhaps,
and some flexible artists' palette knives are useful.
- A container to mix the cement
in. It need not be big because you won't be mixing much at a
time. I use an empty 2-litre ice-cream tub.
- Rubber gloves, if your fussy
about your skin.
- Some sand of varying types for
texturing the surface as well as using in the mix. (Some brands
are supplied ready-mixed, in which case don't add more sand or
you will weaken the structure.
- And some reinforcement.
Reinforcement
Reinforced concrete derives its
strength from the combined high compression strength of the concrete
and the high tensile strength of the steel reinforcement. In
terms relevant to this discussion, imagine a slab of concrete
of any thickness, with a steel wire running through its centre.
If a force is applied to its surface, in order for the slab to
break, the wire must be stretched and the concrete above it must
be compressed. No can do - so the slab is strong. (see diagram
A).
Now, if the wire is almost at
the surface when the pressure is applied, in order for the slab
to break, the wire must be compressed and the concrete stretched.
Easy - one broken slab. (see diagram B).
Okay, so what kind or reinforcement
then?
In sculpture, it is traditional to use chicken wire with fabric
or fibreglass stitched to it as a base for the work, and to smear
the surface with layers of cement. Hit the outside and it appears
strong, simply because the wire is way below the surface. However,
if you were to hit the piece form the inside it would shatter,
because the wire is on the surface being struck, not below it.
This is fine for sculpture but not for outdoor bonsai containers.
Furthermore, fixing relatively
dense fabrics such as fibreglass over the reinforcement to support
the cement will in effect create two separate layers, which will
have no strength at all.
Ideally you need a galvanised
steel mesh with, say, a 25 mm grid and 1.5 mm thick wires - or
thereabouts. Then go find some tulle. Some what? Some tulle -
it's a very fine fibre, usually synthetic these days, woven to
an open mesh. It's the stuff that ballerinas' frillies are made
of. You can use any fine netting, but the type used for grannies
curtains is too closely woven and will weaken the piece. A 2mm
mesh is ideal.
Making the framework
- First of all you will need to
cut the mesh to size and mold it to the desired shape. It is
a good idea to get your experimenting done beforehand by cutting
paper or card patterns. Use your imagination by all means, but
try to keep the shape simple.
- It may be necessary to join
two or more pieces of mesh together. This is perfectly alright
and won't weaken the job, provided that you remove all excess
wire and loose ends. Join the pieces with very fine wire, such
as single strands of telephone cable. Too many wires bunched
together will create a weak point.
- Any free ends around the rim
should be bent to form a more or less continual line, allowing
for about 9mm of cement to be applied before the final dimension
is reached.
- Coat the wire with quick-drying
adhesive. Any kind will do so long as it will stick to the wire.
Smooth the netting over the mesh, ensuring it sticks reasonably
uniformly, especially around the edges. It doesn't need form
a really firm bonding, just enough to stay put for the time being.
Some people stitch the fabric to the mesh. This is entirely
unnecessary and mind-numbingly tedious.
- Trim the edges flush with the
mesh and wait till dry.
The cement
Mixing
Check if the product
you've got is already mixed with sand or not. If not, mix well
with an equal amount of dry builders sand, the type used for
mortar. Add water, stirring continually, until the mix is what
I can only describe as a slop! Too much water may weaken the
mix, too little makes it impossible to work properly in this
context. It needs to be only just runny, no, more. If it is
thin enough to brush on, it's too thin.
Before you proceed
The cement mix is spread onto the frame in layers, which must
be applied before the previous layer has cured - ideally before
it has even begun to change colour from dark (wet) to light (dry).
Bear in mind that making cement containers is a fairly long process,
so you'll probably have to leave the job standing overnight between
layers. This is okay if you cover the job with wet cloth and
then a sheet of plastic, to prevent the surface of the cement
from losing moisture to the atmosphere.
Apply the first layer ...
Using a trowel or palette knife, spread a layer, about 3-4mm
thick, over the netting. Press firmly to force the cement through
the holes in the netting. As the cement passes through the netting,
it spreads a little, so that once it begins to harden, it holds
fast. The same effect happens around the wire as well.
 
As the cement is applied, it
adds weight to the structure and will soon begin to bend the
wire mesh out of shape. To minimise this problem, work around
the edges first and stop as soon as the mesh begins to distort.
Wait until the first section has become firm (but not dry to
the touch) and then fill in the remaining areas, bit by bit if
necessary. Lightly abrade the surface with an old brish to provide
a key for the next layer.
... And the second ...
When the first application begins to lighten in colour, very
carefully lift the piece and turn it over, supporting it as necessary.
Straight away, begin to apply a 3-4mm coat to the second side,
ensuring that it is worked into the spaces created by the pattern
of netting and mesh. It is important that these two layers meet
and bond as perfectly as possible. If you have to do this
layer in sections, make the joints in different places to the
first layer, thus avoiding any weak spots.
As you work around the edges,
begin to extend the rim beyond the mesh framework. When the slab
is finished, the cement should extend beyond the wire by 6 -
9mm. Too much and it may break off easily. Too little and the
temperature-induced expansion and contraction of the wire may
eventually fracture the cement. (see diagram C)
...And the third ... and the
fourth ...
The next layer is applied to the side one, and can be a little
thinner, say 2-3mm. If the base layer has become lighter in colour,
spray it thoroughly a few times with water before you begin.
(Make your mix a little stiffer to compensate.) As soon as this
is firm to the touch, turn the piece over again and repeat.
That part was easy - the next
bit's trickier!
... The final layers
The final layer on each side
is the one where you will create the surface texture. You can
sculpt it with the palette knife; you can add sand or grit; you
can add fine peat; you can stipple the surface with a wire brush
or anything else that takes your fancy. You can also add a cement
dye at this stage. Alternatively, you can colour the slab afterwards
(see below).
Whatever you decide, don't
over-do it. The idea is to make the slab look natural. Too much texture or too strong a colour
will will make that already difficult task impossible. And at
all costs, resist the temptation to try to make your slab look
like a mountainside. It never works and the result will end up
as plain old kitsch. Just try to make your artificial slab look
like a real one.
Once the final textured layer
has become almost dry (lighter in colour but not as light as
the trial piece you did last week) you can colour the surface
if you wish. Use very dilute spirit-based paints or wood stains,
applied in several thin coats. This will soak into the surface
of the uncured cement and will be more or less permanent. Try
a flat layer of one colour, followed by patchy layers of other
colours - browns, greys, muted greens, even a splash of purple
here and there. But always VERY dilute, you want to colour
the cement, not coat it.
Is that it?
Not quite. There's one more thing
you have to do ... Wait!
As I explained earlier, cement
takes several weeks to cure (in fact, in theory, it never completely
'dries'. It continues to harden slowly throughout its lifetime).
You must wait at least a month - the longer the better - before
planting on your slab. The larger the slab, the longer you
should wait. Leave the slab outdoors so the rain will wash the
surface clean of colorant residues. Some recommend sealing the
cement to stop lime from entering the soil. These days there
is very little significant free lime in cured concrete. Anyhow,
if you make your slab in the autumn and leave it out in the weather
all winter, it will certainly be ready for planting come spring.
Oh, by the way, this may sound
daft, but keep the slab frost-free for a month or so. If the
tiny amount of moisture in the concrete freezes before it's cured,
hairline fractures may occur. These won't be a problem at first,
but in years to come they will inevitably begin to deteriorate.
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