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Why a cement container?
As western creativity and tastes play an increasing role in bonsai design, it is inevitable that we should look for alternative, one-off, container designs. Very few have the facilities to fire ceramics, and fewer have kilns large enough to take even medium-sized bonsai pots.

Additionally, large group or forest plantings, as well as some single-tree designs, require a slab rather than a pot. Good slabs of the right thickness and shape are very hard to find.

Making them with cement couldn't be easier!


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.

QUIKRETEŽ Quick-Setting Cement (No. 1240) dries in 10 - 15 minutes, which may be a little too fast for our purposes here.  If you can't find it conveniently, or if you prefer something that dries a little slower,, here's a trick I learned: Get a bag of QUIKRETEŽ Fast-Setting Concrete Mix (No. 1004) - the stuff in the red bag that's used for anchoring fencing posts, available in all hardware stores. Sift out about a few cups full of the fines and mix this with half a bucket of your regular DRY Portland cement.  This will be enough to cause the regular cement to harden in about an hour.


What else will you need?
  • Tools: A small trowel, a putty knife perhaps, and some flexible artists' palette knives are useful. Paper towels and wet rags will also be useful.
  • A container to mix the cement in. It need not be big because you won't be mixing much at a time. A couple of old take-out food containers will do.
  • Rubber or latex gloves, if your fussy about your skin.
  • Some sand of varying types for adding texture 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. Imagine a concrete slab with steel rods running through it, just below center (diagram A). If a force applied from above is to break the slab, the steel rod must stretch and the concrete must compress. Since neither will oblige, the slab will not break.

good
                      reinforcement
Diagram A
bad
                      reinforcement
Diagram B

If the steel rods are too close to the surface, the slab breaks easily because its strength now relies on the weakest properties of both materials. The steel rods will bend and the concrete will fracture (diagram B).

What kind or reinforcement?
In sculpture, it is traditional to use chicken wire with fabric or fiberglass 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 fiberglass over the wire to support the cement will in effect create two separate layers, which will have no strength at all.

Ideally you need a galvanized steel mesh, or 'hardware cloth' with, say, a half by one-inch dimension. Then go find yourself some tulle. Some what? Some tulle - it's a very fine fiber, usually synthetic these days, woven to an extremely open, delicate gauze. It's the stuff that ballerinas' frilly things are made of. You can use any very fine netting, but the type used for grannie's curtains is too closely woven and will weaken the piece. A 2mm mesh is ideal.


Making the reinforced framework
Constructing the mesh
frame and fixing the netting will take several hours at the first attempt, depending on the complexity of shape. Don't rush this stage - make sure the shape's right before "casting it in stone"....

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.


framework                               gauze

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 color 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 gauze. As the cement passes through the gauze, it spreads a little, so that once it begins to harden, it holds fast. The same effect happens around the wire as well - it is fragile, but stable enough to stay in place until the second coat is applied.

As the cement is applied, it adds weight to the structure and will soon begin to bend the wire mesh out of shape. To minimize 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. Using blocks or wedges to support the center of the framework may help.  When almost dry, lightly abrade the surface with an old brush to provide a key for the next layer.

first layer                    underside

... And the second ...
When the first application begins to lighten in color, very carefully lift the piece and turn it over, supporting it as necessary with blocks or wedges. Straight away, begin to apply a 3-4mm coat to the second side, ensuring that it is worked into the spaces created when the first layer oozed through the gauze 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)


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 color, 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 eliminate all the sand or you can add more grit; you can add fine peat or even fragments of live moss; 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 color 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 color will will make that already difficult task almost impossible. Resist the temptation to try to make your slab look like a mountainside on your first attempt. It's very difficult and the result often ends up as plain old kitsch. Try to make your artificial slab look like a real one. Once you have gained experience with finishes and textures you can become more creative.

Once the final textured layer has become almost dry (lighter in color but not as light as the trial piece you did last week) you can color 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 color, followed by patchy layers of other colors - browns, greys, muted greens, even a splash of purple here and there. But always VERY dilute, you want to color the cement, not coat it.

finished

This slab was built using the technique described here and colored with very dilute model-makers' oil-based paints. It has been allowed to weather outside for 12 months.

Experiment a couple of weeks before you construct the slab since the color may change as the cement cures.


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.