Q: How could I make sculpture directly in concrete, without having to go through modeling a form, making a mold, and casting the concrete?
A: Making concrete sculpture directly is a more flexible method than modeling and casting, and enables one to achieve lighter, stronger, and larger pieces; but it is not necessarily going to be quicker, if one intends to do a good job. Making the armature correctly takes time and effort. Start with rebar, or “reinforcing iron rod” if you prefer, bent to shape and welded together to define the basic structure of the intended piece.
This framework is covered as tightly as possible with steel mesh—I prefer the black, unwelded, woven 14 gauge wire type which forms ¼ inch squares or parallelograms. It will smoothly cover a compound curve if the radius isn’t too small. This is (laboriously) tied on as tightly as possible with pieces of tie-wire that are threaded through from the front surface and twisted together, then pushed below the surface.
Alternatively, you can use three layers or so of stucco netting instead of the heavy mesh, offsetting the layers to achieve the effect of a ¼ inch mesh. At this point I like to take the armature to be hot-dipped in a bath of molten zinc. This prevents the armature from rusting under the concrete, which has bad effects from staining to actual shedding of the concrete, since rust takes up more space than metallic iron.
In some cases, it is possible to fill the armature with a light but stiff mix of cement, water, and vermiculite, perlite, or some other lightweight aggregate. This makes the application of the surface mortar easier, as it will have something to adhere to on the back side, instead of falling through. Other configurations will permit working on both sides at once. If neither of these solutions is possible, it sometimes helps to add fiber to the first layer of mortar—various proprietary polymer fibers are available for this purpose.
Mix your mortar using plastic cement or adding lime to achieve the correct degree of stickiness—try 1 part (by volume) plastic cement and ½ part mortar clay to three parts mason’s sand, or 1 part portland cement, ½ part mortar clay, three parts sand, and ½ part lime—but feel free to vary the proportions somewhat to achieve the proper working consistency. Colorants, which must be lime-proof pigments, are added at this point. Premix the dry ingredients in a wide mixing tray using a hoe, then add water, or water plus admix, using as little as possible to wet the mix. It should be stiff enough to spread on the armature and stay put, without slumping down.
Apply a ½ inch coating to the entire form, then scratch the surface all over after it has started to set, promoting the adhesion of the final layer. This should be applied as soon as possible after the “scratch coat” has set—the next day is best. If you wait too long you may have to coat it with concrete adhesive before putting on the final layer.
The last layer forms the surface of the piece, so it should be applied carefully to about the same thickness as the scratch coat and finished by troweling as it starts to set. This pushes projecting particles of sand down into the surface and leaves it smooth, with mostly fine particles showing. While the concrete can be carved a bit as it sets, this will disrupt the trowelled surface, revealing coarse sand and contrasting with the portions of the surface left undisturbed.
The concrete should then be cured for at least a week by being kept covered and damp; this is important for its ultimate strength—concrete left to dry out becomes crumbly and weak. There are proprietary compounds that one can apply that claim to seal in the water and obviate the need for damp curing, but I’ve never tried them.
An interesting alternative is to use crushed marble instead of sand in the final coat. It is available in various colors, and when ground down and polished, gives the effect of a solid terrazzo sculpture. It is possible to paint on and rub off a solution of colorant and water when the piece is still fresh, darkening the low spots. If this is to be done after the concrete has cured, one must use some cement in the solution to hold the pigment.