Create garden pots that rival expensive European antiques by using a round rock to pound a mixture of cement and sand into a sturdy form. Known as "dry-casting," this technique depends on adding only the minimum amount of water. You need to be able to get the mixture to clump, but it cannot sag out of shape. Pounding the mixture into the form produces a glorious surface on the molded face, one more like sandstone than concrete. Because the cement never gets wet enough to create a paste, there's little point in adding pigment to this type of project. The sand color predominates. It's worth searching out tan or yellow sand to give your pots a special look. Even fine details of the mold transfer beautifully. Planters made this way are quite porous. Put plants in a plastic pot and set that into the concrete planter.
1. Find or build a mold to support the exterior of your planter. If the mold flexes, embed it in sand. We used a 20-inch-wide
plastic garden pot and filled parts of the base ring with four rectangles of plasticine clay so that the remaining sections
would become feet on the finished pot. To create a drain hole, drill through the bottom of the mold and insert a short piece
of dowel so it fits tight (tape it from the bottom to keep it in place). Spray the inside of the mold with cooking oil and
wipe off the excess.
2. Wearing a mask and gloves, combine 2 parts sand and 1 part cement. Incorporate plastic fibers (sold as reinforcing for concrete) if you wish. Work in just enough water so you can squeeze the mixture into a ball. Starting at the bottom, pound handfuls against the mold with a round rock. Aim for a layer about 1 inch thick. For especially large pots, walls can be up to 2 inches thick. Cover the pot and keep it moist.
3. Free the bowl from the mold after two days. Invert the mold and tap it with a mallet to free the pot. You may have to hit the mold rather aggressively; aim for the base, not the walls. Remove the drain dowel by drilling through it with an undersize bit; pick out remaining wood with pliers.
4. If the top edge looks ragged, smooth it with a rasp and a sanding sponge. Smooth the interior, if necessary, with the sanding sponge, scrapers, or other tools.
See Sunset's Decorative Concrete book.