...is the name I give to a process which repurposes common trash items as sacrificial mold elements. Although it involves the creative reuse of trash, the process is not strictly concerned with recycling, as the mold material ultimately ends up as waste anyway. Rather, the trash provides a freely-available, spontaneous "found" form which, treated by the aesthetic choices of the caster, can serve to create a beautiful and/or practical work of art. Here the process is demonstrated simply by casting a set of three matching flower pots from a cast-off styrofoam packaging insert. Such inserts, in general, are a wonderful source of forms for use in this process.
These four pieces of 1" PVC pipe are approximately 2.5" long. Pushed through the bottom layers of packed, wet concrete, the pipe sections will serve as mandrels to form the drainage holes in the bottoms of the pots. There are four of them because I'd originally intended to use all four recesses in the styrofoam block and make four pots.
Concrete should be mixed in a 1:1 ratio of aggregate to casting volumes, i.e. you need as much dry aggregate as it takes to completely fill the volume of the finished form. In this case, only 50 lbs of dry aggregate (play sand) were available, which proved sufficient to completely fill only 3 of the 4 recesses in the styrofoam block. Because the concrete formula calls for 3 parts sand to 1 part cement, and because the 4 recesses were identical, the form itself provided the means for measuring out the appropriate volumes. With 3 of the 4 recesses filled with sand and 1 filled with cement, the correct mixture of concrete in a proper volume was easily obtained.
The concrete was mixed by hand in a wheelbarrow. First, the dry ingredients, measured as above, were added and mixed together thoroughly. Only after the dry ingredients (sand and cement) were thoroughly comingled was the 1/2-part water slowly worked into the mixture. This, again, was measured out using the styrofoam block.
With the wet concrete in hand, a 2.5" layer was packed into the bottom of each of three recesses in the styrofoam block using an improvised tamping tool (blue-green rod, above). Any flat-ended rod of about 1" diameter is appropriate for this purpose. These layers will form the bottoms of the three finished pots. Once the tamping was complete, a PVC mandrel was pressed through the wet concrete in each recess to form the drainage hole. These will be knocked out of the dried pots later.
The spaces between the inner and outer molds were packed to overflowing using the tamping tool. During this process some concrete will fall into the inner molds; as long as not too much is wasted, this does no harm.
Using the cheap stainless steel knife pictured, the excess wet concrete was trimmed flush with the tops of the molds.
After trimming, 3 hours were allowed for the concrete to harden, and then the inner molds were carefully lifted out and discarded.
The longer concrete is exposed to water during the curing process, the harder it gets. With the wet pots complete and the inner molds removed, a soaking-wet towel was gently wadded into and draped over the hollow concrete forms. Note here that a towel without coloring agents is preferrable to a towel that has been dyed (see below). The towel and mold were then enclosed and tightly covered by a heavy black plastic garbage bag, to seal in moisture, and left undisturbed for a week.
A week later, the plastic and towel were removed. Some bleeding of color from the towel to the concrete was observed, and although this was later easily removed with a paper towel and mild bleach solution, clearly an undyed towel or other cloth is to be preferred.
After removing plastic and towel, the foam outer mold was slowly broken away using the hands and various simple tools. This was the first time I had a glimpse of the finished forms, and was a very satisfying process. The color gradient visible in the lower photograph is due to uneven moisture content and disappears after a few hours' drying in the sun. All was going well until I had the bright idea to hurry things up by dissolving the foam in gasoline.
The mold block was transferred via wheelbarrow to a well-ventilated outdoor area and approximately 1 gallon of gasoline poured over the central column of the styrofoam block. While this operation did have the effect of removing the foam and freeing the three pots, the foam did not dissolve cleanly, leaving a greenish insoluble goo on the surface of the pots that necessitated much effort to remove even partially. While gasoline or acetone might work to dissolve the foam if one were equipped with a recirculating parts washer, in the absence of such specialized equipment far too much solvent is necessary to completely wash away all traces of polystyrene from the surface. Needless to say, this technique is not recommended. It's best just to break the foam off manually.
When the pots were finally clean and dry, the mandrels were knocked out of their bottoms to form the drainage holes using a hammer and steel punch, as shown above. The PVC pipe itself may or may not stick in the hole; the pot functions fine either way.
And here's what the pots look like when they're finished and planted with some nice succulents. Remember that while you may not have access to exactly the same bits of styrofoam as I did, this process is hightly adaptable and can be used with just about any styrofoam packaging insert having a recessed volume. Very often the shapes of these inserts are extremely convoluted and interesting. What you get depends on how cleverly you exploit your starting material, and that's the really interesting challenge of the process.