Hypertufa How-To

The empty cardboard box outer molds

Most of the time that I make troughs, I'm casting using plywood inner and outer molds, or at least a solid inner core, so that I can achieve a consistency in my results, but I think most people start out making their first troughs using the old-fashioned cardboard box method.  I've made my fair share of troughs this way and really like the lumpen sorts of personalities that results from this low-tech process.  This weekend I set out to make a couple small troughs this way and to document the process as I went. Excuse the cavelike quality of the photos but my garage is a cave partially filled with kids toys.

I started with some small cartons from the stack that I keep in the garage (I'm always on the lookout for a good box).  I think the largest carton was 14" long.  These became the outer molds.  Since I wanted the trough walls to be about 2" thick, next I searched my stack for cartons that were 4" less in length and width than the outer cartons.  I got lucky with the square mold on the right and found the exact carton I needed.  The rectangular inner core I had to cut down from a larger carton that only had one dimension correct.  I then filled these inner cartons with sand to keep them from collapsing during casting.  In the top photo you can see the small 2" pieces of cardboard tube that I cast into the troughs for drainage.

Screening the peat moss

I use a standard hypertufa mix of three parts screened peat moss, three parts perlite, and two parts Portland cement, with alkaline-resistant fiberglass fibers added into the mix as well.  My standard unit of measurement is a gallon plastic pail.  I use a screen made of 1/2" hardware cloth to sort out the obnoxiously large pieces of twigs and such from the peat moss. 

Adding perlite

Adding Portland cement

 Next is a small handful of fibers

Dry ingredients prior to adding water

A crucial step at this point is mixing the dry ingredients very, very thoroughly before adding any water.  It's so much easier to get a homogenous mix at this point than it is when the mixture is heavy and wet.

Next I add just under three parts water (the amount of water can vary widely depending on the ingredients). I like the mixture to be on the dry side.  I think it reduces slumping and the hypertufa cures stronger.  I always add the last bit of water in increments to avoid overdoing it.  To test, I make a ball in my hands and make sure it sticks together, but with very little water squeezing out.  

The mixture should hold its shape when mounded

Starting to fill the molds

Once the mixture is ready to go, I use my hands or a trowel to pack it into the bottom 2" of the outer cartons.  When the hypertufa is packed to the top of the 2" cardboard tube pieces (the drainage plugs) I know that I'm at the correct height for the bottom.  It's important for the trough's final strength to compact the wet mixture as tightly as possible, eliminating the air pockets.  I've reinforced the cartons with tape to prevent them from failing as they get wet and I jam them full of hypertufa.

Just after adding the inner core

Once the bottom is full I place the sand-filled inner core in and continue to fill up the sides. 

 I use a 1x2" piece of lumber to compact as I go.  

The filled molds

After I've packed the hypertufa as much as I can the cartons are usually saturated and at their breaking point.  I don't worry that the cartons have "bellied" or even started to rip out at the bottom corners, like the one on the left.  That all adds personality to the final pieces.

Filled molds sealed in plastic

I made sure to cast these pieces on a large sheet of plastic, which I just folded up over them when I was finished.  The hypertufa cures to full strength in about 30 days and needs to sealed during that time.  The mixture is pretty weak the first day but can be handled and worked after 24 hours. Use caution as I've broken them through careless handling at this point in the process.  I like to be a little rough as I texture them so I waited two days before opening the plastic to continue.

The next post will detail the demolding and finishing of these pieces.