Slowly Freezing ice Cubes

I regularly freeze large ice cubes for my bar, and the strangest phenomenon has been occurring. It is taking 2 full days to freeze the cubes. They are 2 inch x 2 inch cubes in silicone molds, 6 and 8 cubes per mold, in an upright commercial freezer at -4 farenheit. There are 8 silicone molds total. The water I'm using is bottled, Earth 2o brand, begun at room temperature. The molds are stored together in either a stainless steel pan with a lid, or a plastic container with a plastic lid. I cannot, for the life of me understand why it's taking so long to freeze. Can you offer any suggestions?
- Joshua (age 41)
Portland, OR, USA

There are basically two possibilities. Either it's taking an unusually long time to cool the water (poor air circulation?) or the water isn't freezing until it gets much colder than 0°C (32°F), i.e. it's supercooling. (See http://van.physics.illinois.edu/QA/listing.php?id=1618, http://van.physics.illinois.edu/QA/listing.php?id=1618

)My guess is supercooling. Here's an experimental test. Wait one day, so that the water should probably be below 0°C, but would take another day or so to freeze at your current rate. Then drop a little flake of ice in. Does it quickly turn to slush? If so, it's supercooled. If not, it was just cooling very slowly.

Mike W.

(published on 02/16/2017)