Sometimes, finding the answer to what seems a simple question is very difficult (whether it is gardening or really any other aspect of life). So my simple question was, why did I keep finding metal watering cans with bowed out, distorted bottoms? The answer it seems is equally simple and logical when you think about it… ice.
The great thing about metal watering cans is that they can take a bit of punishment, misuse and abuse before they become unusable but looked after they will last a lifetime. Plastic watering cans on the other hand just won’t sustain the same level of working over without becoming very difficult to repair.
Time and time again I keep coming across metal watering cans where the bottom was bowed out but no obvious reason why. Although the bowing makes the can very unstable to rest on the ground in most cast it still seems usable and on the plus side, it holds more water! I had an idea of what the answer might be but couldn’t find a definitive explanation.
So, as ever, a bit of digging around still doesn’t provide the full answer however I did come across this reference in the Haws (makers of Watering Cans) Terms & Conditions:
“3. The Guarantee on metal cans will be invalidated if the can has been left with water inside and allowed to freeze (serious and characteristic distortion of the metal body will occur in such circumstances).”
Given the “characteristic distortion” I have been seeing I think we can conclude that the cause is freezing water.
Well that was easy then – job done and the advice to offer. Don’t leave your watering cans (metal or otherwise) full of water, outside and in freezing weather.
Oh, no sorry, no, not yet my interest has been piqued. Why the bottom, not the top? We know ice expands, why is the top or sides not distorted, just the bottom?
OK, so I am just kind of guessing here based on my physics lessons so don’t take this as gospel:
- Water a bit (4 degrees C?) above 0 degrees C is at is most dense and so sinks below any warmer water.
- Once all the water above is the same temperature (and so density), the top will start to freeze.
- Our watering can is going to be sat on a surface that isn’t freezing (it is insulated by the ground and the water above it).
- As time goes on the water will continue to freeze from the top of the can to the bottom (and I appreciate to a lesser extent from the sides).
- So eventually and lastly, ice will start to form in the base of the can.
- As it can’t move the block of ice above it up and out of the can (unlike an iceberg?), it has only one surface to really push against – the base (a large flat surface) and this results in the base being pushed/bowed out and distorted.
Well if my memory is right this seems very a plausible explanation of what is happening.
Still I kind of of wonder though, why didn’t the bottom of the can just pop out? Well now we are really pushing things. I think we are starting to run up against the amount of expansion ice produces (9%?) over the maximum amount the metal base can expand by and then set against the strength of the base to side seal.
Looking at the picture (above) of the watering can I came across, I would say the distortion could be something around the 9%(ish) mark. Interesting.
Anyway, I have digressed, I think we have come to the same conclusion and regardless – now is the time to empty and put away your watering cans otherwise you are going to have a baggy bottom!