HH -- I do know this about hypertufa, which is similar but without the cloth drape. The longer you let it go through the curing process (gently spraying water then recovering it with a plastic trash bag) the longer it will last outside. I learned this from a friend who lived all her life in Alaska and she had so many hypertufa pots she just left them outside all the time. So the longer they cure, the stronger and more long lasting they become. She also told me to never work in weather that was cooler than 50*F, it won't setup well at that temp. But if you build them in summer and give them about a month to really cure, they last for years in just any kind of weather. My friend passed in 2008 and her sister moved the hypertufa pots to her own garden. The pots are still there and holding strong in Seward, AK.
I suspect the towel or whatever is used for draping will also make these stronger than just regular hypertufa, but I'd say the process is the same. I also read somewhere that some have added a little wood glue to the water before adding it to the concrete mix -- and this was done in a colder weather zone. Can't remember where, though.
