Is cone 6 hot enough?

I have access to a community center potting studio and kiln.  The lady in charge of ceramics there fires to cone 6.  I'm absolutely having a blast trying to build traditional rectangular pots.  After scouring the web last night for more bonsai pot construction info, my heart sank as I realized that so many "name" bonsai potters fire to around cone 10.  For example,  Dale Cochoy, Ron Lang, and Sara Rayner fire their pots above cone 9.  Does this mean that cone 6 just won't get it?    

I'm only about 2 months into this bonsai ceramics hobby and I'm not ready to invest several thousand dollars in my own kiln set up.                                                          


Casey
zone 6 WV

Cone 6 is hot enough if the clay vitrifies (forms some glass in the structure). This is what makes it absorb little or no water so it won't break during freeze thaw cycles. Just make sure you are using a clay body that is formulated for cone 6 - there are many out there. I found that "Mastering Cone 6 Glazes" by John Hesselberth and Ron Roy is a excellent resource.

If this is the case, why would these phenomenal bonsai potters bother firing higher than 6?
Tonight, I dug up a 2 1/2 year old thread on bonsainut in which a bonsai hobbyist with the screen
name "rockm" had this to say:

"I'm not a potter, but have picked up a few things about bonsai pots over the years. Sorry if I sound like a curmudgeon...

Cone 6 ain't gonna cut it if you're living in USDA growing Zone 7 or lower that get temperatures below freezing for months on end. Cone 8 and 9 produce entirely frost proof containers..."

I've read several posts from this fellow rockm. He seems knowledgeable about bonsai and is pretty articulate.
Is the guy off base? I don't mean to belabor things, I just want to make best the quality bonsai pots that I can.

thanks,
Casey

Frost resistance comes down to having a clay body that is matched to the firing conditions (pyrometric cone number). The goal is to get enough glass formation in the body to seal the pores, but not so much that that body slumps and deforms (or in extreme cases forms an odd shaped puddle). You can design clay bodies over a wide range of firing conditions for this - probably over at least 12-14 pyrometric cone numbers.

There is a long history of using cone 9-10 clay stoneware bodies in pottery. This includes lots of firing experience, decoration methods, and teaching with these bodies. I did most of my pottery and glaze development many years ago fired to cone 9.

The cone 6 clay bodies and glazes have seen much development in the past 20-30 years. It takes less energy to fire to the lower temperature and it is easier on the equipment. More programs are using these clay bodies so I think you will see more professional potters using them in the future.

It is probably best to make some pots and test them with any unfamiliar clay body. I would fill the pot with a small grained bonsai soil, water it well for a couple of days, let it drain out, and put it in a nice cold freezer down around 0 F (-18 C). Let it freeze for a couple of days, pull it out and check for damage once it thaws, and repeat - probably about a dozen times. This is a fairly severe test that can be done at home.