[Clayart] durable outdoor planters

[vpitelka at dtccom.net]

Hi Phyllis - 
Val Cushing wrote about this in his handbook.  My copy disappeared while I was still teaching, but if you can acquire a copy, all the information is there.  What I remember is that when it comes to outdoor pots or sculpture failing from hard freezing, the problem claybodies are those with high porosity, and those with very low porosity.  The ones with high porosity have little mechanical strength, and in a hard freeze, absorbed water will cause them to spall or even disintegrate.  The surprise is the ones with very low porosity.  When a vitrified low-porosity clay form lives outdoors full-time, expansion and contraction from normal heating and cooling in warmer weather will cause water to wick into the claybody even with a glazed surface.  It can be an extremely small amount of water, but once it's in there, it stays.  Freezing water trapped in a space develops extraordinary pressure, enough to cause a vitrified clay form to fail.  

Here's another example of how stubborn that impacted water can be.  You have no doubt heard about people trying to re-fire a low-porosity vitrified form after it has been in use.  A good example would be a teapot that someone has been using every day, but then they decide that the glaze would benefit from refiring.  In the firing, such a form can explode from the impacted moisture, and it can destroy the kiln or even cause life-threatening injury.  

The ideal claybody to survive hard freezes is one that has enough porosity to let the pressure of freezing water escape, and high-enough mechanical strength to withstand that pressure.  You are no doubt aware of all the incredible terracotta architectural ornamentation on buildings around the US, including Cleveland, St. Louis, Chicago, Minneapolis, etc., much of which has been experiencing hard freezes for 150 years.  That's a hybrid terracotta body fired to low-midrange - around cone 2-4, and it has the properties mentioned above.  

At the Appalachian Center for Craft, we had a claybody we once used for raku, back in the old days when we did raku.  It contained equal parts Goldart, fire clay, ball clay, and fine grog.  With those components, that claybody was not vitrified even at cone-11, and it actually worked pretty well for outdoor sculpture - enough porosity, and enough mechanical strength.  

But that was in Tennessee, where our hard freezes were in the teens or twenties.  To really do this right, look into Val Cushing's writing and research on the subject. 
- Vince  

Vince Pitelka
Potter, Writer, Teacher
Chapel Hill, NC
vpitelka at dtccom.net
www.vincepitelka.com 
https://chathamartistsguild.org/ 

-----Original Message-----
From: Clayart <clayart-bounces at lists.clayartworld.com> On Behalf Of Phyllis Canupp
Sent: Wednesday, May 11, 2022 12:31 PM
To: Clayart international pottery discussion forum <clayart at lists.clayartworld.com>
Subject: [Clayart] durable outdoor planters

 I have made several planters over the course of my pottery passion. Some of them hold up to the winter (occasional freezing temperatures), and some don't. I would love to be able to pinpoint what I am doing both right and wrong because I would like to make some larger sized planters. But of course I want them to be able to withstand the occasional freezing temperatures that we get in Virginia. Does anyone have any suggestions or tips for making these pots? I do both handbuilding and throwing but I will probably make these planters from extruded coils. As always, thank you in advance for your help.
Phyllis Canupp
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