Sealing ceramic items after glaze firing

[Callie Beller Diesel]

I get similar freeze/thaw cycles here in Calgary. We’re high altitude too. 

I think the risk from water absorption on a clay fired to maturity is much less than water accumulating within the ring in a puddle from melted snow. I had some soda fired stoneware planters that lasted for 15+ years outside, until one year I left it right side up and it collected snowmelt. If it was me, I’d put the glazed side down into the earth, not worry too much about the unglazed rim, and keep water from accumulating inside the ring during the winter somehow. 

[ATauer]

On 6/7/2022 at 6:01 PM, Bill Kielb said:

Epoxy would be my best guess for some longevity. Food grade epoxy if in doubt or turn them right side up, color the top as an accent and clear epoxy just the top. As far as soil pressure, no idea if it will eventually break the ring. If the soil is kept reasonably loose and aerated my total guess is they will last for quite some time and you will have to re - imbed them occasionally as the soil will push them up out of the ground over time.

In my exhaustive search for epoxies and varnishes and sealers that won’t yellow (I make large scale sculptures that have to be fired in sections and put together after glazing, doing cold finishing to cover the seam and match colors and glaze glossiness) I found every day thing yellows except aliphatic polyurethane. Now, your situation that doesn’t matter because they won’t be exposed to UV light. But they are great waterproofers, they withstand chemical attacks so well they are used on murals to protect from graffiti, so they would not degrade from any acid rain getting into the soil or any other chemicals. They are waterborne and not made of petrochemicals, don’t put of VOCs (or at least so low they aren’t detected). It is what I use to protect my outdoor sculptures, especially ones made out of adobe. The only real caveat is you should reapply every 5 years, which with what you said with the plants growing over them might not be possible. But I’d also guess, knowing what I know about these chemicals, that being face down in the dirt would lengthen its effective time and that I believe the waterproofness lasts a lot longer than the rest of its properties. You could also do what someone else suggested with flipping them over and coloring them with honestly anything, and then you would be able to reapply this if you felt you needed to. You could also do a little study and leave some that you don’t reapply and so how they do. 

There are not very many products made of this stuff, it is usually used as a topcoat for epoxy or concrete flooring or ramps, so it can be hard to find it in other than very expensive gallons (two part product, it has an activator). I have found that Lowe’s in the US carries a quart size version, which is very affordable. I’ll be experimenting with whether I can just mix portions of the two parts together so I don’t have to use all of a container up at once with the gallons.

I would also look definitely into how vitrified your clay is and the ASTM’s porouness test. It is generally considered that it is ok to have up to 5% absorption in clay that is outdoors in freezing weather, but you should remember this is really based on bricks and they will still have a huge amount of spalling even if they are within the guidelines. In order to avoid that I prefer to vitrify my clay to 0.5% or less and seal it, as it would be horrifying for me to have one of my 6 foot sculptures that took so much work have chunks fall off it ruining them forever. You aren’t really in that situation. But, vitrifying and sealing would be my suggestion if you don’t want little chunks of your collars coming off over the years…it wouldn’t be terrible, but I’m sure you want them to stay as nice as possible…