Question for potters...

[Bonsai Nut]

Is there a reason why, on a glazed pot, the inside of the pot is not glazed?

 

[Anthony]

Supposedly the g;lazed inside makes the roots slip.

Also if the pot is vitreous doesn't need glaze on the inside.

Sometime the glaze is expensive.

If your pot is very thin and the glaze / body expansion is off an internal glazing
will break the pot.

Hope this helps confuse
Good Day
Anthony

 

[GrimLore]

Is there a reason why, on a glazed pot, the inside of the pot is not glazed?

I can only quote this from Charles Ikers' FAQ on his site -

"Are your pots glazed on the inside?

None of my pots have glaze on the interior. There are varied opinions regarding interior glaze. I can say for sure that it has nothing to do with the plants ability to breath or moisture. A correctly fired pot won’t allow moisture or air to move through the clay body anyway. The biggest advantage of unglazed interiors is the “tooth” that the raw clay surface has. This gives the roots an opportunity to grip the container and anchor itself better in the pot."

Crystal does have pots with interior glaze that are mass produced so I suspect it is easier/cheaper in production. None of them are suited for our Winters but work out good for Orchids and such

Grimmy

 

[Eric Group]

Glasing the inside of a pot would be like painting the inside of a trash can!

It wouldn't be seen anyway, and would just get all dirty as soon as you use it.

Might be a neat twist for the guys who make the pots so fancy people just stick them on a shelf and view them like art!

 

[Bonsai Nut]

Glasing the inside of a pot would be like painting the inside of a trash can!

It wouldn't be seen anyway, and would just get all dirty as soon as you use it.

Might be a neat twist for the guys who make the pots so fancy people just stick them on a shelf and view them like art!

I am truly curious if there is a functional reason... because my daughter gave me a hand-made bonsai pot and she glazed the inside. Because I had never seen a glazed interior, I assumed there was a "reason" why you wouldn't want to do so.

 

[Stickroot]

Glaze is EXPENSIVE!
And the rough tooth of the stoneware is desired by roots.

 

[mrcasey]

I have doubts about the value of roots "gripping" the unglazed walls. It reminds me of that horticultural hooey about needing sharp soil particles in order to split and ramify roots. If rough containers were even marginally healthier, plant and soil scientists would have the billions-of-dollars-a-year nursery industry using plastic nursery cans with rough interiors.

The most expensive thing in glazes with which I'm familiar are the metallic oxides used for coloring. I'm assuming that most of the traditional bonsai pots were high fired in reduction. If that's the case, greens, blue greens, oribes, celadons, and certain reds could be obtained using fairly inexpensive copper and iron. In our studio, the colorants that really cost money are cobalt, tin, and chrome. These give us our dark blues and reds firing in oxidation.

My only other idea for this convention is that maybe someone way back when felt that the fluxes used in some glazes would be unhealthy for plants.
Lead comes to mind. Don't know if they used lithium, barium, or any of the other nasty stuff. I don't even know if these metals are unhealthy for plants.

Also, even if it's a myth, an uneducated buying public will keep demanding that potters make bonsai containers unglazed on the inside. It's hard enough to
sell my pottery glazed in the traditional manner; I just keep the interiors unglazed, my mouth shut, and I move on.

 

[jacob letoile]

There was a hood maker who used to make beautuful, well fitting falconry hoods. The outside would be decorated pleasantly but on the inside of special hoods, usually on one of the eye panels he would do the most amazing, intricate drawings. When in use only the bird could see it. And I have never found a falcon who had anything resembling good taste. I always liked this idea, something special between the artist and recipient. The question in the OP made me think of that.

 

[MrFancyPlants]

I always thought it was so the roots could breathe better. I would think glaze in the inside would be like polyester underwear. Not real scientific, I know.

 

[Stickroot]

I was not suggesting that it is healthier for the roots. It just seems that traction for the roots would be helpful. I am completely positive that Chojubai do much better when they have resistance on there roots. Begei and other potters have gone to the trouble of actually carving traction in the walls off pots to help aid roots from circling.
I have found myself carving traction and never even thought about glazing the inside of a pot.
Polyester underwear for sure! Just seems totally yuck to me.
I think a test is in order!

 

[rockm]

I have doubts about the value of roots "gripping" the unglazed walls. It reminds me of that horticultural hooey about needing sharp soil particles in order to split and ramify roots. If rough containers were even marginally healthier, plant and soil scientists would have the billions-of-dollars-a-year nursery industry using plastic nursery cans with rough interiors.

The most expensive thing in glazes with which I'm familiar are the metallic oxides used for coloring. I'm assuming that most of the traditional bonsai pots were high fired in reduction. If that's the case, greens, blue greens, oribes, celadons, and certain reds could be obtained using fairly inexpensive copper and iron. In our studio, the colorants that really cost money are cobalt, tin, and chrome. These give us our dark blues and reds firing in oxidation.

My only other idea for this convention is that maybe someone way back when felt that the fluxes used in some glazes would be unhealthy for plants.
Lead comes to mind. Don't know if they used lithium, barium, or any of the other nasty stuff. I don't even know if these metals are unhealthy for plants.

Also, even if it's a myth, an uneducated buying public will keep demanding that potters make bonsai containers unglazed on the inside. It's hard enough to
sell my pottery glazed in the traditional manner; I just keep the interiors unglazed, my mouth shut, and I move on.

The idea that an unglazed interior allows roots to "hold onto" the pot isn't as sketchy as it sounds. Good bonsai pots, especially those from Japan, have great clay with "tooth"-- rougher texture. I've noticed over the years that pots made with clay with a lot of "tooth" tend to be a bit happier. That grainy texture DOES hold onto roots better than a smoother texture -- glazes aren't just smoother, they're slippery as glass.

Dan Gould, one of the first excellent American bonsai potters, researched ancient Asian pottery for his glazes and techniques. His glazed pots have simple designs etched into their unglazed interiors, on the bottoms and walls. A simple series of wavy lines, combined with straight lines, etc. I asked him why he did that one time. He said "it gives the roots something to look at." He said that was a traditional thing in China hundreds of years ago. I don't know if that's true, but he did the research. I notice when I pull a plant out of one of those pots with the etchings that the roots at the edge of the rootball have to be pulled off the sides and bottom to a certain extent.

Just sayin...also I'd bet the main reason is glaze on the inside is wasted as it's not seen. some bonsai potters glaze only the top inch of pot interiors, as that is what shows when a tree is planted correctly in it...

 

[Starfox]

Is there any reason to think that the pot may crack more easily when firing a fully glazed interior as well?

Funny this was brought up, I just picked up a pot that has glazing on the underside which doesn't seem to be done much either.

 

[Anthony]

@Starfox .

the original pots were easily obtained from potters in China. So for example we still get porous pots,
like the original eartheware pots, that drain away and evaporate to cool, and a glaze only on the
sides.

We found the Chinese serissa grows best in these pots.

We have also found the J.B pines prefer earthenware / porous containers. Today you can get porous glazes so
you can change the colour of pinky red earthenware to anything.

The other day a stoneware pot just touched the concrete and beacause it was castware at this thickness ( )
it split in half.
Just break out the Gorilla glue

Now imagine if it had also been glazed and the glaze expansion had broken it

Often Chinese pots have the insides combed for added root pleasure.
Good Day
Anthony

 

[michaelj]

I am truly curious if there is a functional reason... because my daughter gave me a hand-made bonsai pot and she glazed the inside. Because I had never seen a glazed interior, I assumed there was a "reason" why you wouldn't want to do so.

I wondered the same thing last year when my daughter also made me a nice little blue bonsai pot with glaze on all surfaces.

 

[garywood]

If you think how pots were made and fired historically, the two important aspects were cost and efficiency. Pots were generally stacked in the kiln with or without wadding between pots to keep them separated. Glazed pots needed separation. So the inside and the bottoms were not glazed otherwise they couldn't fire but one layer in the kiln. Not very cost effective or efficient.

 

[twisted trees]

I always thought it was so the roots could breathe better. I would think glaze in the inside would be like polyester underwear. Not real scientific, I know.

I breathe just fine in my polyester underwear.

 

[MrFancyPlants]

I breathe just fine in my polyester underwear.

Yeah, but do your "roots"?