Bonsai Pots - Glazed vs. UnGlazed

[NaoTK]

The gold standard for freeze resistance in ceramics is described here: https://digitalfire.com/article/outdoor+weather+resistant+ceramics

Here is a simple version for us bonsai folk. You can test this yourself at home.

Your pot will fit into one of these groups:
1. The open porosity is <5%. This means it simply wont absorb more than 5% water. So when it freezes there is not enough water in it to burst the pot = frost resistant.
2a. The open porosity is >5% and the porosity coefficient is >0.78. This means the freezing water has nowhere to expand into and the pot will crack when it freezes.
2b. The open porosity is >5% but the porosity coefficient is <0.78. This means there is enough open space in the pores for water to expand into and the pot will not crack when it freezes. I suspect some longer lasting terracotta pots must fit in this group

 

[ForrestW]

The gold standard for freeze resistance in ceramics is described here: https://digitalfire.com/article/outdoor+weather+resistant+ceramics

Here is a simple version for us bonsai folk. You can test this yourself at home.

Your pot will fit into one of these groups:
1. The open porosity is <5%. This means it simply wont absorb more than 5% water. So when it freezes there is not enough water in it to burst the pot = frost resistant.
2a. The open porosity is >5% and the porosity coefficient is >0.78. This means the freezing water has nowhere to expand into and the pot will crack when it freezes.
2b. The open porosity is >5% but the porosity coefficient is <0.78. This means there is enough open space in the pores for water to expand into and the pot will not crack when it freezes. I suspect some longer lasting terracotta pots must fit in this group
View attachment 532147

Thank you very much for sharing this-- I only ever looked at the closed porosity. I am curious to test and see what my coefficient is. Thanks you-- Rob

 

[JeffS73]

The gold standard for freeze resistance in ceramics is described here: https://digitalfire.com/article/outdoor+weather+resistant+ceramics

Here is a simple version for us bonsai folk. You can test this yourself at home.

Your pot will fit into one of these groups:
1. The open porosity is <5%. This means it simply wont absorb more than 5% water. So when it freezes there is not enough water in it to burst the pot = frost resistant.
2a. The open porosity is >5% and the porosity coefficient is >0.78. This means the freezing water has nowhere to expand into and the pot will crack when it freezes.
2b. The open porosity is >5% but the porosity coefficient is <0.78. This means there is enough open space in the pores for water to expand into and the pot will not crack when it freezes. I suspect some longer lasting terracotta pots must fit in this group
View attachment 532147

I do these tests for all my clays, made or bought. I've got a box of samples in the freezer that I occasionally thaw by pouring boiling water on them. Most are around 2-8% closed porosity, the more open ones have a good coeff., Haven't had any break yet, so I haven't learnt any limits yet.

 

[JeffS73]

There's a lot of variety in Chinese and Japanese pots. I received a carved Bigei that had broken, tested the clay, totally vitrified < 2% closed porosity. Pour water on a Gyoun, Tadahiro or some Mazan pots, you can watch them absorb the water. Along with Chinese pots, I wouldn't let any of those freeze, but I do like buying and using them! They get sheltered over Winter. Try sprinkling water on your pots if concerned, might learn something. Most Tokoname pots seem fairly well vitrified.

 

[HENDO]

The gold standard for freeze resistance in ceramics is described here: https://digitalfire.com/article/outdoor+weather+resistant+ceramics

Here is a simple version for us bonsai folk. You can test this yourself at home.

Your pot will fit into one of these groups:
1. The open porosity is <5%. This means it simply wont absorb more than 5% water. So when it freezes there is not enough water in it to burst the pot = frost resistant.
2a. The open porosity is >5% and the porosity coefficient is >0.78. This means the freezing water has nowhere to expand into and the pot will crack when it freezes.
2b. The open porosity is >5% but the porosity coefficient is <0.78. This means there is enough open space in the pores for water to expand into and the pot will not crack when it freezes. I suspect some longer lasting terracotta pots must fit in this group
View attachment 532147

You have no idea how valuable this information is to somebody who has to literally bury their expensive pots underground for the Winter. I will definitely be performing these tests on my pots moving forward before using them for over-wintered plants. Perhaps treating them will thr recommended sealer beforehand. Thank you so much!

 

[NaoTK]

The caveat is if the shape of the pot is concave such that the soil is a solid block of ice with nowhere to go, that pot can crack no matter the clay absorption. I had that happen the first time this year. My soil is usually loose but this one pot was dense with roots and it cracked the pot in half.

 

[Gabler]

What people sell as terracotta pots are just low fire earthenware clay bodies for the most part.

Is that not the definition of terra cotta?

 

[BrightsideB]

Is that not the definition of terra cotta?

Yeah it’s a definition of it. And I understand it’s primarily used to define the red/orange earthenware. I use the term to define the color rather than pottery. I use earthenware to define the ceramic. Really just because earthenware can be any color. And a stoneware body can be a terracotta color.

 

[A Bonsai Brit in Spain]

There are no advantages or disadvantages to either, comparatively, only aesthetics and personal taste. There are some who still believe the myth that unglazed pots "breathe" or allow moisture to move through the interior to the exterior of pot walls because of porosity in the clay, while glazed pots can't do that because the glaze blocks it. None of that is really true for the high fired stoneware used in quality bonsai pots.

Unless the clay has a 0 absorption rate then the pot will for want of a better word breathe. Many of theclays used have an absorption rate of 5%. Especially if grogged, so they do absorb water and evaporate water through their walls and floor. If you want frost free pots High fired stoneware clay with less than 5% absorption is the way forward. Having said that. The clay I use for wheel throwing is super fine and will only have an absorption rate of less than1%. Porcelain is 0 % as far as I can see, most good clays will have all this information on the label.

 

[Deep Sea Diver]

Yeah it’s a definition of it. And I understand it’s primarily used to define the red/orange earthenware. I use the term to define the color rather than pottery. I use earthenware to define the ceramic. Really just because earthenware can be any color. And a stoneware body can be a terracotta color.

Ran across this article titled “What is terracotta”.

Quite a bit different from what is being mentioned here.

btw, Tokoname Grow Pots, made in Japan do not break when frozen, unlike the ones made down south. (Their clay must be raw terra cotta, which doesn’t stand up. ). Have over 100 years Tokoname grow pots with one type of plant or another in them. Was 15F last winter. Absolutely no damage… there was that one that broke because I dropped it when slipping on ice.

Cheers
DSD sends

 

[BrightsideB]

Ran across this article titled “What is terracotta”.

Quite a bit different from what is being mentioned here.

btw, Tokoname Grow Pots, made in Japan do not break when frozen, unlike the ones made down south. (Their clay must be raw terra cotta, which doesn’t stand up. ). Have over 100 years Tokoname grow pots with one type of plant or another in them. Was 15F last winter. Absolutely no damage… there was that one that broke because I dropped it when slipping on ice.

Cheers
DSD sends

This is a great resource to look at to understand ceramics. Not everything on the site is perfect but is one of the best resources. At the bottom of my post.

I’ve read that the grain structure in the terra cotta fired clay defines terra cotta as well.

I tend to think the Japanese training pots are a low fired stoneware with 4% or less absorption. And are slip casted in a production facility.

Terra Cotta

A type of red firing pottery. Terra cotta clay is available almost everywhere, it is fired at low temperatures. But quality is deceptively difficult to achieve.

digitalfire.com

 

[Leo in N E Illinois]

Just to confirm or echo @NaoTK 's warning;

Even if your pot is freeze thaw proof, if the soil is fine grain, and holds a lot of water, or is packed with roots, the pot can be shattered by expanding frozen root ball or soil mass.

This has forced me to winter my trees in "good" pots, pots that are worth money or are difficult to replace, these get wintered in my well house. The well house stays above freezing.

An alternative is to slip pot trees into plastic pots for winter.

Shape of pot makes a difference. Pots with walls that angle out at (example 45 degrees ) the soil mass can rise up as it freezes and expands. Vertical walls will break. So shape of pot is critical too.

Summary of different solutions to freeze breakage

*Above freezing winter storage

*Slip pot to plastic

*Shape of pot, outward sloping walls, or slabs.

*Open coarse potting media, fine media expands more

*Vitrious or impervious, high fired ceramic pots

 

[rockm]

Ran across this article titled “What is terracotta”.

Quite a bit different from what is being mentioned here.

btw, Tokoname Grow Pots, made in Japan do not break when frozen, unlike the ones made down south. (Their clay must be raw terra cotta, which doesn’t stand up. ). Have over 100 years Tokoname grow pots with one type of plant or another in them. Was 15F last winter. Absolutely no damage… there was that one that broke because I dropped it when slipping on ice.

Cheers
DSD sends

It's not any different than what has been discussed here. There are more than one kind of terracotta--depending on the firing and type of clay used. This is indeed a great link

Terra Cotta

A type of red firing pottery. Terra cotta clay is available almost everywhere, it is fired at low temperatures. But quality is deceptively difficult to achieve.

digitalfire.com

 

[Deep Sea Diver]

Didn’t see the durability of Tokoname grow pots mentioned by name previously. Use of these have saved me lots of money and to use less plastic…. (and still haven’t found a potter that makes… wants to make…. good pots for growing and keeping Satsuki in the US yet…likely someone out there?)

Yet not thinking plastic pots do not have a place in stages of development.

Cheers
DSD sends

 

[NaoTK]

Somebody please test the absorption of one those standard Tokoname terracotta grow pots (the ones that say "トコナメ" on the bottom.) I have some but they are in use.

Put it in the sun for a week so it it super dry. weigh it. Do the overnight soak then the boil soak and calculate.

 

[BrightsideB]

Somebody please test the absorption of one those standard Tokoname terracotta grow pots (the ones that say "トコナメ" on the bottom.) I have some but they are in use.

Put it in the sun for a week so it it super dry. weigh it. Do the overnight soak then the boil soak and calculate.

I want to guess 3-4%. Want to add a space heater works too if it is rainy.

But does it pass the lick test? Lol

 

[Gabler]

But does it pass the lick test? Lol

That's how I learned to do it in archaeology field school. Dig up a pottery sherd and lick it to determine whether it's earthenware, stoneware, or porcelain.

 

[BrightsideB]

That's how I learned to do it in archaeology field school. Dig up a pottery sherd and lick it to determine whether it's earthenware, stoneware, or porcelain.

It’s very scientific

 

[BrightsideB]

That's how I learned to do it in archaeology field school. Dig up a pottery sherd and lick it to determine whether it's earthenware, stoneware, or porcelain.

You’re also onto something if it taste like dirt

 

[Leo in N E Illinois]

Didn’t see the durability of Tokoname grow pots mentioned by name previously. Use of these have saved me lots of money and to use less plastic…. (and still haven’t found a potter that makes… wants to make…. good pots for growing and keeping Satsuki in the US yet…likely someone out there?)

Yet not thinking plastic pots do not have a place in stages of development.

Cheers
DSD sends

I didn't mention Tokoname, because I can't read Japanese. I do know that there are kilns in Japan that make bonsai pots that are not part of the Tokoname collective (or historical district). Pots from Seto, and a few others are independent from Tokoname, but very high quality too. True, Tokoname is FAMOUS for exceptionally high quality display pots. And Tokoname also produces production grade azalea pots and training pots specifically for bonsai. Generally all seem to have a dense enough clay body to withstand freeze thaw cycling if the shape of the pot does not lock the expanding soil mass in place. Outward sloping walls are good, wider at the top than the base.

High quality pots with high quality clay bodies can come from a variety of places. Yi Xing, China has excellent clay quarries, famous kilns, and many great potters. Sadly Yi Xing also has many mass produced low quality pottery production factories that are flooding the market with junk. On the internet, it is difficult to sort the crap from the choice premium high end pots. I have a couple good Yi Xing pots, and in person, it is easy to feel and see the quality difference.

There are a number of North American and European potters making fantastic, high quality pottery. Most are not making training pots. rather display pots. The good ones are great. Quality is all over the map, Internet auctions don't display the quality of the clay bodies, but if you start with buying only one from an artist, then go back for more if their pot proves to be well made. I have found Sara Raynor pots to be very freeze thaw resistant if you keep shape in mind. Her pots with outward sloping walls survive winter no problem. Walls that curve back inward at the lip, lock in the soil ball, which prevent it from lifting as it expands, these will break if frozen it they are full with a fine particle media mix.

So because good pots come from many places, I did not specify Tokonome, as they are not the only game in town.