Calcined Clay defined

[Bonsai Nut]

I was about to write this in another thread, but I decided it might be worth its own thread given the number of questions about this product.

Products (of various brand names) made from calcined clay are quite common in bonsai soil mixes. Though the raw clay product occurs naturally, calcination as a process occurs artificially (via heating) and can differ based on the elemental makeup of the "clay" used, the amount of calcination, the particle size, and other factors.

There are different families of clay depending on how the clay was created and where it occurs naturally. Akadama and many other clay products comes from Kaolinite clay, the primary component of which is Aluminium Silicate. In the case of akadama and other similarly colored clay products, the next most prominent component is Iron Oxide (which gives the product its orange color). Akadama is approximately 80% Aluminium Silicate and Iron Oxide. Because iron oxide is not desired for some industrial uses, some artificial calcined clay products contain fewer components and a more concentrated makeup of up to 90%+ Aluminium Silicate (this product is grey in color).

So what is "calcined" clay? In nature, clay is formed by the gradual chemical weathering of rocks or through hydrothermal activity. It usually contains a high quantity of water and dissolved organics, and is plastic when wet (ie you can form it, and it will hold shape). As you remove water, it becomes less plastic. As you add water it becomes more plastic until it becomes a slurry, or liquid with suspended solids. If you were to use clay (by itself) as a potting material, it would tend to absorb water, become more plastic, compress, and then harden as it dried. Over numerous wet/dry cycles, you would find that your bonsai is planted in a solid block of clay - and with limited/no access to oxygen the roots would suffer or die.

The process of calcination is to heat a substance to a high temperature, but below the melting or fusing point. This not only removes water, but oxidizes any organic compounds, and hardens the clay so that it no longer turns plastic when wet. The higher and longer the clay is heated, the harder the calcined clay end-product becomes. Because of the naturally occurring void space (caused by the evaporation of water during heating), calcined clay has high absorptive properties, which is why it is used in gardening, to sop up oil spills, to coat the baselines of baseball fields, and for numerous other applications.

Not all calcined clay products are identical. In addition to trace elements in the clay itself, it can be softer or harder, and particle size has a significant impact on the degree to which it will absorb or release fluids, and the ease with which oxygen will penetrate. Even hard calcined clay is brittle and has a tendency to crumble (albeit slowly) and reduce in particle size with time. As a component in a bonsai mix, it tends to play the role of "water reservoir" - absorbing water while preventing saturation - and slowly releasing it as needed by the plant or tree. In the case of akadama, wet pellets are significantly darker than dry pellets, making it easier to tell at a glance whether a tree needs watering.

It should be noted that calcined clay is inorganic. You will still need to fertilize in order to provide nutrients for your tree - just as when you are using pumice, lava, decomposed granite, or other inorganic particles in your mix.

Additionally, there are many clay products that LOOK like calcined clay, but are not - some cat litter being an example. These products may simply be heat-dried clay, which when wet will simply break back down into raw clay, and should be avoided for bonsai soil mixes.

Discuss

 

[EPM]

Is akadama actually "calcined"? Maybe it depends on the definition of "calcination". In terms of effects on the clay substance at hand, do you consider calcination to do more than thermal drying, but less than what would happen if it were "fired" like pottery? Does calcination cause silica or other constituents to fuse in the granules? Maybe some pottery experts can explain to us what happens to clay at different temperatures.

I just feel like the term calcination needs to be better defined before any discussion begins. Certainly different "calcined" products have very different properties.

The akadama I've played with was no where near as hard some something like this:

https://epminerals.com/products/safety-absorbent-and-safe-t-sorb

which is not nearly as hard as real Turface.

What is the source of heat that caused the deep akadama to be harder? I thought it was formed from voclanic ash and other volcanic sediment. Is it actually heated by nature after it was deposited or is it just pressure/compression (which could create heat I suppose) from the depth?

Sorry, my post is just a bunch of questions but hopefully they will further discussion.

 

[Bonsai Nut]

What is the source of heat that caused the deep akadama to be harder?

/EDIT I am editing out this post because my original post was completely wrong - and suggested that akadama is naturally calcined. Only some akadama is calcined - and it is an artificial process that occurs after mining.

 

[River's Edge]

I was about to write this in another thread, but I decided it might be worth its own thread given the number of questions about this product.

Products (of various brand names) made from calcined clay are quite common in bonsai soil mixes. Calcined clay occurs naturally, or can be produced artificially, and can differ based on the elemental makeup of the "clay" used, the amount of calcination, the particle size, and other factors.

There are different families of clay depending on how the clay was created and where it occurs naturally. Akadama and most other calcined clay products comes from Kaolinite clay, the primary component of which is Aluminium Silicate. In the case of akadama and other similarly colored calcined clay products, the next most prominent component is Iron Oxide (which gives the product its orange color). Akadama is approximately 80% Aluminium Silicate and Iron Oxide. Because iron oxide is not desired for some industrial uses, some artificial calcined clay products contain fewer components and a more concentrated makeup of up to 90%+ Aluminium Silicate (this product is grey in color).

So what is "calcined" clay? In nature, clay is formed by the gradual chemical weathering of rocks or through hydrothermal activity. It usually contains a high quantity of water and dissolved organics, and is plastic when wet (ie you can form it, and it will hold shape). As you remove water, it becomes less plastic. As you add water it becomes more plastic until it becomes a slurry, or liquid with suspended solids. If you were to use clay (by itself) as a potting material, it would tend to absorb water, become more plastic, compress, and then harden as it dried. Over numerous wet/dry cycles, you would find that your bonsai is planted in a solid block of clay - and with limited/no access to oxygen the roots would suffer or die.

The process of calcination is to heat a substance to a high temperature, but below the melting or fusing point. This not only removes water, but oxidizes any organic compounds, and hardens the clay so that it no longer turns plastic when wet. The higher and longer the clay is heated, the harder the calcined clay end-product becomes. (This can be seen naturally with different grades of Akadama or Kanuma soil - the soil that is closer to the surface and experienced less intense heating is softer and crumbles or breaks down more easily, while the "deep" akadama is harder). Because of the naturally occurring void space (caused by the evaporation of water during heating), calcined clay has high absorptive properties, which is why it is used in gardening, to sop up oil spills, to coat the baselines of baseball fields, and for numerous other applications.

Not all calcined clay products are identical. In addition to trace elements in the clay itself, it can be softer or harder, and particle size has a significant impact on the degree to which it will absorb or release fluids, and the ease with which oxygen will penetrate. Even hard calcined clay is brittle and has a tendency to crumble (albeit slowly) and reduce in particle size with time. As a component in a bonsai mix, it tends to play the role of "water reservoir" - absorbing water while preventing saturation - and slowly releasing it as needed by the plant or tree. In the case of akadama, wet pellets are significantly darker than dry pellets, making it easier to tell at a glance whether a tree needs watering.

It should be noted that calcined clay is inorganic. You will still need to fertilize in order to provide nutrients for your tree - just as when you are using pumice, lava, decomposed granite, or other inorganic particles in your mix.

Additionally, there are many clay products that LOOK like calcined clay, but are not - cat litter being one example. These products are simply heat-dried clay, which when wet will simply break back down into raw clay, and should be avoided for bonsai soil mixes.

Discuss

Well written and I appreciate the clarity. I have often referred people to the appendix entitled " Japanese Bonsai Soil Types" written by Maskuni Kawasumi II and his son Maskauni Kawasumi III. The appendix is contained in the text " The Secret Techniques of Bonsai" 2005. It references the volcanic origin of Akadama ( shared by pumice and lava) as well as documents some of the important characteristics for Bonsai purposes.
Comparing grain size, dry weight, Water retentitvity and PH values.
A good basic reference to understand the origins and uses of Akadama, Kanuma, Kyru sand, Fuji sand, Granite sand, River sand and Regular mountain sand.
I have found using the term "clay" confuses most beginners and none too few experienced practitioners of Bonsai. Your article helps to clarify the degree of clay like properties that are present. Essentially the key difference in my mind is the particle size and how it functions. When it truly becomes " clay " as in minute particle size, it no longer functions for Bonsai purposes. I have found most definitions of clay include very small particle size.
That is why, i also believe that other types of substrate can be very suitable for Bonsai if they are selected for appropriate characteristics and functions. The varying recipes that account for tree and climate differences are invaluable for success in Bonsai.

Understanding form and function in Bonsai " soil" is fundamental to healthy Bonsai. Thank you for putting forth this thread.

 

[EPM]

Akadama is mined from areas with high volcanic activity. Japan is, for all intents and purposes, a chain of volcanoes.

I was aware of it's volcanic origins and the fact that Japan is made up of volcanoes. But there is a big difference from say the heat next to a lava tube, versus some ash landing on the ground and getting buried under more ash and just sitting there far from from high, relatively speaking, heat. People talk about occasionally finding organic material in their akadama like roots, which would lead me to believe that if it was subjected to heat it was not high enough to cause any appreciable carbonization.

I'm just trying to get to the bottom of it. Akadama is a strange substance. It's definitely like night and day compared to the clay in my yard (if dried and crumbled up like you discussed).

 

[papymandarin]

and regarding cat litters, if most of the brands are indeed what you told, there are also brands indeed being high fired and usable for bonsai. I use one and it's not breakng down since i bought my stock in 2009-2010

 

[EPM]

and regarding cat litters, if most of the brands are indeed what you told, there are also brands indeed being high fired and usable for bonsai. I use one and it's not breakng down since i bought my stock in 2009-2010

Probably similar to that Saf T Sorb which I posted above. How does this cat litter compare to Turface? Does the product still resist being crushed between your fingers or has it softened over time?

 

[EPM]

Well written and I appreciate the clarity. I have often referred people to the appendix entitled " Japanese Bonsai Soil Types" written by Maskuni Kawasumi II and his son Maskauni Kawasumi III. The appendix is contained in the text " The Secret Techniques of Bonsai" 2005. It references the volcanic origin of Akadama ( shared by pumice and lava) as well as documents some of the important characteristics for Bonsai purposes.
Comparing grain size, dry weight, Water retentitvity and PH values.
A good basic reference to understand the origins and uses of Akadama, Kanuma, Kyru sand, Fuji sand, Granite sand, River sand and Regular mountain sand.
I have found using the term "clay" confuses most beginners and none too few experienced practitioners of Bonsai. Your article helps to clarify the degree of clay like properties that are present. Essentially the key difference in my mind is the particle size and how it functions. When it truly becomes " clay " as in minute particle size, it no longer functions for Bonsai purposes. I have found most definitions of clay include very small particle size.
That is why, i also believe that other types of substrate can be very suitable for Bonsai if they are selected for appropriate characteristics and functions. The varying recipes that account for tree and climate differences are invaluable for success in Bonsai.

Understanding form and function in Bonsai " soil" is fundamental to healthy Bonsai. Thank you for putting forth this thread.

Thanks for pointing us to that reference. Sounds very interesting, I'll check it out.

 

[Bonsai Nut]

What is the source of heat that caused the deep akadama to be harder?

One other addition to this comment - under pressure, the heat necessary to calcine clay drops significantly.

 

[EPM]

One other addition to this comment - under pressure, the heat necessary to calcine clay drops significantly.

Interesting tidbit. So maybe some combination of pressure/temperature, combined with the volcanic origin is responsible for it's uniqueness. Where is the resident geologist when you need them

 

[Bonsai Nut]

Interesting tidbit. So maybe some combination of pressure/temperature, combined with the volcanic origin is responsible for it's uniqueness. Where is the resident geologist when you need them

Calcined clay is currently a "big deal" in the cement / construction industry. They use clay sources that don't contain iron oxide. There are a number of technical papers floating around out there that, honestly, are beyond my basic chemistry knowledge.

 

[papymandarin]

it's red , for comparison it's similar to what Harry Harigton is using. If you walk on it or crush it hard it will indeed crumble (like a terracota little pot will) but in pots it does not change at all, as i said i'm still using what i bought in 2009-2010 and reuse it since then

 

[River's Edge]

I was aware of it's volcanic origins and the fact that Japan is made up of volcanoes. But there is a big difference from say the heat next to a lava tube, versus some ash landing on the ground and getting buried under more ash and just sitting there far from from high, relatively speaking, heat. People talk about occasionally finding organic material in their akadama like roots, which would lead me to believe that if it was subjected to heat it was not high enough to cause any appreciable carbonization.

I'm just trying to get to the bottom of it. Akadama is a strange substance. It's definitely like night and day compared to the clay in my yard (if dried and crumbled up like you discussed).

There is an interesting article on a manufacturers website that explains the differences in distance material is thrown from the volcanic activity and the exposure to the elements. This evidently affects the speed of the cooling effect which in turn creates different characteristics such as pore size which in turn affects water retention. The website reference is CanLava corporation.

 

[chansen]

... People talk about occasionally finding organic material in their akadama like roots, which would lead me to believe that if it was subjected to heat it was not high enough to cause any appreciable carbonization.

The roots came after the heat .

 

[Wires_Guy_wires]

My cat litter is calcined clay. It does not break down when wet and it is not plastic.

I'm missing the part about ion exchange; the ability of clay to hold ions based on water pH. Excess of H+ releases positive ions, which in the clay are replaced by said H+. An excess of OH- releases negative ions.
Excess is when there's more than equilibrium of something.

 

[Cosmos]

When you stop to think of it - akadama, pumice, lava, even decomposed granite - all come naturally from volcanic sources. DE (diatomaceous earth) does not, but that is a subject for another thread.

You can add perlite to the list.

About cat litter, AFAIK, some brands sold in North America make theirs with zeolite, and these are probably harder/more suitable for our purposes than the clay equivalents.

 

[hemmy]

The higher and longer the clay is heated, the harder the calcined clay end-product becomes. (This can be seen naturally with different grades of Akadama or Kanuma soil - the soil that is closer to the surface and experienced less intense heating is softer and crumbles or breaks down more easily, while the "deep" akadama is harder).

In general a very good write up, thank you!

Although as @EPM noted, I don’t think I would consider Akadama and Kanuma to be naturally calcined and I think the mining process usually only involves heating for sterilization and to drive off excess moisture.
I don’t believe that the depth related hardness of the deeper Akadama and underlying Kanuma is geothermal temperature related but rather dependent on the alteration of the harder ash silicates into clay minerals from the action of meteoric (rain) water infiltration. Somewhere I have seen a composition breakdown that shows that Akadama has more aluminum and iron oxides at the expense of the harder silicates, while Kanuma has a higher silicates composition (not as altered).

But I could be wrong and will certainly defer to @markyscott on this one, since he has done the research:

https://www.bonsainut.com/resources/inorganic-soil-reference-sheet.28/

Also related would be the vitrification process which heats a material past the calcining temperature, for example expanded shale (haydite) heated to harden and expand/create air pockets but the outside is shiny (vitrified). I guess you calcine (?) shale but it would have less porosity and be a poorer aggregate for our purposes than calcined clay.

Anywho, thanks again for taking the time to write this up.

 

[Bonsai Nut]

Also related would be the vitrification process which heats a material past the calcining temperature, for example expanded shale (haydite) heated to harden and expand/create air pockets but the outside is shiny (vitrified). I guess you calcine (?) shale but it would have less porosity and be a poorer aggregate for our purposes than calcined clay.

Yes. I was reading a paper on calcined clay and it actually goes through more than one stage as it is heated, depending upon the composition and the behavior of the underlying elements. It also reacts differently based on the pressure when it is heated, and how it cools. Calcination occurs around 1400 degrees F while vitrification occurs around 2000 degrees F. Vitrified clay, because the exterior was heated until it becomes glass-like, has low water absorptive properties.

 

[Jester217300]

and regarding cat litters, if most of the brands are indeed what you told, there are also brands indeed being high fired and usable for bonsai. I use one and it's not breakng down since i bought my stock in 2009-2010

it's red , for comparison it's similar to what Harry Harigton is using. If you walk on it or crush it hard it will indeed crumble (like a terracota little pot will) but in pots it does not change at all, as i said i'm still using what i bought in 2009-2010 and reuse it since then

My cat litter is calcined clay. It does not break down when wet and it is not plastic.

I'm missing the part about ion exchange; the ability of clay to hold ions based on water pH. Excess of H+ releases positive ions, which in the clay are replaced by said H+. An excess of OH- releases negative ions.
Excess is when there's more than equilibrium of something.

It's probably diatomaceous earth. There are many cat litter brands that use DE in Europe at a reasonable particle size. This is not available in the US or Japanese market.

 

[sparklemotion]

Thanks for starting this thread @Bonsai Nut.

I've been beyond frustrated at how hard it is to come up with consistent definitions for some of the terms that people throw around when talking about soil components.

Can you share some of the sources that you used in putting together the original post? In particular, this is the first time that I've ever heard the notion that akadama/kanuma are "naturally" calcined. Every definition that I've seen for calcination talks about it as a human-driven transformation.