Firing/Density

[Chip]

... or the teacher can teach again!

[JBaymore]

Kidding here .... Paul is a VERY skilled potter.

best,

..................john

[paul haigh]

The funkier I get, the less dense my pieces are, and my work is fired to very high heat (like well over cone 13 in the middle of the kiln). Some of the wilder clays have lots of organics that burn out, making them less dense when fired.

Paul, perhaps it would be helpful to readers if you would share what cone 13 or any cone designation means?

Thanks!

Oops! Sorry, I sometimes forget that this isn't a forum of potters!

The "cone" system is a method of measuring the amount of heat work put into the kiln. Much in the way that getting the oven to 325F doesn't mean that the turkey is cooked (you need 20 mins per pound), getting a kiln hot doesn't mean that the pottery is fired to the level that it needs for the clay to vitrify (become stone-like) or the glaze to melt properly.

So, there are "cones" that are formulated to melt after they've been exposed to a certain amount of heat. Think of them as turkey pop-ups for really high temps; looking into the kiln and seeing which cone is melted tells you where you are in the firing. You might get enough heat by getting the kiln really hot, or by getting it less hot for a longer time- but the results will be different for those two cases, just as cooking BBQ low and slow gives different result than high-heat.

These cones are numbered; high-fired wares are normally cone 6 or cone 10. At the low end of the scale, a zero is put in front of the cone number (think of it as a negative-sign), so that the scale runs 016, 015, 014... and then 1,2,3... as temp increases. Wood firing gives a slightly higher than expected result, as the ash helps cones to melt a bit as well.

Here is a pic of a cone pack that I made to look like a dead cockroach. I arranged it so that the cones would hit the legs, rather than slump all the way over- just for fun. That last one is cone 12- most "cone packs" that I have- cone 12 is pretty flat, meaning that it's been fired to maybe more like cone 14. The front of the kiln, near the fire box gets significantly more heat. Hope that is all clear.

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Here is a cone pack before the firing- this one is very basic- when I think ahead, I make them look like skulls with the cones on it's tongue, a mountain with the cones melting down a river, etc. They are different colors before firing.

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[paul haigh]

I knew that I had better before and after pics. You can see the numbers on the sides- I use 012,010, and 10,11,12. I change kiln conditions after 012

Before

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After

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[wert]

Thanks paul haigh! Very informative, I learned alot.
Zisha is also wood fired in old times too, right? I doubt they were fired with that degree of control then.

[JBaymore]

wert,

EVERYTHING was woodfired when you go back enough.

Prior to the advent of cones (Hermann Seger, about in 1886)...... judging the firing completeness was done by a combination of "draw trials" (small pieces pulled from the kiln chamber) and by the color of the light coming from the chamber.

best,

.................john

\PS: Good job, Paul.

[paul haigh]

As John-sensei said- it was all wood fire at one time. (coal and oil have been used in some places, as well)

When I describe wood firing to potential customers, I'm always amused when they say "Wow- I never heard of that before! Is it a new thing?"

[Tead Off]

The funkier I get, the less dense my pieces are, and my work is fired to very high heat (like well over cone 13 in the middle of the kiln). Some of the wilder clays have lots of organics that burn out, making them less dense when fired.

What does 'funkier' mean? What does 'dense' mean to you? Why does the burning out of organic matter make a clay less dense? I don't think anyone has approached the original topic, firing/density, and the meaning and effect one has on the other. Please elaborate if you can. You, too, John, in lay terms.

[chrl42]

It's a pretty complicated topic.

Porosity (if Yixing) is very important factor when it comes to brewing tea, because porous teapots earn less stain, smell less or more sanitized...but we don't want our pots fired at lower temperature either，which ultimately get more porosity.

Yixing teapots made of better clays are easier to use for this reason, I just close the lids right away everytime I finished rinsing the teapots, but recent cheap Yixings easily smell like x if I use them carelessly. Factory-1 pots and master pots are fired at good temp. but have a very good porosity. I don't know the chemistry behind this difference though.

Yixing potters, from my understanding, don't know much about the clay, many time they just purchase from the clay manufacturers (of course there are exceptions). And my experience with 'clay' is quite limited to Yixings

[paul haigh]

The funkier I get, the less dense my pieces are, and my work is fired to very high heat (like well over cone 13 in the middle of the kiln). Some of the wilder clays have lots of organics that burn out, making them less dense when fired.

What does 'funkier' mean? What does 'dense' mean to you? Why does the burning out of organic matter make a clay less dense? I don't think anyone has approached the original topic, firing/density, and the meaning and effect one has on the other. Please elaborate if you can. You, too, John, in lay terms.

Density means the weight per volume. Two pieces may be the same exact size, but one that weighs more is more dense.

By funkier, I meant in how I approach making clay- adding materials to it to give it character. When organic materials burn out they leave tiny voids- since these are areas in the clay that are not occupied by actual clay after the burn out, the clay is less dense.

In general, ceramic gets denser as it approaches it's maturity temperature- the top temperature to which it is meant to be fired, but as John pointed out there are too many variables to make an absolute statement that higher firing equates to more density in all cases.

[Tead Off]

The funkier I get, the less dense my pieces are, and my work is fired to very high heat (like well over cone 13 in the middle of the kiln). Some of the wilder clays have lots of organics that burn out, making them less dense when fired.

What does 'funkier' mean? What does 'dense' mean to you? Why does the burning out of organic matter make a clay less dense? I don't think anyone has approached the original topic, firing/density, and the meaning and effect one has on the other. Please elaborate if you can. You, too, John, in lay terms.

Density means the weight per volume. Two pieces may be the same exact size, but one that weighs more is more dense.

By funkier, I meant in how I approach making clay- adding materials to it to give it character. When organic materials burn out they leave tiny voids- since these are areas in the clay that are not occupied by actual clay after the burn out, the clay is less dense.

In general, ceramic gets denser as it approaches it's maturity temperature- the top temperature to which it is meant to be fired, but as John pointed out there are too many variables to make an absolute statement that higher firing equates to more density in all cases.

Thanks, that was very clear.

Now, what does porosity mean in a clay? Does firing temp have any effect on porosity?

[JBaymore]

Porosity is the way that the clay body will allow moisture (and technically also gases) to penetrate into the (supposeldy) solid mass. It is also a bit related to its bulk density (relationship of volume to weight).

The easiest way to test this property is to take a piece of clay body, and put it in a hot oven for a long time...... like a number of hours.... at a temperature OVER 212F (100C). This is to assure that it is totally and compeltely dry.

You immediately take the piece of clay out and weigh it on a gram based scale that is accurate to at the MINIMUM point one grams (1/10 of a gram). Better if the scale is accurate to 1/100 of a gram.

Then you place the piece of clay into a water bath and boil it at a low rolling simmer continuously for at least 2 hours (refresh evaporating water as needed).... longer is better here....... but if you are doing comparisons... you must use the same exact timeframe.

You immediately take the piece of clay out of the water and qucikly and basically dry off the exterior surface with a cloth.

You weigh it again. The difference in the weight is the amount of water that the clay has managed to absorb into the pores / voids inside the walls. It indicates how easily WATER moves through the struture of the fired clay, and the amount and size of pores that exist there.

To put this in percentage format (the standard) you plug the numbers into this equation:

Boiled weight - original dry weight / original dry weight X 100 = apparent porosity.

Example: So if the sample weighed 100 grams when bone dry, and 102 grams after boiling, it would have an Aparent Poriosity of 2%.... it changed in weight 2 grams out of 100 grams.

You could do this with any sized sample...... even an entire yixing teapot if you wanted to.

Organic matter, like bits of paper, leaves, roots, microbes, and so on all exhibit a physical mass. They have some "volume" themselves. So when contained within a clay wall, they take up a little tiny bit of space.

Early in the firing process, when the general ignition temperature of carbonaceous matter has been reached (about 451F.....remember the book title?) IF there is sufficient oxygen present in the kiln at that point, the material then combusts. At this early part of the firing the clay walls are gase permeable to stuff like oxygen, carbon dioxide, and water vapor. So the carbon compounds go off as CO2 gas, and the hydrogen compounds go off as water vapor... and the physical size of the impurity shrinks to the tiny mass of what we'll call "inorganic ash" left over. The rest of the original volume is an empty "pore" left in the clay wall.

If not filled in by melting glassy phase materials from the clay body's chemistry (like fiberglass resin permeating fiberglass cloth), these pores contribute to the overall porisity of the clay body.

Hope that helps.

best,

....................john

EDIT: Weird... a posting in between the one above and mine disappeared...... which I was responding to here.

[Chip]

Thank you for the reply and photos on cones, Paul. I did not know pretty much any of that. I figured it was just another temp scale used by potters.

Not sure what happened to the post you are referring to, John ... but thanks for the lesson!

[TIM]

Porosity is the way that the clay body will allow moisture (and technically also gases) to penetrate into the (supposeldy) solid mass. It is also a bit related to its bulk density (relationship of volume to weight).

The easiest way to test this property is to take a piece of clay body, and put it in a hot oven for a long time...... like a number of hours.... at a temperature OVER 212F (100C). This is to assure that it is totally and compeltely dry.

You immediately take the piece of clay out and weigh it on a gram based scale that is accurate to at the MINIMUM point one grams (1/10 of a gram). Better if the scale is accurate to 1/100 of a gram.

Then you place the piece of clay into a water bath and boil it at a low rolling simmer continuously for at least 2 hours (refresh evaporating water as needed).... longer is better here....... but if you are doing comparisons... you must use the same exact timeframe.

You immediately take the piece of clay out of the water and qucikly and basically dry off the exterior surface with a cloth.

You weigh it again. The difference in the weight is the amount of water that the clay has managed to absorb into the pores / voids inside the walls. It indicates how easily WATER moves through the struture of the fired clay, and the amount and size of pores that exist there.

To put this in percentage format (the standard) you plug the numbers into this equation:

Boiled weight - original dry weight / original dry weight X 100 = apparent porosity.

Example: So if the sample weighed 100 grams when bone dry, and 102 grams after boiling, it would have an Aparent Poriosity of 2%.... it changed in weight 2 grams out of 100 grams.

You could do this with any sized sample...... even an entire yixing teapot if you wanted to.

Organic matter, like bits of paper, leaves, roots, microbes, and so on all exhibit a physical mass. They have some "volume" themselves. So when contained within a clay wall, they take up a little tiny bit of space.

Early in the firing process, when the general ignition temperature of carbonaceous matter has been reached (about 451F.....remember the book title?) IF there is sufficient oxygen present in the kiln at that point, the material then combusts. At this early part of the firing the clay walls are gase permeable to stuff like oxygen, carbon dioxide, and water vapor. So the carbon compounds go off as CO2 gas, and the hydrogen compounds go off as water vapor... and the physical size of the impurity shrinks to the tiny mass of what we'll call "inorganic ash" left over. The rest of the original volume is an empty "pore" left in the clay wall.

If not filled in by melting glassy phase materials from the clay body's chemistry (like fiberglass resin permeating fiberglass cloth), these pores contribute to the overall porisity of the clay body.

Hope that helps.

best,

....................john

EDIT: Weird... a posting in between the one above and mine disappeared...... which I was responding to here.

Thank You So Much. All your answers are the best information so far on TC!
Best ~ T

[JBaymore]

This is SUCH a complex subject............ one more point (out of MANY)......

So if the clay body contains grains of SAND........

The clay materials are small hexagonal plates. When I say small I mean SMALL x10 23 power! Between any two of these plates is a thin film of water. Between the electrical charges on the outer molecules of these clay crystals and the chemistry of the water between and their interactions, this is what makes clay "plastic" (malleable / fomable) like it is.

As the water evaporates, the space it takes up is disappearing. So the CLAY material shrinks in drying.

Back to those sand grains. Sand does not shrink. So around every sand grain is a layer of clay that is shrinking as it dries. The sand grain isn't and something has to give there. So the drying clay forms little microscopic stress cracks around each little sand grain. These too are voids that exist in the clay body.

It gets worse. As the clay begins to fire, it also continues to shrink. So those strain cracks cvontinue to grow and enlarge.

As I said.... this subject is complicated beyond belief. This is why god created ceramic engineers .

best,

........................john