low fired stoneware vs mid fired

[Kakes]

Hi everyone, I posted this question in Clay and glaze chemistry and got no replies so I'm trying here. I hope that's ok.

Is there a difference in fragility and durability between vitrified low fired stoneware (04) and vitrified cone 6 stoneware?  I make non-functional sculptures so I'm primarily concerned with handling and shipping.  What would be the advantages or disadvantages of using either clay for sculpture?

thank you!

[Min]

Pete Pinnell did a test of claybody types many years back and one of the things they tested for was modulus of rupture, don't know how relevant that would be to your question although it's an interesting read. Link here to it. I would hazard a guess that how you pack and ship your sculptures might be more germain.  

Sorry your question didn't elicit any responses in the chem section.

Edited April 13, 2023 by Min
grammar

[neilestrick]

If it's an indoor sculpture, any clay body is strong enough regardless off the firing temp. How you pack it is much more important. In terms of deciding how hot to fire, that will depend on what types of surfaces you're after, and what materials are needed to achieve that. You'll also need to consider the shape of the piece and how the clay will respond to different firing temps. Porcelain is more likely to warp and sag when fired to maturity, low fire temps won't have that problem.

[Callie Beller Diesel]

If you’re concerned about the fragility of a sculpture as opposed to a pot, how the piece is constructed is going to have more of a bearing on the durability than the clay body. A thin walled piece constructed with the best methods described in Min’s link above will still break sooner than a brick made using the exact opposite principles. If your sculptures are thin walled and unglazed, making them out of earthenware glazed with a well fitted glaze will add strength, but you’ll still need a lot of packing padding. If your pieces are solid, they’ll withstand more impact. But even construction bricks break if they’re dropped from enough height.

[Jeff Longtin]

Hey Kakes,

The first thing to mention is your terminology.  Typically stoneware clay is not considered "low-fire". It used to be considered "high-fire" (fired to cone 10) but more recently many bodies have been mixed to vitrify at a "mid-fire" temperature. (cone 6) 

Earthenware clay is more typically considered "low-fire". ( fired to cone 06) The two local companies, Minnesota Clay and Continental Clay, both sell earthenware clay but I know several potters who like to fire their earthenware pots to cone 2.

In a sense any vitrified clay is highly durable. In my experience, however, earthenware tends to be a bit more brittle than stoneware or porcelain clay, after firing.

As to which is best, that, I think, would be more about color range you want to work with. If you're finishing the pieces in glaze earthenware temps give you brighter colors and a broader color range.

Hope this helps?

 

[Kakes]

On 4/13/2023 at 12:13 PM, Min said:

Pete Pinnell did a test of claybody types many years back and one of the things they tested for was modulus of rupture, don't know how relevant that would be to your question although it's an interesting read. Link here to it. I would hazard a guess that how you pack and ship your sculptures might be more germain.  

Sorry your question didn't elicit any responses in the chem section.

Thank you for this link - fascinating! I'm surprised that grog weakens clay and that red earthenware fired to 04 is the strongest. Do you think MOR would also indicate chipping resistance?

[Kakes]

On 4/13/2023 at 1:03 PM, Callie Beller Diesel said:

If you’re concerned about the fragility of a sculpture as opposed to a pot, how the piece is constructed is going to have more of a bearing on the durability than the clay body. A thin walled piece constructed with the best methods described in Min’s link above will still break sooner than a brick made using the exact opposite principles. If your sculptures are thin walled and unglazed, making them out of earthenware glazed with a well fitted glaze will add strength, but you’ll still need a lot of packing padding. If your pieces are solid, they’ll withstand more impact. But even construction bricks break if they’re dropped from enough height.

That's exactly my situation - I make thin walled slab built pieces with a lot of unprotected edges. Thanks for this info - i need to investigate the type of glazes I like to see if they fit. 

[Kakes]

On 4/13/2023 at 1:40 PM, Jeff Longtin said:

Hey Kakes,

The first thing to mention is your terminology.  Typically stoneware clay is not considered "low-fire". It used to be considered "high-fire" (fired to cone 10) but more recently many bodies have been mixed to vitrify at a "mid-fire" temperature. (cone 6) 

Earthenware clay is more typically considered "low-fire". ( fired to cone 06) The two local companies, Minnesota Clay and Continental Clay, both sell earthenware clay but I know several potters who like to fire their earthenware pots to cone 2.

In a sense any vitrified clay is highly durable. In my experience, however, earthenware tends to be a bit more brittle than stoneware or porcelain clay, after firing.

As to which is best, that, I think, would be more about color range you want to work with. If you're finishing the pieces in glaze earthenware temps give you brighter colors and a broader color range.

Hope this helps?

 

Thank you - very helpful! I'd like to experiment with earthenware clay fired to 04. It feels a bit daunting because I'll need to find a different set of glazes.

[Min]

7 hours ago, Kakes said:

Do you think MOR would also indicate chipping resistance?

I'm not so sure it would. For testing MOR a three point support system holds the sample piece while pressure is applied in the middle of the sample until the sample breaks. (I simplified the process) In real life this isn't what happens. We bang or drop a piece and it chips or cracks. Lots of variables as have already been mentioned plus there are factors such as sintered vs vitrified, clay density, particle size etc. If you have concerns then look into which couriers cover breakage plus what the cost and coverage is. There are many articles online about how to best pack ceramics for shipping.

[Kakes]

Thank you, everyone, for your helpful replies. I think i'll experiment with earthenware and see how that goes. 

[neilestrick]

Keep in mind that Pinnell's test was just one of several different types of tests that could be done to determine the strength of clay, and that most clay bodies are plenty strong for what we do with them. If strength was really an issue then we'd all be using low fire red clay, yet it's one of the least used bodies there is among ceramic artists. And although grog weakens bodies, it's still used in a large percentage of stoneware bodies because it has other benefits and doesn't weaken it enough to matter. Pinnell's tests are incredibly interesting and the results surprised most of us, but they're more academic than practical for the typical potter. The far bigger issues when determining the strength of a piece are the form, thickness, and construction technique. Little fingers on a figurative sculpture are going to be easy to break off regardless of what type of clay body you use. We select clay bodies based on firing temp, texture, and color, not MOR test results.

[Callie Beller Diesel]

It should also be mentioned that there’s a narrower window where nice looking and well melted, well fitted glazes overlap at lowfire than at cone 6 or cone 10.  The lower you go, the better versed in ceramic tech you need to be to get it to work. You won’t do it just out of the box with commercially made materials. 

[Kakes]

2 hours ago, neilestrick said:

Keep in mind that Pinnell's test was just one of several different types of tests that could be done to determine the strength of clay, and that most clay bodies are plenty strong for what we do with them. If strength was really an issue then we'd all be using low fire red clay, yet it's one of the least used bodies there is among ceramic artists. And although grog weakens bodies, it's still used in a large percentage of stoneware bodies because it has other benefits and doesn't weaken it enough to matter. Pinnell's tests are incredibly interesting and the results surprised most of us, but they're more academic than practical for the typical potter. The far bigger issues when determining the strength of a piece are the form, thickness, and construction technique. Little fingers on a figurative sculpture are going to be easy to break off regardless of what type of clay body you use. We select clay bodies based on firing temp, texture, and color, not MOR test results.

Very good points. I'm probably not a 'typical' ceramic artist in that my goals run counter to what most people want (some of the time), especially in terms of glaze. But I'm attracted to low fire clay because of less warping and distorting of forms. And the practical advantages of less wear and tear on my kiln. However, my patience for endless experimenting is limited, so I'll have to take that into account too!

[Kakes]

1 hour ago, Callie Beller Diesel said:

It should also be mentioned that there’s a narrower window where nice looking and well melted, well fitted glazes overlap at lowfire than at cone 6 or cone 10.  The lower you go, the better versed in ceramic tech you need to be to get it to work. You won’t do it just out of the box with commercially made materials. 

This is something I find really frustrating about ceramics. I come from a background in painting, so it's been a major shift in working methods and especially my attitude towards these technical demands. It's overwhelming at times. I realize there's no shortcut. Sorry, just venting. I do appreciate the technical expertise offered here and have learned some very important tips that have made a huge difference!

[Jeff Longtin]

1 hour ago, Kakes said:

I'm probably not a 'typical' ceramic artist...

Truth be told Kakes you are exactly a "typical ceramic artist".  No need to apologize. The major ceramic companies, Duncan and Amaco, designed their product lines to serve customers just like you. The ceramic choices at 06-04 are much more diverse than the choices at cone 6/10.

I work with a number of young people, recent ceramic grads, who are not concerned with durability or process but instead with simply making "interesting/unique" ceramic objects. I applaud their ambition as I applaud yours. 

Your results may not be, exactly what you want them to be, but if you lower your expectations a notch you will find some amazing results nonetheless.

[neilestrick]

3 hours ago, Kakes said:

But I'm attracted to low fire clay because of less warping and distorting of forms. And the practical advantages of less wear and tear on my kiln.

All very good reasons to work with low fire clay!

[Juxtaposie Jen]

On 4/16/2023 at 2:08 PM, Kakes said:

Very good points. I'm probably not a 'typical' ceramic artist in that my goals run counter to what most people want (some of the time), especially in terms of glaze. But I'm attracted to low fire clay because of less warping and distorting of forms. And the practical advantages of less wear and tear on my kiln. However, my patience for endless experimenting is limited, so I'll have to take that into account too!

I find myself in the same boat. I am repeatedly drawn to low fire clay and processes and I’m not really sure why.  This forum hada shown me I’m not alone.  

[Hulk]

Looks like many options for bright colored glazes in low fire as well.