Porcelain bisque temperatures for low and high fire glazes

[nhendel]

I've been working with my kiln for about a year and I've been struggling recently with a lot of crazing... I'm not sure what I'm doing wrong, and would like some tips on temperatures for bisque firing porcelain for cone 04/05 glazes as well as recommended bisque temps for cone 5/6 glazes... Is that what would be causing all of the crazing issues? Additionally, I made some mugs with Spectrum's pistachio low fire glaze on porcelain, which resulted in crazing, and I 'm wondering if it is a bad idea to use them for coffee?

Thanks for any and all help!

 

[neilestrick]

The cause of crazing is the fit between the clay and the glaze. If the glaze shrinks a little more than the clay, it crazes. The only way to fix it is to adjust the recipe of the clay or glaze (glaze, usually), which is not possible if you're using commercial products.

In my experience, low fire glazes do not fit well on porcelain. You would be better off using a white earthenware if you're going to glaze fire at cone 05/06. You don't want crazing on low fire work, because liquids can soak through the cracks in the glaze and into the wall of the pot.

At cone 5/6, make sure you're using a cone 5/6 porcelain, not one that will go to cone 10. Even then you will get crazing with some glazes, but it's not really an issue from a safety or functionality standpoint because the clay will be vitrified and won't soak up liquids through the crazed glazes. No glaze will fit all clay bodies, and vice versa. It will take a lot of trial and error to figure out which glazes fit your clay bodies.

Bisque at cone 04 regardless of glaze firing temp. If you bisque hotter, the clay won't be porous enough to glaze easily.

[nhendel]

Thank you!! So if I can hear my mugs absorbing liquids, do you think I should toss them and start again?

[Mark C.]

3 hours ago, nhendel said:

Thank you!! So if I can hear my mugs absorbing liquids, do you think I should toss them and start again?

no change -clay bodies

[nhendel]

1 hour ago, Mark C. said:

no change -clay bodies

Okay! But, with the current finished mugs I have, if they are absorbing liquids, do you think they are still food/drink safe for coffee?

[Benzine]

12 hours ago, nhendel said:

Okay! But, with the current finished mugs I have, if they are absorbing liquids, do you think they are still food/drink safe for coffee?

Maybe a time or two.

The problem is, the porous clay body absorbs liquids from food and drink, which then give a place for bacteria and mold to grow.  A person could then get sick, from using those wares. 

It is next to impossible to clean the porous ware, to prevent this.  

So it is best to just not use them.

[Mark C.]

Porus pots are mantle piece pots-for looks not for use.

Get better clay and make more-pots are Not precious -you can just make more as clay is cheap really,I would smash those pots and start over-your time has some value here.

[nhendel]

1 hour ago, Mark C. said:

Porus pots are mantle piece pots-for looks not for use.

Get better clay and make more-pots are Not precious -you can just make more as clay is cheap really,I would smash those pots and start over-your time has some value here.

Thanks!! What would you recommend as a good clay body for mugs and other dinnerware? Stoneware maybe?? I'm in Minnesota, so I get all of my clay from Continental Clay which is a pretty good business! Maybe domestic porcelain is just not the right way to go for functional ware?

[Benzine]

17 hours ago, nhendel said:

Thanks!! What would you recommend as a good clay body for mugs and other dinnerware? Stoneware maybe?? I'm in Minnesota, so I get all of my clay from Continental Clay which is a pretty good business! Maybe domestic porcelain is just not the right way to go for functional ware?

Nearly any clay that is properly vitrified, will make for good dinnerware.  Stoneware, porcelain, etc.  Low fire/ earthenware clay CAN work, as there are some that vitrify at Cone 1 or so.  Also, as Neil mentioned, if you properly glaze some porous low fire ware, it does seal the clay.  I use low fire, in my classroom, and we can make functional wares. 

The other aspect to "dinnerware safe" is glazing.  A porous ceramic body can make you sick, from microorganisms, that are growing on it.  A glaze that leaches heavy metals, or other such things, into food and drink, is a far worse thing. 

[nhendel]

13 hours ago, Benzine said:

Nearly any clay that is properly vitrified, will make for good dinnerware.  Stoneware, porcelain, etc.  Low fire/ earthenware clay CAN work, as there are some that vitrify at Cone 1 or so.  Also, as Neil mentioned, if you properly glaze some porous low fire ware, it does seal the clay.  I use low fire, in my classroom, and we can make functional wares. 

The other aspect to "dinnerware safe" is glazing.  A porous ceramic body can make you sick, from microorganisms, that are growing on it.  A glaze that leaches heavy metals, or other such things, into food and drink, is a far worse thing. 

Thank you! So, with the low fire glaze, it says to paint 2-3 coats, which I did and then I fired my bisque cone04 porcelain to cone 05 as the glaze directed... So, if my pots crazed... what do you think I did wrong?

[MartinB]

5 hours ago, nhendel said:

Thank you! So, with the low fire glaze, it says to paint 2-3 coats, which I did and then I fired my bisque cone04 porcelain to cone 05 as the glaze directed... So, if my pots crazed... what do you think I did wrong?

Is the porcelain you are using a cone 05 porcelain? Most porcelains are cone 6-10 (1200c - 1300c), cone 05 is 1046c so you are likely a long long way off your body being vitrified.

If you are using 04/05 glazes you need a body that is made for 04/05 temps...

[Benzine]

10 hours ago, nhendel said:

Thank you! So, with the low fire glaze, it says to paint 2-3 coats, which I did and then I fired my bisque cone04 porcelain to cone 05 as the glaze directed... So, if my pots crazed... what do you think I did wrong?

 

4 hours ago, MartinB said:

Is the porcelain you are using a cone 05 porcelain? Most porcelains are cone 6-10 (1200c - 1300c), cone 05 is 1046c so you are likely a long long way off your body being vitrified.

If you are using 04/05 glazes you need a body that is made for 04/05 temps...

Exactly what Martin said.

The glaze is crazing, because it doesn't fit the clay body.  It is trying to shrink more than the clay body does. 

You either need a new clay body, that is low fire, to go with the low fire glazes you have.  Or, you need to get some mid to high fire glazes (Depending on what Cone your porcelain is) and fire your higher. 

What is the max temp on your kiln?  Personally, I would just get some new glaze and fire hotter, if possible.  That way the clay is vitrified.  If you want to stick with low fire, get a new clay body, because that porcelain, just aint gonna work. 

 

[nhendel]

6 hours ago, Benzine said:

 

Exactly what Martin said.

The glaze is crazing, because it doesn't fit the clay body.  It is trying to shrink more than the clay body does. 

You either need a new clay body, that is low fire, to go with the low fire glazes you have.  Or, you need to get some mid to high fire glazes (Depending on what Cone your porcelain is) and fire your higher. 

What is the max temp on your kiln?  Personally, I would just get some new glaze and fire hotter, if possible.  That way the clay is vitrified.  If you want to stick with low fire, get a new clay body, because that porcelain, just aint gonna work. 

 

Okay! Thank you! I guess I've never really thought to ask what temp the clay bodies are! My kiln goes to Cone 10, so I will probably just go with high fire glazes then. Would you still recommend bisque firing to cone 05 with the porcelain and then using a high fire glaze and firing to cone 5/6?

[Benzine]

6 hours ago, nhendel said:

Okay! Thank you! I guess I've never really thought to ask what temp the clay bodies are! My kiln goes to Cone 10, so I will probably just go with high fire glazes then. Would you still recommend bisque firing to cone 05 with the porcelain and then using a high fire glaze and firing to cone 5/6?

I would shoot for a Cone 6 or Cone 8 clay.  Your kiln CAN go to Cone 10, but you will burn through your elements very quickly doing so.

Find a good clay body you like at those temperatures.  If you opt for Cone 6, those would be more of a mid-fire glaze.

Can you post a link to the the clay/ supplier you are using, or just give their details?

[MartinB]

14 hours ago, nhendel said:

Okay! Thank you! I guess I've never really thought to ask what temp the clay bodies are! My kiln goes to Cone 10, so I will probably just go with high fire glazes then. Would you still recommend bisque firing to cone 05 with the porcelain and then using a high fire glaze and firing to cone 5/6?

Around 1000dec C (cone 06) is a common bisque temp for mid (cone6/7) and high fire work(cone9/10), you might go a bit higher or bit lower for various reasons but consistency is more important so just aim for 1000 to start with.

Then find a clay body you are happy with that matures at the temp you've decided to work at. Most places do 1kg sample packs, order a few in and try throwing/building with them and fire them. Stoneware and Porcelain should not be absorbing water, or very little, if they are fully matured and vitrified.

[MartinB]

Some more info on clays/shrinkage and absorption

https://www.ceramicindustry.com/articles/84673-ppp-clay-body-shrinkage-absorption

https://digitalfire.com/4sight/glossary/glossary_functional.html

 

[Roberta12]

I just wanted to add to this thread by saying I have tried a lot of the continental Clays.  They are very nice, throw well.  Most, not all, of their clay bodies are rated for a wide range of temps.  Super White is rated 5-9.  I have used it and loved working with it, but all the glazes I used on it crazed.  If not right away, then later.  Their mid fire white is also quite nice to use.  It crazed with the clear I was using.  It crazed with all clears that I had mixed up.  But did not craze with other glazes.   Their mid range oxidation and the mid range oxidation with manganese are both very nice clays.  I did not have crazing problems with them.    I have tried to stay with clays that have a narrower range of firing temps.  I seem to have fewer problems that way.  I have my work and glazes dialed in with  3 Laguna clays right now.  And mainly that is because it is easier for me to get those clays than the Continental clays.  I use porcelain for a lot of my work, which is functional.  Porcelain can be a very good everyday use clay.  All of my pots go in the dishwasher and micro.  

I agree with Benzine about midfiring. (cone5/6)  kiln repair/replacement is a cost of doing business, but you don't want to have to do it more often than necessary.  Firing to cone 10 will accelerate that schedule! 

Roberta

[nhendel]

21 hours ago, Benzine said:

I would shoot for a Cone 6 or Cone 8 clay.  Your kiln CAN go to Cone 10, but you will burn through your elements very quickly doing so.

Find a good clay body you like at those temperatures.  If you opt for Cone 6, those would be more of a mid-fire glaze.

Can you post a link to the the clay/ supplier you are using, or just give their details?

Here's the link!

https://www.continentalclay.com

[Benzine]

On 9/6/2018 at 9:01 AM, Roberta12 said:

I just wanted to add to this thread by saying I have tried a lot of the continental Clays.  They are very nice, throw well.  Most, not all, of their clay bodies are rated for a wide range of temps.  Super White is rated 5-9.  I have used it and loved working with it, but all the glazes I used on it crazed.  If not right away, then later.  Their mid fire white is also quite nice to use.  It crazed with the clear I was using.  It crazed with all clears that I had mixed up.  But did not craze with other glazes.   Their mid range oxidation and the mid range oxidation with manganese are both very nice clays.  I did not have crazing problems with them.    I have tried to stay with clays that have a narrower range of firing temps.  I seem to have fewer problems that way.  I have my work and glazes dialed in with  3 Laguna clays right now.  And mainly that is because it is easier for me to get those clays than the Continental clays.  I use porcelain for a lot of my work, which is functional.  Porcelain can be a very good everyday use clay.  All of my pots go in the dishwasher and micro.  

I agree with Benzine about midfiring. (cone5/6)  kiln repair/replacement is a cost of doing business, but you don't want to have to do it more often than necessary.  Firing to cone 10 will accelerate that schedule! 

Roberta

 

15 hours ago, nhendel said:

Here's the link!

https://www.continentalclay.com

I have always used Continental, in my classroom.  I'm happy with their product, and they are one of the closest, large-scale suppliers.

@Roberta12 like you said, they list a wide range of firing temps, which is odd.  They also don't list specifications, like other suppliers do (shrinkage, absorption, etc.)  As many others here have said, a clay body can't actually have a wide firing range.  It will either be underfired, at the low end, or overfired at the high end. 

I would like to use them, for my own personal work, but I'm not sure they have what I need, based on the information they provide.

I will note, that I've had almost no crazing issues, with  the Continental Low Fire White, I use in my classroom.  The only crazing I've encountered are some of the decoration only, textured glazes, where it doesn't matter.

[MartinB]

2 hours ago, Benzine said:

Like you said, they list a wide range of firing temps, which is odd.  They also don't list specifications, like other suppliers do (shrinkage, absorption, etc.)  As many others here have said, a clay body can't actually have a wide firing range.  It will either be underfired, at the low end, or overfired at the high end.

Yeah I don't really trust a clay body that's listed as cone 6-10...

I know there's a few mid fire cone 6 porcelains in the US but I can't find any in the UK so its cone 10 for me.