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Bisque firing earthenware & stoneware


Emma Louise

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I remember some people having strong feelings that e/w & s/w should be bisqued at
different temperatures, in order to have the same absorbancy when glaze-dipping.
[In the context of fairly high-fire s/w.]

I suppose that it makes sense if you have high throughput and are only using two clays.

Regards, Peter

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  • 2 weeks later...

Based on the replies above, obviously what I think I know is dead wrong and I need to be educated.  Please educate me!  See, I use low fire (04) clay AND medium fire (cone 6) clay and it would never occur to me to bisque them together.  How could the cone 6 clay mature in an 04 firing?  Wouldn't it still need to be fired to cone six before it could be glazed?  (Gee, can you tell I am self-taught? I suspect I "know" a bunch of stuff that is totally incorrect!)

:huh:

Rusty

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The only reason, that I can think of, where you'd not want to fire low fire and mid to high fire together, would be if you want to keep the mid/ high fire *more* porous. 

The low fire glazes I use, recommend bisquing the clay to 04, and then glaze firing to 05.  So if someone didn't want to risk having their low fire slightly underfired, or their mid/ high fire over fired, in regards to the bisque, then I can see them advising against it. 

Honestly though, I do not have enough experience and expertise to know how much one Cone difference can make, when it comes to absorption. 

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2 hours ago, Rusty Hills Farm said:

Based on the replies above, obviously what I think I know is dead wrong and I need to be educated.  Please educate me!  See, I use low fire (04) clay AND medium fire (cone 6) clay and it would never occur to me to bisque them together.  How could the cone 6 clay mature in an 04 firing?  Wouldn't it still need to be fired to cone six before it could be glazed?  (Gee, can you tell I am self-taught? I suspect I "know" a bunch of stuff that is totally incorrect!)

:huh:

Rusty

The typical method for studio potters is to bisque everything at low fire temps, so it is porous for glazing. After glazing, the pieces are fired to whatever temperature is needed to mature the clay and glaze, whether that be another low firing, or hotter like cone 6. If you bisque to the maturing temperature of mid-high fire clay bodies, they will no longer be porous, and will be very difficult to glaze.

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9 hours ago, neilestrick said:

The typical method for studio potters is to bisque everything at low fire temps, so it is porous for glazing. After glazing, the pieces are fired to whatever temperature is needed to mature the clay and glaze, whether that be another low firing, or hotter like cone 6. If you bisque to the maturing temperature of mid-high fire clay bodies, they will no longer be porous, and will be very difficult to glaze.

Okay, I think I see WHY I am misunderstanding this!  Currently I make bonsai pots with my cone 6 clay and apply Renaissance glaze and fire once to have a finished pot.  I was ASSUMING that if I did this as a 2-step process, both firings should be at cone 6.  But what you are showing me here is that the piece would be resistant to the glaze, and I'd get better results from the low fire bisque, then glaze and fire at cone 6 to mature the clay and the glaze together--am I now understanding this correctly? 

LOL You'd think by now I'd know better than to assume stuff!  Prior to this I only worked with low-fire clay.  These current pots are my first attempts with stoneware, which I thought would make stronger pots for my bonsai.  I've never had a pot break from winter cold, but even though I'm in the south, the climate is definitely changing. 

Thanks for all your help.  There is so much to learn!  But that is part of what makes this hobby so enjoyable.

Rusty  :D

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Methinks there's a misunderstanding here. No matter the clay, once it's been bisqued it's all earthenware. It's not until you high fire the bisqued piece doe sit become stoneware.

For instance, a raku or saggar pot remains earthenware when it's complete. A glazed (or not galzed) pot that's fired to stoneware temps like cone 6 becomes stoneware.

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better yet, apply the label "stoneware"  when the moisture absorbance of the fired clay becomes nil, not when a predetermined firing temperature is reached.   

I used to fire a commercial low-fire clay body (cone 04) to cone 3.  Those pieces had zero water uptake and were stronger and had less water uptake than some commercial mid-range clay bodies fired to cone 6.  The same goes for glazes.  I have a colleague that uses a commercial low-fire (04) clear glaze as his base glaze for all of his cone 10 reduction work. He has never had a problem with the glazes in the 8 years I have worked with him. 

It is my opinion that the pottery community focuses too much on cone number labels and not enough on the maturity of the clay bodies and glazes we use.  

LT
 

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Hi I am a newbie to the community and novice when it comes to ceramics too. 
So on this topic can I ask what difference it would make to my pots if I bisqued to 03
and then used 06 glazes and tried to fire them to say cone 2 -
will this improve the strength of the pot  and what effect would it have on the glazes - 
would they melt off?  because they are too hot?

 

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Hi Hils and welcome to the forum!

What you are asking might work or might not, every clay and glaze is going to behave differently. There isn't a one size fits all answer to this.

I noticed you are in the UK where broad firing range clays seem to be quite common. Which clay are you using (or do you have the firing range for it)? It is quite common to bisque fire earthenware to a hotter cone than the glaze firing as the clay will still be porous enough to take the glaze and this enables more of the organics to burn out of the clay prior to glaze firing where off-gassing can cause pinholes in the glaze. If your glaze will only go to cone 06 without running then a hotter bisque fire might be in order, but many glazes have some wiggle room in how hot they can go. Again it depends on your clay and glazes and testing to see what they do at different cones.

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Quote

 

Hi Min thank you for you reply   the clay we use is a School buff which is listed at Potclays  as a stoneware clay that fires between 1100  and 1200   but we have only ever bisque fired it at cone 04  and then painted with Duncan underglazes and dipped in clear glaze  and fired at cone 06 -   the people at potclays seemed shocked that we had not had a problem with the glaze not slipping off  so I am confused because I thought the school buff was an earthen ware clay in the first place and so  am obviously totally muddled.  Would like to know if I could still bisque fire it at 04  and then fire to 02 to give it more strength  or whether that would be a problem  and as it seems to be a stoneware clay anyway is it okay to use it as I have been.

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@Hils, hmmmm, so it seems you do have one of those broad firing range claybodies. 1100C up to 1200C is approximately going from cone 02 all the way up to cone 5 1/2. The clay can't be tight at earthenware temps (like 02) all the way up to cone 5. At the lower temps it's going to be more porous and weaker than if it was fired to it's maturity at cone 5. If the pots are non functional it won't be much of an issue but if you are making functional pots then chances are they will leak sooner or later (and develop crazing). 

Re your question about firing to cone 02 to give it more strength. It will be marginally stronger than your 04 firing, but not appreciably so. If you want to stick with lowfire temperatures can you find a clay (probably from a different manufacturer) that is just for those temps, or fire up to cone 5 with your existing clay? I don't know what your glazes will do at cone 02, you would have to try them and see. If you do fire hotter it's a good idea to put a very thin waster cookie of clay under the pots so if the glaze does run the waster cookie will protect your kiln shelves.

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On ‎12‎/‎20‎/‎2018 at 8:53 PM, Min said:
21 hours ago, Min said:

I don't know what your glazes will do at cone 02, you would have to try them and see. If you do fire hotter it's a good idea to put a very thin waster cookie of clay under the pots so if the glaze does run the waster cookie will protect your kiln shelves.

 

Hi Min thankyou for your reply I think I am understanding it a little better now - so basically if I want to make functional items like mugs or bowls I should bisque fire to 04  as usual but then glaze fire to cone 5  so my glazes would need to be cone 5 glazes.    My concerns on firing to cone 5 are simple the wear and tear on the elements  as we run a Paint a Pot Studio and only glaze  fire the bisque we buy to cone 06   so the kiln has never been taken to the higher temp ranges  - it does have a top temp of 1300 ( cone 10 I believe)  but everyone I talk to  says that higher cone temps hammer the elements. I am assuming that s thin wafer cookie is a small  thin disc of clay that has been bisque fired  Thanks again for you help

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At a combo of low fire bisque and cone 5/6 glaze, you can expect to get about 150 firings from a set of elements. Doing low fire only, you'll get double that. it's just one of the things you have to accept if you want to make vitrified, durable ware. Just make sure your clay and glazes all mature at cone 5. If you run a low fire white earthenware body to cone 5 it will melt.

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Yeah, on the North America side of the pond earthenware is not made to fire to the midfire range but in the UK PotClays Earthenwares have a wide firing range. From the Potclays site they have "earthenwares" that have a firing range from 1100C - 1250C, so approximately cone 02 - 8 for one of their bodies, and 02 thru 5 1/2 and 6  for others. 

 

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On 12/21/2018 at 12:21 PM, Hils said:

Hi Min thank you for you reply   the clay we use is a School buff which is listed at Potclays  as a stoneware clay that fires between 1100  and 1200   but we have only ever bisque fired it at cone 04  and then painted with Duncan underglazes and dipped in clear glaze  and fired at cone 06 -   the people at potclays seemed shocked that we had not had a problem with the glaze not slipping off  so I am confused because I thought the school buff was an earthen ware clay in the first place and so  am obviously totally muddled.  Would like to know if I could still bisque fire it at 04  and then fire to 02 to give it more strength  or whether that would be a problem  and as it seems to be a stoneware clay anyway is it okay to use it as I have been.

Hi @Hils.  Like you I thought the "school buff" was a low-fire clay 04-06.

We switched away from that to Valentines Special Fleck Stoneware, and fire it to ^04-06 for decorative stuff, and I occasionally fire it to ^6 for bonsai pots, ovenware and outdoor stuff that needs to be freeze-resistant.

It seems quite happy at ^04, sometimes pings, but most of the stuff made at the centre is flat or slab-built and so far, it all survives.

I know it's wrong to fire to the wrong cone, but.......  the SF is the cheapest we can buy from either of my local suppliers, so ........

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27 minutes ago, Min said:

Yeah, on the North America side of the pond earthenware is not made to fire to the midfire range but in the UK PotClays Earthenwares have a wide firing range. From the Potclays site they have "earthenwares" that have a firing range from 1100C - 1250C, so approximately cone 02 - 8 for one of their bodies, and 02 thru 5 1/2 and 6  for others. 

 

Sorry I should have specified. I meant low fire white talc bodies like we use here.

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1 hour ago, neilestrick said:

 If you run a low fire white earthenware body to cone 5 it will melt.

 

6 minutes ago, neilestrick said:

Sorry I should have specified. I meant low fire white talc bodies like we use here.

I have the evidence of a low fire earthenware melting at ^6.  It wrecked the bottom of my kiln.  (Luckily I don't have to be reminded as I didn't take photos, and the floor of the kiln is now covered with a shelf.

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31 minutes ago, Chilly said:

 

I have the evidence of a low fire earthenware melting at ^6.  It wrecked the bottom of my kiln.  (Luckily I don't have to be reminded as I didn't take photos, and the floor of the kiln is now covered with a shelf.

I've had to throw out many customer's kilns that have had full loads of white talc go to cone 4 instead of 04.

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21 hours ago, Chilly said:

Hi @Hils.  Like you I thought the "school buff" was a low-fire clay 04-06.

We switched away from that to Valentines Special Fleck Stoneware, and fire it to ^04-06 for decorative stuff, and I occasionally fire it to ^6 for bonsai pots, ovenware and outdoor stuff that needs to be freeze-resistant.

 

Hi Chilly  thanks for that  I am looking to switching to the white earthenware that Cromartie sell as it seems to be a better bet for low fire work that we do mostly but wanting to throw a few mugs and bowls (when I can get a little more competent)  and not wanting to have the confusion of different clays to reclaim etc.  I think it is likely to be similar to the Paint a pot bisque ware we buy from them so should be okay for most of what we want to do but I will look up the Valentines Special fleck too.  Thxs

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23 hours ago, neilestrick said:

At a combo of low fire bisque and cone 5/6 glaze, you can expect to get about 150 firings from a set of elements. Doing low fire only, you'll get double that. it's just one of the things you have to accept if you want to make vitrified, durable ware. Just make sure your clay and glazes all mature at cone 5. If you run a low fire white earthenware body to cone 5 it will melt.

Thanks for that Neil - I am shocked that it is so few firings  - elements are pretty expensive aren't they   would it be likely that they would have to be all be replaced at once?   I think we would probably try to keep the higher firing to a minimum  but it still sounds like it might get costly.   Cheers

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