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Low-Fire Fritware Question


arcadiax

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Hi Everyone,

 

Just a simple quick question about low fire fritware from a noob :P.

 

I remember reading somewhere that when bisquing, it is better to fire 2 cones above your glazing cone to get a better burn off during maturation so you can minimise pin-holing issues and some other minor glaze issues during glaze firing.

 

Example: Bisque to cone 02, glaze fire to cone 04.

 

Reason being if you hit the same temperatures again in glazing, there might be burns offs happening still - the source explained.

 

Ok, so my question is related to low fire fritware as fritware has higher glass body material and if it specifies the body is for low fire cone 04, can I bisque to cone 02? I don't want the glass fritware body to sag as I know glass tends to sag if you over fire - especially with a body that has lots of glass content.

 

It was a recipe introduced to me in my other thread and i thought I'd start a new thread specifically asking about this question.

 

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This is the original recipe by Joel:

 

5% talc, 33% Frit 3124, 12% Minspar 200, 25% EPK Kaolin, 25% #6 Tile, 1% Macaloid.

 

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Updated Version by Nerd:

 

7% talc, 30% Frit 3124, 12% Minspar 200, 25% EPK Kaolin, 25% #6 Tile, 0.5% Macaloid.

 

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These recipes I have turned into slip casting bodies already and they will be ready to go after I create 2 more other test recipes.

 

Thanks for reading, any advice really appreciated!

 

- Steven

 

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Stephen:

 

The recipe I provided only has 4.17% alkali, and 18.64% total fluxes. Low fire recipes commonly run 20-24% total fluxes: nothing to be concerned about. Frits can be fired all the way up to cone 10. Small, delicate, very thin pieces might show some sag, but that is unlikely.

 

Nerd

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You're talking about a body that vitrifies at 04, correct? If you bisque to 04 or above, glazing will be difficult, because you want a porous surface for applying glaze. You'll need to figure out what temp to bisque to, something below 04. You want to get as hot as possible so everything gets burned out that needs to, but not so hot that it's no longer porous. Just a guess, but I'd start at 012 and go from there.

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You're talking about a body that vitrifies at 04, correct? If you bisque to 04 or above, glazing will be difficult, because you want a porous surface for applying glaze. You'll need to figure out what temp to bisque to, something below 04. You want to get as hot as possible so everything gets burned out that needs to, but not so hot that it's no longer porous. Just a guess, but I'd start at 012 and go from there.

 

Yes, I would think so that this is partially vitrified...? I've been doing some reading on and off and I guess mostly everyone is saying that true vitrification does not occur in low fire clays and most low fire clays are talcy blend in nature...? To be honest, my experience in describing ceramic knowledge is still quite general lol...I think I am more focused on the artistic application of Ceramic Design although I realize ceramics is also a very geologically chemical process =P...

 

http://ceramicartsdaily.org/ceramics-monthly/article/unity-form-and-surface-a-new-approach-to-fritware/

 

That is the recipe I am following along with its augmented version by Nerd.

 

If I was to bisque the ware lower for glaze application, would I need to soak the wares at a longer maturation period during glaze firing? 

 

- Steven

 

Stephen:

 

The recipe I provided only has 4.17% alkali, and 18.64% total fluxes. Low fire recipes commonly run 20-24% total fluxes: nothing to be concerned about. Frits can be fired all the way up to cone 10. Small, delicate, very thin pieces might show some sag, but that is unlikely.

 

Nerd

 

Sounds good Nerd, thanks for always following up in my threads. You're a true life saver^^.

 

- Steven

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If you look at the recipe at the bottom of the article, he shows that the absorption rate is 0.4% at cone 04. That's vitrified. This is not a typical white talc earthenware body that remains porous at cone 04. If you bisque to 04, it will be just like trying to glaze a piece of porcelain that went to cone 10. You have to figure out a lower cone for bisque if you want glaze application to be easy.

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If you look at the recipe at the bottom of the article, he shows that the absorption rate is 0.4% at cone 04. That's vitrified. This is not a typical white talc earthenware body that remains porous at cone 04. If you bisque to 04, it will be just like trying to glaze a piece of porcelain that went to cone 10. You have to figure out a lower cone for bisque if you want glaze application to be easy.

 

Oh I see Neil!

 

Ok, so (Just repeating to clarify for myself as well), vitrified usually means we're done, the body has been compressed to the desired compression due to material transformation right? vitrification also usually means the ware is safe usually for liquids and food because there is no microscopic holes for bacteria to hide right? The absorption rate you mentioned kind of made me realize that absorption means the ware can no longer really absorb any moisture when it's rate is only at 0.4%? Usually within what absorption range is considered vitrified - anything under 1% or...?

 

I think now I do somewhat understand what you mean by the porous aspect as glazes needs something to seep and cling into so having a bisque not at maturation yet provides such pores.

 

If that is the case...you mentioned in your previous post starting at cone 012. That's around decal firing range, can I go slightly higher for bisque? I was thinking around cone 08? Of course...I am totally guessing right now.

 

- Steven

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If you look at the recipe at the bottom of the article, he shows that the absorption rate is 0.4% at cone 04. That's vitrified. This is not a typical white talc earthenware body that remains porous at cone 04. If you bisque to 04, it will be just like trying to glaze a piece of porcelain that went to cone 10. You have to figure out a lower cone for bisque if you want glaze application to be easy.

 

Oh I see Neil!

 

Ok, so (Just repeating to clarify for myself as well), vitrified usually means we're done, the body has been compressed to the desired compression due to material transformation right? vitrification also usually means the ware is safe usually for liquids and food because there is no microscopic holes for bacteria to hide right? The absorption rate you mentioned kind of made me realize that absorption means the ware can no longer really absorb any moisture when it's rate is only at 0.4%? Usually within what absorption range is considered vitrified - anything under 1% or...?

 

I think now I do somewhat understand what you mean by the porous aspect as glazes needs something to seep and cling into so having a bisque not at maturation yet provides such pores.

 

If that is the case...you mentioned in your previous post starting at cone 012. That's around decal firing range, can I go slightly higher for bisque? I was thinking around cone 08? Of course...I am totally guessing right now.

 

- Steven

 

 

You've got it! I think cone 08 will be too close to the peak temp to leave it porous enough to glaze well. Cone 012 will put at or just below the melting point of the frit, so I thought that would be a good starting point. Nerd will definitely have a more educated answer on that. 012 is pretty low to ensure that everything gets burned out in the bisque, so if you can go hotter than that you you should. But I have a feeling that once the frit starts to melt it's going to tighten up the body pretty quickly. You may need to slow down the glaze firing a little bit to complete the burnout process. The plus side is that that body doesn't have anything particularly dirty in it to cause big problems. A hold at the end of the bisque, probably at a slightly lower temp so as not to increase the cone, will also help. There's a lot of maybes and what ifs at play here.

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If you look at the recipe at the bottom of the article, he shows that the absorption rate is 0.4% at cone 04. That's vitrified. This is not a typical white talc earthenware body that remains porous at cone 04. If you bisque to 04, it will be just like trying to glaze a piece of porcelain that went to cone 10. You have to figure out a lower cone for bisque if you want glaze application to be easy.

 

Oh I see Neil!

 

Ok, so (Just repeating to clarify for myself as well), vitrified usually means we're done, the body has been compressed to the desired compression due to material transformation right? vitrification also usually means the ware is safe usually for liquids and food because there is no microscopic holes for bacteria to hide right? The absorption rate you mentioned kind of made me realize that absorption means the ware can no longer really absorb any moisture when it's rate is only at 0.4%? Usually within what absorption range is considered vitrified - anything under 1% or...?

 

I think now I do somewhat understand what you mean by the porous aspect as glazes needs something to seep and cling into so having a bisque not at maturation yet provides such pores.

 

If that is the case...you mentioned in your previous post starting at cone 012. That's around decal firing range, can I go slightly higher for bisque? I was thinking around cone 08? Of course...I am totally guessing right now.

 

- Steven

 

 

You've got it! I think cone 08 will be too close to the peak temp to leave it porous enough to glaze well. Cone 012 will put at or just below the melting point of the frit, so I thought that would be a good starting point. Nerd will definitely have a more educated answer on that. 012 is pretty low to ensure that everything gets burned out in the bisque, so if you can go hotter than that you you should. But I have a feeling that once the frit starts to melt it's going to tighten up the body pretty quickly. You may need to slow down the glaze firing a little bit to complete the burnout process. The plus side is that that body doesn't have anything particularly dirty in it to cause big problems. A hold at the end of the bisque, probably at a slightly lower temp so as not to increase the cone, will also help. There's a lot of maybes and what ifs at play here.

 

 

Sweet Neil. 

 

I will wait for Nerds reply, hopefully he sees this thread again lol...

 

- Steven

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Neil said: You've got it! I think cone 08 will be too close to the peak temp to leave it porous enough to glaze well. Cone 012 will put at or just below the melting point of the frit, so I thought that would be a good starting point.

 

I think Neil's recommendation is spot on: and for the exact reason of the early melt of frit.

 

You need to be less concerned about carbons because frits have already burned them off. Carbon burn off usually occurs between 1100-1300F; when the kiln is starting to turn red. EPK is fairly clean, the only real carbon would be the talc. You can calcine your talc to 1800F, to burn off carbon before you add it. You can also switch to a white talc after you deplete current stock.

 

A more specific answer about glazing a piece that is already vitrified. It has to do with porosity, but it also has to do with the flux melting and sealing the face of the piece. The clay/glaze interface is a combined reaction of the clay bonding to the glaze, and the glaze bonding to the clay: its an equal reaction as this picture shows.

 

Clayglaze interface

 

Notice the white body of the clay is pulled into the glaze, as well as the glaze is pulled into the body. So there is a mechanical structure that forms when it is properly fired. I

 

Nerd

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Neil said: You've got it! I think cone 08 will be too close to the peak temp to leave it porous enough to glaze well. Cone 012 will put at or just below the melting point of the frit, so I thought that would be a good starting point.

 

I think Neil's recommendation is spot on: and for the exact reason of the early melt of frit.

 

Nerd

 

 

YES! NERD APPROVAL! :D I was thinking the same thing about the talc being the only potential issue in there with bisquing so low. Nerd, do you know of a white talc that is considered to be as safe as the dirty Texas talc? I hate the color of that stuff. It just makes clay bodies UGLY. Also, would the calcining of the talc make any difference in the workability/feel of the clay- does the particle structure of the talc change in some way during calcining that would be noticeable?

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Neil:

 

I do not know of any on the open market: there are technical grades that come already calcined. I have not noticed any change in properties, nor have I seen anything in technical papers that suggest a physical change in properties. I have not looked hard at Sierra, but on my list to check out. The LOI on talc are all within a small fraction of each other: which pretty much tells the carbon story.

 

Nerd

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You can calcine the talc yourself at around 900C. I've found that some materials can clump together if you calcine them - a session in a ball mill if you have one will fix this, but try it and see.

As regards vitrification and porosity, the best thing is to test it yourself. Make some numbered test bars say 12cm x 2cm x 0.5cm. Then fire at a range of temperatures - one every couple of cones is a good starting point - and weigh them when they come out of the kiln and have cooled. Then put into water for 5 minutes, remove and dab off the surface water, and weigh again. This gives you how porous they are. If you plot this out, you'll see quite a kink when vitrification starts - keep below this when bisque firing, then go above it for your final firing if you want it vitrified.

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