Why do they bisque firing at cone 010?

[Tabathos]

Hello. 

I live in Costa Rica, and in the college where I took some pottery classes, they bisque firing at cone 010.

Almost every book I read says that bisque firing should be done at cone 04-06. Any reason our clay is bisqued fired at lower temperature? It is a red clay, with a high content of iron. Our teacher says that iron acts as a flux, so I wonder if that is the reason they decide to fire it at cone 010.

I want to make functional pottery and glaze fire at mid range (cone 4-6), because of all the sanitary needs. Stoneware clay is commonly used for these purposes, but it seems that our native clay is earthenware. They add refractory grog to red clay, so I suppose it is a way to make it more resistant to higher temperatures that are required to glaze fire at mid range, but still bisque fired at cone 010.

As a topic, there is a trend here to reduce firing temperatures, as it seems to be more practical regarding economic and ecological factors. Functional ware is fired at cone 2, instead of higher temperatures, but I've seen test tiles of cone 6 hanging there. I wonder if this reduction in glaze firing has implications in the quality of functional ware. 

I'm seriously confused, and I can't ask anyone as we're in quarantine lockdown. Thanks in advance. 

 

[neilestrick]

It could be that the red earthenware clay gets too tight to take glaze well if bisque fired hotter than cone 010. And they probably glaze fire to cone 2 because it vitrifies fairly well at that temp. Red earthenware usually isn't fired above cone 3-4, even with grog. If you want to fire to cone 6 you'll probably need a different clay body.

Some clay bodies have problems when bisque fired that low, but some will do fine. In general we like to bisque to at least cone 06 to make sure everything gets burned out that needs to, otherwise it can cause problems with the glazes. Is there any chance you'd be able to bisque fire your work separately, so you can go to whatever cone you want? Like if you filled the kiln with just your work?

Reduction firing will have no impact on the durability of the work.

[Tabathos]

Thank you very much for your insights, Neil. 

I have my own kiln. I bisque fire at cone 010 and glaze fire at low temperatures, and it seems to work fine. But I want to mix my own glazes and make functional ware, so I bought this book from John Britt called "The complete guide to mid-range glazes: glazing and firing at cones 4-7", and I was thinking of just trying the recipes as they currently are. It seems it won't be as easy as I thought it would be. 

I guess there are certain ways to reduce the temperature of the glazes of the book using a software, but I guess it doesn't hurt to make some tests with the earthenware I have. We don't have a ceramics supplier that sells stoneware clay, so I think I have to adapt to what I have here. For example, we are unable to make porcelain using the native kaolin, as it has very low quality. I suppose it is something related to the fact that geologically our land is relatively new, compared to the mines from China, or USA. 

[Mark C.]

When I was in Costa Rica in the middel 80s I recall all the clay was red earthenware fired to lower temps. So that sounds like its still that way..I do not recall any high temp clays or even mid range clays.

[neilestrick]

If you can get the native kaolin, I would try mixing it with your red clay and seeing if you can make a body that vitrifies at cone 5. Bringing those glazes down to cone 2 will be difficult.

[Tamas]

I have a related question. I have been watching some Japanese videos about making kohiki ware lately (like this one: https://www.youtube.com/watch?edufilter=NULL&v=bdwDsUNL0f8) and noticed that they bisque fire to 700 centigrade (which is like cone  018?) and glaze fire to 1230 degrees (cone 6-7) in reduction. Is their clay that different, or is there some other consideration behind this technique?

 

Thanks.

[Mariesa]

Holaaa, yo soy de Costa Rica y también experimenté con cono 010 por primera vez hace unas semanas. Nunca en mis 3 años haciendo cerámica había horneado a este cono hasta que conocí a unas estudiantes de la UCR. A mi gusto me quedo con 04, exponer la pieza a tanta porosidad puede generar grietas a la hora de esmaltar por inmersión. Si quisieras hablar más me podes escribir a @hechodearcilla Ig  

[Min]

2 hours ago, Mariesa said:

Holaaa, yo soy de Costa Rica y también experimenté con cono 010 por primera vez hace unas semanas. Nunca en mis 3 años haciendo cerámica había horneado a este cono hasta que conocí a unas estudiantes de la UCR. A mi gusto me quedo con 04, exponer la pieza a tanta porosidad puede generar grietas a la hora de esmaltar por inmersión. Si quisieras hablar más me podes escribir a @hechodearcilla Ig  

Google translation: 

Hi, I'm from Costa Rica and I also experimented with cone 010 for the first time a few weeks ago. Never in my 3 years making ceramics had I baked this cone until I met some students from the UCR. To my liking I prefer 04, exposing the piece to such porosity can cause cracks when immersion enamelling. If you want to talk more you can write to me @hechodearcilla Ig 

 

[Anjy Cameron]

Hi,   I am trying to research if there are any supplies of white earthenware clay in Costa Rica. I would like to do some slip casting with a white clay body.

Thanks Anjy