Low-Fired (Cone 04) Porcelain Characteristics

[Soren]

If porcelain is only fired to cone 04, how does it compare to earthenware clays?

Particularly:
How strong is it?
What is the shrinkage rate?
How porous or absorptive is it?
How easy is it to color with oxides and stains to get bold colors?

I understand this probably varies for different porcelains, but a general idea would help me to know if I should even consider this.
What I am trying to determine is if I can use porcelain low-fired to obtain colored bodies but still have lower shrinkage and higher porosity/absorption.

Thanks,
Soren

[glazenerd]

Just now, Soren said:

If porcelain is only fired to cone 04, how does it compare to earthenware clays?

Particularly:
How strong is it? Weak, as porcelain goes compared to full vitrification. Yet, close to the strength of any other body at cone04. At cone 04, clay does not vitrify: it begins to vitrify at 2050F.
What is the shrinkage rate? At cone 6, porcelain averages 12-13%, with some blends as low as 10%, a few higher than 13%. At cone 04, shrinkage is around 6-7% pending formulation.
How porous or absorptive is it? High degree of both.
How easy is it to color with oxides and stains to get bold colors? Easier than most other bodies.

I understand this probably varies for different porcelains, but a general idea would help me to know if I should even consider this.
What I am trying to determine is if I can use porcelain low-fired to obtain colored bodies but still have lower shrinkage and higher porosity/absorption.

Thanks,
Soren

Frit ware, also called stone paste in some parts, can be composed of porcelain (kaolin). The definitions of clay bodies have seen a shift in the last decade: instead of porcelain, stoneware, terra cotta, or earthenware: there is a generalization of high, medium, or low fire. Frit ware is low fire, and formulation typically includes up to 35% frit. The frit serves as the body flux. Frit begins to melt at 1475F, long before typical fluxes used in clay and glaze. By cone 04, the frit is fluid enough to seal up the pores of the body: creating a "pseudo" vitrification.

in your other threads you were asking me the differences between commercial clays. I am not well versed in all the different brands: I have not purchased premade clays in a very long time. 

T

[neilestrick]

If you fire cone 10 or cone 6 porcelain to cone 04, it is not very strong, is very porous, and low fire glazes do no fit it very well. It's not the best way to do a low fire white. If it was, we wouldn't need white earthenware bodies. I don't think it'll be all that much better for coloring if you're not firing it to vitrification. Colors get bolder as they go into melt, either in the glaze or in a vitrified body. At cone 04 there is very little melting going on in the body.

If you're looking for something that is glassy and tight like porcelain, but only fired to cone 04, then a fritware body like Nerd described is the way to go. They are not commercially available, though, so you'll have to formulate your own and either mix it yourself or pay a clay company to mix it for you (which will require a minimum order of about 2000 pounds).

Any reason you can't fire up to cone 5/6?

[Soren]

Thanks, Tom.  You answered my questions thoroughly.
I hope I can continually learn more about the terminology and science behind all this.

6 minutes ago, glazenerd said:

...I have not purchased premade clays in a very long time. ...

I may someday reach the point where I make my own clays, but that seems too much for me at this time as a small hobbyist with too little time

2 minutes ago, neilestrick said:

Any reason you can't fire up to cone 5/6?

I have capabilities to fire to cone 5/6, but I make ocarinas.  The instrument is benefited by porosity to allow for water absorption so the moisture in breath does not condense in the airway, which hinders the tone.
The only firing limitation I have is cone 8 (maximum cone for all three kilns I have).

If there is some way to have the higher strength and better coloration of firing to cone 5/6 while maintaining the absorption of cone 04 earthenware, I would be very interested.  Another note is that the pitch of an ocarina is based on chamber size, so lower shrinkage rates are preferred.  I could experiment with initial size to adapt to a higher shrinkage rate, but it incorporates more chance for the ocarina to end up off pitch after firing.

The only reason at this time that I would personally consider porcelain for ocarinas would be if my colors would be bolder.

At this point, I am pretty sure that my best route is to stay with low-fire earthenware and just do the best I can at achieving the colors I would like with oxides and stain.  If I use terra sig, it may help bolden the colors with higher percentage colorant but lower use of colorant mass.  This would cover general coloring, but does not help if I try agateware (preferably a mix of white, dark navy blue, and dark red).

Thanks for all the help!  I hope I am not a nuisance with delving into the deeper science while I am still such a novice ceramicist.

[Magnolia Mud Research]

Soren,

Take the white clay body you have and conduct a set of firings from say cone 05 to cone 3 or to the max temperature you kiln will reliably (meaning repeatedly hit the target).  evaluate the porosity, color, strength, etc. Then, on the basis of your experiments, choose the set of conditions that produces the best product.  Also keep in mind that you can bisque to a higher temperature and the glaze fire at a lower temperature.  For instance take a cone 10 porcelain bisque it to cone 5 and then add stains glaze, slips, engobes, etc. and fire to cone 04.  
 
There is no rational reason for you to follow the herd to the firing sequence, (or raw materials), or some target that does not produce the product you want to make.  You know the characteristics you want in your product.  Keep tweaking the materials and the processing schemes to achieve your goal.  

I learned long ago, that  I can take control and adjust the system to use the materials and equipment I have to do the job; most of my work has been breaking the rules to achieve success. In other words, use your engineering background to guide your ceramics creativity.  

LT
(Ja, Ik ben een ingenieur!)