Cone 04 Bisque? Newbie Potter

[Earthwood]

As a newbie potter who is having trouble with glazes, I was referred to "Mastering Cone 6 Glazes"

 

In the book, the authors reccommended bisque firing to cone 04. I was kind of surprised by this, since I've learned to do cone 06/05. I was wondering if anyone else bisques to cone 04?

 

If I have a glaze that crawls, would bisquing to cone 04 make it worse because the clay wouldn't be as porous and would absorb less glaze? On the contrary, if I've been having issues with crazing on nearly all of my work (fired to cone 6 glaze), would firing to cone 04 help? Any experts out there who can share their experience?

 

Thanks!

 

- S.K.

[MuddyMomma]

i'm not as experienced as alot of others here, but i've always bisqued to ^04 and i glaze at ^6. i can't help you w/ the crazing, but i've gotten some crawling lately and so far it's happened where i have 2 glazes overlapped and layered on too thickly. not much help i know but hopefully somebody else can chime in and answer your questions a little better. good luck!

 

 

[Guest JBaymore]

S.K.,

 

Cone 04 is not an uncommon bisque temperature. And it is generally a good idea for many clay bodies. Your bisqued ware will likely be SLIGHTLY tighter (less porous) than at your usual 06/05...... but adjusting the viscosity of the glaze batch can easily take care of any issues with that (see below).

 

The purpose of bisque firing is kinda' two-fold. It is first to make the ware more easily handleable for the application of surface decoration and glazes. Secondarily it is to allow certain reactions within the clay body to go to completion so that the final fired product is as strong and durable and without defects as it can be.

 

It is important to note that you technically CAN single fire work ....... IF the glazes are adjusted to do so, and if the beginning stages of the once-firing are handled as if they were what we would consider a good bisque firing cycle. For most people.... the adjustments are not worth it . Green glazing is an "art" for sure.

 

The slightly higer bisque cone endpoint is to make sure that all of the chemical reactions that need to take place in the body are able to do so. One of the big ones that has to happen is the oxidation of any carbonaceous matter in the clay body. To do this you need also to have a lot of excess oxygen present (air through the kiln) in a bisque firing. Otherwise it can cause defects that only show up in the finish firing.

 

Too many "packed really tight" electric kiln bisque firings end up with not enough oxygen present..... and this causes glaze defects. Compound this lack of air with a lower cone end point... and the lieklyhood of problems out of the glaze firing goes up. BTW... many people don't seem to realize it these days (thanks to the prevalence of and convenience of the computerized controllers on electric kilns) .....you CAN bisque in a fuel firied kiln too if you want to. You just have to make sure to run oxidation all the time. (I fire all of my bisque in a propane fired gas kiln.)

 

 

As to the crazing issue.............

 

Crazing is caused when the glaze shrinks more than the clay body after the liquid glaze has "set". Glass is strong in compression.....but weak in tension. The craze lines are the "stress relief" that the glaze gets to allow it to stay on the body. In more technical terms, the Coefficient of (reversible) Thermal Expansion of the glaze is greater than the COE of the clay body.

 

This is typically fixed by adjusting the COE of the glaze chemical formula (not the glaze recipe) so that there are a higher percentage of oxides there that "shrink less" (have lower COEs) than what was originally present. It is typically a pretty easy fix to do. The quick and dirty "material oriented" approach to this problem is to do a line blend in which you SLIGHTLY increase the Flint/Quartz content of the original glaze recipe a bit. (If this fails... then you'd be better off using molecular calculation to fix the issue.)

 

As to if the higher bisque will fix the crazing........... almost for sure not. The higher bisque will not likely affect the final matured COE of the clay body.

 

At a REAL hair-splitting technicality, if currently your bisque cone and firing atmosphere is allowing some carbonaceous material to remain, AND if that material is then causing some of the red iron in the body to end up getting reduced to the FeO state, AND that FeO is actually remaining that way in the early stages of the second firing....... the glassy matrix in the body could be a tiny, weenie bit different because the FeO is a flux on silica whereas the red Fe2O3 is not. But this change, if it ever happened, would not likely affect the COE much if at all. It might affect the strength of the body, and maybe the fired color. And cause other glaze defects.

 

 

Now to adjust the viscosity of the glaze slurry..........

 

In your case it likely is just adjusting the water to solids ratio of the glaze slurry slightly. Generally speaking, you'll want just a tad less water in a given liquid volume of the glaze to account for the lowered absorbency of the bisque during glaze application. When you mix your glazes from dry.... just add less water.

 

While many people suggest using a hydrometer to adjust glaze "thickness"...... at the Specific Gravities that the typical glaze materials are at, and given the nature of the typical glaze slurry batch (suspension, not solution) a better device to use is a viscometer. A viscometer measures a couple properties of the slurry at once. You can think of it as the "flow" of the liquid glaze slurry. A viscometer is easy to make for personal use.

 

First of all experiment with using less water in your glaze so that you get the application characteristics that you want for the new bisque temperature.

 

Take a decent sized tin can or plastic container. Drill a clean 1/2" hole in the center of the bottom. Mark a clear line on the inside top area of the can a little below the rim. Put your finger over the hole. Take your glaze to be tested and fill the can up to the mark on the inside. Using a stop watch, time how long it takes for the liquid to all flow out the hole. Record this time right on the side of the bucket that you keep the glaze in. When you mix up your glaze, if you keep this time number constant, the application thickness in a unit of time for a specific body bisqued to a specific cone will be the same.

 

You can also just increase your dipping time into the exisiting slurry to get the same thinckness of application.

 

 

As to the crawling issues........

 

There are many causes of crawling. To diagnose this, it would be very helpful to see the glaze recipes.

 

 

Hope this helps.

 

best,

 

..................john

 

PS: Great book too. Study it.

[Earthwood]

John,

 

Thank you so much for your detailed reply! I am so new to the world of pottery (coupled with not having had a formal education in the art) that I have not attempted to mix my own glazes. I am using AMACO Potter's choice glazes. Some of which have come out fine in a firing, but the one I like the best, Blue Rutile, coupled with a transparent on the inside is what has been having issues with crazing/crawling.

 

I also use pre-mixed clays. After reading Mastering Cone Six Glazes, I was worried it might be a glaze/clay fit problem. This would mean I would have to change the clay I was using (which is an overwhelming decision in and of itself because there are so many different kinds of clay - I wound up purchasing 7 different types and trying them all to see how I liked the ease of working with them), change the glaze, or start trying to mix my own glaze.

 

Without having had a formal education, I don't know the first thing about mixing glazes, or even how to read glaze recipes! (I assume in the book, each ingredient was listed as a percentage of what the whole make-up should be). Some things I am most unclear on are what the colorants are, what colors they produce and how much you are supposed to add to the base. And are the bases listed in the book all transparent when fired? Also, what suppliers sell the raw ingredients?

 

I will try the cone 04 bisque and the firing schedule the authors recommend. It is becoming aparent that I also need to try mixing my own glazes.

 

Can anyone recommend a good beginner book on mixing your own glazes?

 

Thank you again for all your replies and generous information. This forum is a godsend!

 

- S.K.