Bisque firing of terracotta clay with underglaze

[PaulineNL]

I created some small ornaments with Amaco terracotta clay no. 77 which has firing temp range of cone 05-5. Recommended temperature is cone 5. I let the pieces dry for a week and have just added a couple layers of underglaze and a layer of clear glaze.

I would normally fire my Skutt kiln on an auto program  of cone 05 medium speed with no ramp/hold time. But I have never made my ornaments with terracotta and am uncertain about firing. Will all be OK at cone 05?

[Bill Kielb]

Terracotta COLORED stoneware mature fires to cone 5. So  I would glaze fire it to cone 5. I also would bisque to 04/05 but I think you. Have glazed it already. So maybe once fire  to cone 5 using a slow / medium bisque schedule. Of course assuming you used a cone  5 glaze and not a lowfire cone 05 product. If you glazed with a cone 05 product and have not bisqued, I really don’t have a good idea other than remove the glaze someway and bisque fire to 05, then glaze with a cone 5 product.

It is not functional and fragility may not be an issue, so trying one of them to cone 05 bisque schedule might get an acceptable result.p for now.

Edited December 7, 2023 by Bill Kielb

[PaulineNL]

So don’t bisque fire since I have already glazed with an underglaze and clear glaze?

[Bill Kielb]

What cone is your glaze? The clay is basically cone 5 red clay so the glaze needs to be cone 5 for them to mature together. If you bisque this clay itt will be a fragile sintered bisque which most likely would not work well with a cone 05 glaze. So what cone does the glaze mature at?

Edited December 7, 2023 by Bill Kielb

[PaulineNL]

Amaco Velvet underglaze says fire to cone 04-05. So I I think should be ok. I’m so new to all of this and there is so much to learn technically.
There is no detail work on any of the pieces. They are all just one or two colors. So there is no risk of the clear glaze smudging anything. I wiped the bottom edges so hopefully nothing will stick to the kiln shelf. I did add only clear glaze to the interior of each piece, so hopefully that does not cause the clay to expand or crack. We will see…

[Bill Kielb]

5 minutes ago, PaulineNL said:

Amaco Velvet underglaze says fire to cone 04-05. So I I think should be ok. I’m so new to all of this and there is so much to learn technically.

The underglaze is fine and will fire higher, specifically what clear glaze is on the piece? Velvet under glaze will basically go to cone 10 btw.

Edited December 7, 2023 by Bill Kielb

[PaulineNL]

It’s Georgie’s PG630 Zinc Free Clear Gloss Glaze that fires to cone 6

[Bill Kielb]

1 hour ago, PaulineNL said:

It’s Georgie’s PG630 Zinc Free Clear Gloss Glaze that fires to cone 6

So If you fire to cone 05, the clay will be bisque fired only, so porous and weak and the glaze will be severely underfired. The normal progression would be decorate with underglaze, then bisque fire. Then glaze with a glaze matching the cone of the clay and fire to cone 5/6 so the clay and glaze melt and mature as expected.

I think the confusion here is the clay is not really firing at typical terracotta temperatures, it’s just called that and is a midfire clay that matures at cone 5.

The good news is this may work just fine for a single fire to let’s say cone 5, just use a slow / medium bisque schedule but still fire to cone 5. Definitely something I would try and test. The clay should off gas and burnout as a normal bisque firing and then go on till maturity (cone 5). If your glaze tolerates the off gassing, then it will heal and be just fine when mature. Definitely worth a test IMO.

This is often referred  to as once firing and many combinations do just fine which saves one whole firing. Going slower than a normal glaze firing (bisque firing speed) is precautionary to allow enough time for the organics to burn out before proceeding onward to cone 5.

Edited December 7, 2023 by Bill Kielb

[PaulineNL]

I had already set the kiln running the Cone 05 program yesterday afternoon. I will open the kiln this evening and see what I have. I have reglazed and refired pieces that did not work out before. It’s a risk, but it’s the only option I have at this point - add more clear glaze and fire to cone 5.

Thank you for you advice because I will now be more careful to check what cone to fire all the products I put in the kiln.

[Bill Kielb]

If everything is intact, your glaze should be very underfired, but if it has stayed put and intact then I would test one piece in a cone 5 glaze fire. The worry would be if the glaze has come off or developed cracks that will not  heal in the glaze firing. So as you remove those, very careful handling is probably really important. Firing a second time from cone 05 is very different than refiring. Right now your clay and glaze are not fully baked, so very different than a second glaze firing.

[PaulineNL]

 OK, I will cross my fingers and be very careful.

[PaulineNL]

Well nothing blew up. Everything is intact. Obviously the clear glaze is more translucent rather than glossy because the kiln did not get hot enough. So my question is whether I should add a new coat of clear glaze or whether I should just fire to cone 05. 

[Bill Kielb]

As a suggested test I would take one piece that is very intact with no defects and fire to cone 5, not cone 05. Your clay and glaze are midfire, not low fire.

Edited December 8, 2023 by Bill Kielb

[Min]

@PaulineNL, link to cone information firing temperature chart from Orton in this thread.  Have a look at how cone numbers go up as the temperature goes up. As you can see there is a big difference between cone 05 and cone 5. What you are doing is like trying to bake a roast that should be baked at 350F at only 150F, it  just will never be "done".

Amaco Velvet underglazes can often be fired hotter than cone 05 without too much difference in the colour. Link here to a field guide from Amaco for their Velvet Underglazes, fired with one of their clear glazes, to cones 05, 5 and 10.

There is no point firing a cone 6 glaze, like the one you have applied to this work to just cone 05, this is far too cool and why your glaze is not fully melted.

If there is no risk of the glaze running onto the shelves then fire the kiln to cone 5. If there is a risk of the glazes being to close to the bottom of the pieces then I would sit each one on a small flat pad of clay (a clay cookie) that way if the glaze runs it will stick to the cookie and not the shelf. These clay cookies need to be totally bone dry before firing the kiln and can be reused.

Going forward match the clay maturity firing temp / cone to the glaze. Not all glazes will work well over all underglazes so it's a good idea to test first on test tiles.

 

 

 

[PaulineNL]

Oh my, that was a typo. Yes of course I will be firing to Cone 5 this time, not Cone 05. I’m new to ceramics but not daft or dumb.

I am willing to risk firing them all at once because firing one tiny thing in a big kiln has more downside to me that just going for it.