Question about correct firing temps

[Wichy]

Hi

I have a bone dry bowl made of crank stoneware clay that has a firing range of 1200-1300C.

I want to fire it first and then apply underglaze decorations and a transparent gloss glaze over everything.

I have transparent stoneware glaze with a firing range of 1200-1280C and some Amaco underglazes with a firing range of 1040-1220C

Can anyone please advise on what temps to set for the first and second firing to avoid problems and also what general way to ramp up the kiln temp to avoid any physical or glaze problems?

Thanks for any advice.

[Bill Kielb]

The general rule is to match your clay and glaze as close as practical. Clay is usually fired by cone, so cone 5-1/2 is about 1200c and cone 12 is about 1300c so a very large spread which is a bit unusual. Most folks try and fire to maturity so the clay is vitrified and as strong as it can be and as non porous as it can be. Most electric kilns will not fire  to cone 12 so that is a bit bewildering. Clay is usually bisqued first which is at a lower temperature where it remains porous and ready to decorate. The bisque firing is usually in the range of cone 06 (998c) to cone 04 (1063c) and takes enough time to burn out all the organics and chemically dewater the clay from which it can never return to its previous clay state. This is typically 10-14 hours.

Glaze firings are usually done to the maturity of the clay and take approximately 7 hours give or take, but again cone 12 seems an odd number.

without going further, could you double check your clay and maybe provide information on your kiln, many kilns have an automatic firing sequence and this would save you a whole bunch of immediate learning. Welcome, I see this is your first post. Add the above information and it will likely make this easier to sort out for many folks here.

[Min]

Unless things have changed in the last while my understanding is crank clay is mainly used as a sculptural claybody. It's name originates from the cranks (shelves, saggars, posts) used in kilns in the UK and now means the claybodies these were made from. So basically a sculptural body which doesn't need to be vitrified and can take a lot of abuse. As a learning experience I'd go ahead and underglaze it and then glaze fire to your glazes maturity but it's likely going to have a fairly high absorption rate which means any unglazed surfaces will take up water and cause issues.

Some Amaco underglazes can fire hotter that 1220C but it will take testing to see if your ones do. There is a caveat to that though, underglazes (or glazes) that include cadmium inclusion stains shouldn't be fired hotter than manufactures recommendations as they can become unstable. (these are often the bright orange, pinks, reds and some yellows)

Going forward I'ld suggest looking for a claybody that matures at the same top temperature / cone as the glazes you are using. If you can find a claybody that matures (top of the firing range) at cone 6 (approx 1220C) that will be much easier on your kiln and elements than firing to cone 10.

Welcome to the forum!

[Chilly]

I (generally) find that underglazes work better on unfired clay.

The only way you will learn and understand is to make make lots, and treat them in different ways, and make copious notes.

Then you will find a process or choice of processes that work for you.

 

I would:

underglaze

fire to ^04

glaze

fire to ^6.

 

Make sure you know the difference, the zero in ^04 is very important.

 

Clay and glaze here (UK) is often sold with a wide firing range, which isn't always a good thing.  You need to find a clay and glaze that fit well together, and suit your making requirements.  You will only find that with testing.  Even if we both used the same clay and glaze, we will not necessarily get the same results, as each kiln can produce different results.

Testing can feel daunting, when all you want to do is make finished pots.   It's a bit like learning to drive, you spend hours learning, but don't actually make any real journeys.   An oft quoted Q&A goes:  how long did it take you to make this mug?  25 years and 6 minutes.

Enjoy the learning process.