Expert: Sam Kelly - 8/26/2008

Question
QUESTION: Hi Sam,

I'm trying to understand the temperatures used in glazing a piece of ceramic.  If I fire something at cone 6, wouldn't the glaze firing need to be 6 or below?  Couldn't I use an 02 glaze and just fire it to that temperature?  Or does it need to be as hot as the initial firing?

Thanks,
Joe


ANSWER: Hi Joe, I will try to keep this as simple as possible for you

1) the glaze firing process requires 2 seperate firings. A bisque firing to turn the clay hard, but not hard enough so that a glaze will not suck into it and stick there. This firing is traditionally 1000DegC cone 06.

2)The second process is the glaze firing. The clay body manufacturer usually states the maturation temperature of the clay on the bag it comes in. Firing below this temp will mean the clay has not vitrified(hard) and firing above will cause slumping or melting.

A glaze, wether it is store bought or made by the individual also have maturation temperatures for correct fit to the clay body and visual apperance. An example being a cone 9 glaze is fired to cone 9 on a stoneware clay body for correct fit and apperance. A cone 6 glaze fired on the same body to cone 9 will result in the glaze overfiring, become fluid and run off the pot. A cone 9 glaze fired to cone6 on a cone 6 clay body will result in underfiring of the glaze and it will be a flat, dry, surface indicating underfiring of the glaze.

Now both processes can be used if you want, overfiring or underfiring of a glaze ia acceptable if that is the look you are looking for and it suits the use of the pot. Neither would be suitable for eating from.

Sam







---------- FOLLOW-UP ----------

QUESTION: Thanks Sam.  Very helpful.

Do you see more change of shape or shrinkage during the final firing?

Answer
There is more shrinkage in the final firing. measure your pot at some stage BEFORE firing occurs, then measure it again after the FINAL firing.
Formula is not fired minus fired divided by not fired multiplied by 100.

Example being if length was 98mm and came out fired at 84mm:
98 minus 82, divided by 98, multiplied by 100, equals percentage of Shrinkage.

You will find that shrinkage in height is usually more than shrinkage in width. I measure my new pots freshly thrown on the wheel head. This allows me to set up at a later date to throw more that will come out at the same size as those made previously. At stoneware my shrinkage is 17% in height and 15% in width.

Sam