overfiring glazes. Why not?

[Michael D]

I know that overfiring clay, say a terracota to C6, can cause the body to bloat, etc. 

But besides burning out colors, or perhaps creating a mismatch with a body intended for midfire, is there any fundamental problem with firing a lowfire glaze way beyond its intended cone?

thanks

Michael

[liambesaw]

Running, crazing, crawling, take a look at issues that crystalline folks run into.  They overfire by a few cones on purpose to grow crystals.

[Magnolia Mud Research]

I use commercial cone 05 clear glaze at cone 10 and have never had a problem!   Also have used it successfully at cone 3 and cone 5 and on raku which is a fired a smitten above bisque temperature.  YMV.  Have also used that clear as a base glaze for other ingredients such as rutile, iron,  kaolin, etc. tests at cone 10  so far nothing unexpected has resulted.  Crazing can be adjusted with silica most of the time.   
 

I also use low fire clay bodies as a decorative slip on cone 10  reduction fired ware;   sometimes low fire red/dark clay body will run if applied too thick.  Testing is necessary!  

[Bill Kielb]

1 hour ago, Michael D said:

I know that overfiring clay, say a terracota to C6, can cause the body to bloat, etc. 

But besides burning out colors, or perhaps creating a mismatch with a body intended for midfire, is there any fundamental problem with firing a lowfire glaze way beyond its intended cone?

thanks

Michael

If we define an overfired glaze as one that exhibits some bad behavior when fired beyond a certain temperature, then generally glazes become runny. One test for a true matte glaze can be when overfired it becomes a runny matte.   Testing is the only way to know though as glazes often fire well in a tested range. Generally glazes  run because they cannot resist the pull of gravity at some point.

The earth is basically a cone ten machine though and since we make everything basically from earthly geologic products cone 10 is often fine. Cone 6 and 05 melts can be reached several ways but a common simple way is to add boron to a cone 10 glaze to get it to easily melt in the lower cone ranges. Firing them to cone 10 for many, may still be fine. Many of the lowfire commercial stroke and coat glazes claim to be fine from 05 through cone 10.

Testing is likely the only way to know for sure.

[Mark C.]

13 hours ago, Michael D said:

I know that overfiring clay, say a terracota to C6, can cause the body to bloat, etc. 

But besides burning out colors, or perhaps creating a mismatch with a body intended for midfire, is there any fundamental problem with firing a lowfire glaze way beyond its intended cone?

thanks

Michael

Only issue is if it runs off-pinholes, crazes badly,crawls, does not fit body-if you test it and like the results you are fine