Over-Cooked Glaze Firing?

[Frank Hott]

Valuable lessons can be learned from mistakes and unplanned events (I should be a wealthy person by now!).  I have forty pieces that are meant to be lessons in glazing (I'm a very new potter).  The clay and the commercial glazes are matched at cone 5/6. I carefully documented every glaze and glaze combo (learning how easy it is to misremember how many coats I brushed - 2 or 3?).

 

The pottery store did the glaze firing and the unplanned event turned out to be a bad bottom element in the kiln.  After firing for about 16 hours (a cone-sitter controlled kiln), it was discovered that the kiln never reached cone 5.  After cooling, the pieces were moved to a working kiln and fired to cone 6.

 

I haven't seen the results yet (not cool enough from the second firing).  With the centuries of experience here, can anyone speculate whether or not my careful documentation will reflect a true evaluation of the glazes and combinations I used?

 

I appreciate your fellowship and willingness to share from your experience and knowledge.  Perhaps someday, I'll be able to contribute to the answer from someone else's question.  In the meanwhile, I am very much enjoying my new activity.

 

Thanks for your input.

[neilestrick]

It depends entirely on how hot the pots got in the first firing. If the first firing got close to cone 5, like within a couple of cones and they actually melted a bit, just didn't get totally mature, then I would say that the results will not be completely accurate in terms of color. As far as melt, they should be fine since the first firing did not totally mature. Of course, it all depends on the glazes, too. Some glazes do not change appearance when refired. Either way, they are likely close enough to use as a reference when choosing glaze combos.

[GEP]

I think most cone 6 glazes do not mind being fired twice (or almost twice). But there are some recipes that will be way overfired. You should probably wait until your next firing before you trust the color results. You should still document these results anyways, because this type of incident can happen again! btw, that type of careful record keeping really matters. Having taught many students, some who keep careful notes and some who don't, I can see what a difference it makes. 

[Pres]

Agree with GEP, as most of the time a refiring in the 5-6 rainge will not result in major problems. However, if the glaze is a cone 5, it may be a little shinier than matt, or may move slightly more.  Most glazes will take this as different parts in the kiln can be one cone difference.  I only know one glaze that was a ^5 that would be dissastrous at ^6 unless care was taken, it could glue the piece to the shelf even with an inch of bottom unglazed.