Candleing and firing Slip Ware to Bisque

[LeslieV]

Hello, 

I am hoping for some advice on drying my slipware before firing to bisque and a recommended bisque firing speed.  I understand many potters candle their work at under 100c for some hours before then firing.  I have not tried this with my slipware but am wondering if there is any reason to not candle it.  Slipware dries relatively quickly anyway but on some occasions I would like to speed up the process. Hope someone can advise on this. 
I have also read in Andrew Martins excellent book on Moulds and slipware where he advises it is ok to use a fairly fast firing speed for slipware as the clay is very homogeneous and doesnt have the problems that regular clay might experience if fired too fast. Currently I fire to bisque over a 6.25hr period but would like to shorten this time if possible and advisable. 
I also now have Jonathon Kaplan’s excellent Mold Making Manual but cant find any firing recommendations in there yet but I am still working through and enjoying the book so they may be there waiting to be found.

Hope someone can advise. 
 

[Bill Kielb]

2 hours ago, LeslieV said:

Currently I fire to bisque over a 6.25hr period but would like to shorten this time if possible and advisable

Just thought to mention

The speed one fires is based upon the tolerance of the clay and any effect it may have on the glaze. Bisque firing is designed to burn out organics so for many clays, speed and time at or above temperature is a primary function of the firing. So for may clays, the bisque firing is in the 8-12 hour range and is designed to allow enough time for organics to fully burn out. Firing to bisque temperatures also leaves the clay in a sintered state so it is sturdy to handle and decorate and yet will absorb glaze predictably. If you are bisquing in 6.25 hours (about 2/3 to 1/2 the time of traditional bisque) and not experiencing any glaze defect issues, your clay is likely fairly clean and your glazes tolerate anything remaining to be burned out. 

This may not be the case for future clays and glaze, but this has already proven tolerant at a very very fast speed. I thought you should know, you presently are firing at what many would be considered very fast.

An old Ceramic arts publishing that explains a bit about bisque you may find helpful https://ceramicartsnetwork.org/ceramics-monthly/ceramics-monthly-article/Bisque-Firing-101#

Edited July 9 by Bill Kielb

[LeslieV]

Thank you Bill, that information is most helpful. I was aware that clays used for hand building and wheel work need to fire slowly to ensure the unwanteds are burnt off but thought the 6+hours I am using would work ok. I now know otherwise. In the case of slip ware I thought maybe the amount of unwanted particles was very minimal so was thinking the bisque firing could be shortened to 4-5 hours but that sounds very unlikely. 
I have been firing bisque to 1000 over 6+ hours then glaze firing to 1220 with no ill effects so maybe so far I have been lucky. I will follow the link you provided. 
regards

[Hulk]

I'll run the kiln up to ~200°F the night before bisque fire, then save a few hours the next day*; the wares are typically fully dry going in, but any damp wares are bone dry after a night in the hot->warm kiln (still well over ~145°F by the following morning).

I'm running a powered kiln vent, which should help with providing adequate oxygen for burnout.

I'd added an hour+ hold at 1500°F when struggling with "dirty" clays - it did help - and left the hold against much less frequent, but still annoying defects in "cleaner" clays.

Ware thickness may be an important variable; I'm seeing higher defect rate where thicker...

*Our electric rates go up at 4:00 p.m.!
Curtailing the candling segment allows some flexibility...