Slow Bisque Concern re: Iron Bearing Bodies

[nairda]

Using an L & L kiln with a vent. Have always used Slow-Bisque - Cone 06 (@13 hours) and Slow Glaze - Cone 6 (@ 8 or 9 hours) with good results. I use Highwater Little Loafers (white body) and their Brownstone and Redstone bodies (brown and darker brown  but nothing like Standard Clay Umbria).  Generally have a mixed load of these bodies in every firing.  

With scorching outdoor temps these days, I'd like to try a Fast Bisque, but stick with Slow Glaze.  Should I be concerned that I'll have the normal amount of gases/minerals/etc. burned out with a Fast Bisque so that my glazed ware won't be affected?  Ware will be very dry before bisque firing. I always load pretty evenly, 7" stilts on bottom, then staggered shelves above with shortest shelf height at 4".  Always have at least 1 element radiating into each area to help with even firing and air-circulation.  Thanks!

Edited July 16 by nairda
Needed to fix a typo!

[Hulk]

Good question.

For the clays that require more time for "burn out," an abbreviated bisque could make a difference, especially where the wares are thickest, and particularly if it is time above about 1500°F that is being curtailed (per my experience).

I cut back my bisque schedule in order to finish before "peak" rates (4 p.m.) one time last Fall; there were more faults in the red clay (at thickest sections, per my notes).
I'm running a bisque load right now, with a full hour at 1500°F, half on the way up, half on the way down, and a slow climb over the last 200°F to target/peak.

[Bill Kielb]

2 hours ago, nairda said:

With scorching outdoor temps these days, I'd like to try a Fast Bisque, but stick with Slow Glaze.

Bisque schedules are slower to ensure there is enough time to burnout the organics. From an energy standpoint a fast bisque likely will consume slightly less energy - shell losses - than the slow bisque so I am unclear what the outdoor temps have to do with it. If your clay is relatively clean and you are trying to save some time and a bit of energy then testing first is something I would suggest.

FYI - Plus one for even radiation, as for air circulation, it has very little effect as all the air in the kiln and mass of this air contribute very little to the heating of the kiln. The air just cannot contain a bunch of energy.  For even heating the losses through the shell top and bottom and speed being fired  are usually very significant though

Edited July 16 by Bill Kielb

[Jeff Longtin]

Air circulation - I'll pick up on Bill's point - air circulation is only important in fossil fuel kilns: wood, propane, natural gas. As the fuel itself, creates an air flow, how you control this flow greatly affects your firing results. (How tall, or short, is your chimney / when do you open the damper, and how much, all affect air flow.)

Electric kilns, for the most part, do not have "air flow". (At least to the same degree.) Making sure that no kiln shelf "blocks" an element is a good way to maintain heat gain but you're not really hampering any kind of "flow". (In electric kilns the heat is "radiating" not so much "flowing".)

Several things affect how uniform a kiln will fire. One big element is tightness of the kiln pack. Leaving room around each pot/form allows air/heat to envelope it. This creates more uniform results.