Jump to content
Sign in to follow this  
irenepots

Looking For A Bisque Schedule

Recommended Posts

Hello helpful potters!  

 

I need help with an 04 bisque firing schedule.  I'm having problems with pinholing in the 266 Standard Ceramic cone 4-6 dark brown clay that I'm using.  About 50% of the time I'm getting a few pinholes, generally on the bottom but also on the sides.  When I slow glaze, my cone 5 witnesses goes all the way down and the 6s are at about 2:00.  The glaze I'm using is Potters Choice for cone 5/6, 3 coats as suggested by Amaco Brent, and this problem is happening pretty much just on the 266 and not the lighter color clays I use.  

 

I know pinholing can be a symptom of a variety of things, but I'm ready to try a slower bisque schedule in case they're being caused by gases or other impurities that haven't completely burned out during the bisque.  Maybe it should go faster to 1100 and then slower for the next 1000 degrees?  Or have a hold in there somewhere?  I've tried to work out a schedule myself but it always ends up being about 18 hours long.  Here's the cone 04 preset firing schedule I'm using in my LL kiln: 

 

80 per hour to 250  (2 hrs)

200 per hour to 1100 (4 hours)

100 per hour to 1100 (1 hour)

180 per hour to 1676 (3 hours)

80 per hour to 1926

I also add a 10 minute hold at the end. 

 

When I use this cycle it tells me the end temp will be 1945 instead of 1926, ( 1926 is what it says on this firing schedule.  It does end up at around 1945.  I unplug the vent when it's done firing.

 

Any ideas will be greatly appreciated, as would any suggestions about other things I may be doing, or not doing,  that could be causing the pinholing.

 

oh also I don't stack my bisque in the kiln.  Would stilting the pieces give better air flow or deform them? 

 

Thanks!

 

Irene

 

 

Share this post


Link to post
Share on other sites

That clay will do all sort of strange things to a lot of glazes regardless of your bisque firing. But to make sure you're getting a good bisque, put that schedule in as a Vary-Fire program, but change the last ramp to 50 degrees per hour, and put in cone 04 as the last temperature setting rather than an actual number. Use the 'other' button to choose a cone rather than a number. You could probably also change the 180 ramp to 240 so it's not such a long firing. The high end is what's really important. Also keep your vent on until the firing is cooled down, since stuff may still be burning out even when the kiln is off.

Share this post


Link to post
Share on other sites

Everything Neil says is spot on. I adjusted my Bisque schedule with his and others help here on the forum. I can't say much to help with your bisque schedule as I do tumble stack my Bisque and really pack it in there...aka Mark C style.

 

I can maybe suggest something else to try during your glaze firing. Again the forum was super helpful in figuring out what to adjust. I was having issues with pinholing, especially with the Amaco Potters Choice glazes. I did a cone offset on my cone 6 slow glaze schedule and then added a hold at the end. Doing this dropped the overall set temperature of every cone 6 firing and then I get the heatwork I need by adding a hold at the end. I tried just adding a hold to the regular cone 6 firing but it made the cone 6 overfire and I had some other issues develop. The offset with a hold allows those Potters choice glazes to smooth out and flow just enough without running and all my pinhole issues went away. My ^6 cone bends perfectly every time.

 

If the bisque adjustments don't get you what you need you might want to try adjusting your glaze firing instead.

 

T

Share this post


Link to post
Share on other sites

Neil, I am firing to cone 04 and not to a temperature.    When I review the program before firing, the cone and also the target temp come up as it scrolls.   I'll definitely try making a program with that last ramp change and also leaving the vent on.  I didn't realize  realize there could  still be stuff burning out at the end.  I was trying to keep the heat in a little longer.     

 

Pugaboo, I am actually entering cone 5 as my slow glaze to get a firing that's close to cone 6.  I'd tried changing the thermocouple offset and then the cone offset when the cone 6 setting kept completely bending my cone 7 witness, but I couldn't get it to cone 6.  I finally just started firing to cone 5 instead, and that seems to be ok, it comes in at about 5 1/2.   Meanwhile the 04 was underfiring and I had changed the offset on that but now it's back at 0 and the cone is bending nicely.  I never understood why at the lower temp it was firing cooler and at the higher end it was hotter.  

 

I wonder if maybe the glaze firing needs to be closer to cone 6 than 5 1/2.  How much is your cone offset and how long is your hold?   And do you use commercial glaze, and if so, what type do you like?

 

The other thing I have going on is that my kiln is 18" deep and 28" round, and the only way I can get an even firing is to use 2 half shelf rows in it instead of 3.  The kiln has 4 elements and 2 thermocouples and unless there are 2 elements and 1 thermocouple for each shelf it fires very unevenly.  I put the extra 2 half shelves at the top, like a cover.  Maybe I shouldn't put the extra shelves on the top?  I've  tried all kinds of staggering as well, but nothing helped.   Do you think the fact that there is less pottery being fired could be contributing to the pinholing?  Sometimes I add extra kiln posts so there's more mass.  I know it's not as economical as having pots on 3  shelves but I'd rather get an even firing of fewer pieces.  My cones come out the same now.  

 

Thank you both for your suggestions, I greatly appreciate them.  I'm going to try them my next firings.

Share this post


Link to post
Share on other sites

The greyer the clay; the more carbons in the clay body.

The browner the clay: the more carbons, iron, and feldspars.

 

I think you will find that in this case your bisque schedule will do little to cure pin holing.

In your glaze firing; use your schedule up to 2050F, and then drop to 125-130F up to cone 5.5 with a 15 minute hold. Your dark brown clay is full of iron and potassium: it is the potassium burping gas. Naturally occurring dark brown ball clay tends to have larger particles of feldspar minerals in them: which takes a little more heat to gas off.

 

Nerd

Share this post


Link to post
Share on other sites

Thank you Nerd, I'll try that.    Do you think I ought to stick to 5.5 or go up to 6?  I could change the con offset a bit.  I had stuck with the 5.5 after reading that some of the glazes were happier there.  

 

Irene

Share this post


Link to post
Share on other sites

I just checked the preset cone 6 slow glaze schedule and this is it -

 

150 per hour to 250

400 per hour to 1915

120 per hour to 2199   

 

I've been firing to cone 5 with a 15 minute hold (which the schedule says is 2165 I think), but this seems to be very close to what you're suggesting.  How would you tweak this?  Slow down the last segment even more?  

 

Thanks again.

Share this post


Link to post
Share on other sites

 

150 per hour to 250   Slow preheat to drive off atmospheric moisture... keep it.

 

400 per hour to 1915  Fast fire to high bisq - on pieces already bisq'ed.  keep it.

 

120 per hour to 2199   for porcelain with a hold, it works fine.

                                    for white stoneware with a hold, it should work as well.

                                    for dark stoneware bodies: 120F to 2230F with a 10 min. hold.  20 min hold if kiln is bigger than 6 CF. 30 min. if over 10 CF

 

Grey'ish colored clays have more carbons: so higher bisq with hold usually resolves carbon issues.

Brown'ish  colored clays have more carbons, iron, and potassium feldspars: these are the bodies that need the slower high end ramp, to a higher a temp, with a short hold to burn off larger particle feldspars that occur naturally in high iron clays (brown)

 

Nerd

Share this post


Link to post
Share on other sites

Sorry it's taken me so long to reply.   

 

Thanks Nerd, for the info about the differences between the clays.   It really is important to know how they should be fired differently, and why.   Once again, the more I learn, the more I need to learn.  Pottery is fascinating.

 

Again, I appreciate the help.

 

Irene

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Sign in to follow this  

×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.