Kiln Firing Discussion

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Discussion on kiln firing

This is a transcription of a discussion between leebc and his friend spoony about how she fires her kiln. It is edited for content.

Intent is to create a write-up that presents the information in a more linear format.

Thursday, May 12, 2022

leebc — Today at 10:47 AM
Because someone going to ask, what was "School" for you for this?

spoony — Today at 10:50 AM
Ceramics 1&2 at spc

leebc — Today at 9:40 AM
How do you feel about "firing a kiln unattended"?  How much do you "attend" your kiln?  What do professionals say?

spoony — Today at 9:44 AM
I generally just make sure not to fire it if no one is going to be home in the 8 hours it’s firing

leebc — Today at 9:50 AM
hummmm...  And if someone is home?  Do they check it periodically?  Or just listen for the fire alarm? 

spoony — Today at 9:51 AM
At school someone would just come check periodically through the weekend (their firing cycles were generally like 48 hours)
Listen for the fire alarm
I do check it now and then but mostly to make sure it’s heating/cooling as needed
(Yesterday that paid off cause it turned out I forgot to set the timer to the max time allowed before shutdown, so it shut down 3/4ths through heating - which is just one of the fail-safes)

leebc — Today at 9:58 AM
What were you firing to?   The slip-mold-clay Ryan is working with wants Cone6

spoony — Today at 10:02 AM
Build -> fire to 04 -> glaze -> fire to 6
Or whatever the glaze requires (4-6 for cone 6 clay)

spoony — Today at 10:07 AM
I would start with a low fire clay. Still same process but the glaze fire is 05-06 rather than 4-6
I would also check to make sure the slip cast clay is actually 6 not 06

leebc — Today at 10:08 AM
Well, he's already made several of these slip castings and wants to fire them.   I'll encourage him to fire some low temp stuff first.

spoony — Today at 10:08 AM
Have him send a link to the exact clay?

leebc — Today at 10:10 AM
Highwater Clays
White Stoneware Casting Slip (Cone 6)
The White Stoneware Casting Slip is a smooth pouring stoneware slip which releases easily.  Excellent for utility ware. Cone 6. At Cone 6:  Shrinkage 13.0%;   Absorption 2.0%. IMPORTANT NOTES: This item cannot be shipped via ground shipping.  It must be picked up in our retail store or shipped freight (freight quantity
White Stoneware Casting Slip (Cone 6)
OH!  it's stoneware. 
tbf, he can probably call them for a walkthrough....

spoony — Today at 10:17 AM
Well firing is circumstantial so whatever you asked before might not match anyway
But yeah he needs to fire his slip casts to 04
Then glaze with a glaze that fires from 4-6
NOT 04-06: 4-6
And the initial fire is 04
No glaze before then
No glaze on the bottom (if he’s dipping, he needs to wax the bottom)
And wipe off the glaze from the wax

leebc — Today at 10:20 AM
Roughly how long should an 04 firing take?    How long should a 6 take?

spoony — Today at 10:20 AM
Depends on the kiln and speed. Mine does 04 at med speed in 6-8 hours
I’ve never fired to 6 - at school we loaded on Friday and unloaded Monday

leebc — Today at 10:21 AM
did they attend it all 48 hours?

spoony — Today at 10:21 AM
Nope - just people would come in over the weekend to check on it
Unless it was a soda fire (you have to manually set the flame)

leebc — Today at 10:22 AM
and that was electric?   or gas/wood?
to 6

spoony — Today at 10:22 AM
Most electric, some wood and gas for soda/salt

leebc — Today at 10:23 AM
main question being: the electric ...may be able to get up to 6? 

spoony — Today at 10:23 AM
Those you have to keep watch on, turn the flame up and down, and at one point open it and throw in salt
What’s the kiln? And what’s it’s sitter?

leebc — Today at 10:25 AM

spoony — Today at 10:26 AM
Oh good I use a similar sitter
It’s usually got pre-programmed things and should be pretty straight forward. Click button to start, takes you through a menu that you can scroll through options with the up/down arrows, last click is ‘start’
Yeah cone 6 is its max temp looks like
Typically you want your kiln to be rated to fire one cone hotter than you want
Oh though I guess technically it can get to 7 so you’re probably good

leebc — Today at 10:37 AM
ok, cool., rather.

spoony — Today at 10:37 AM
Actually teeeechnically it can get to 9 depending on heat speed
Slow has a lower final temp than fast
And it can get just to the final temp of 9 at medium speed
You’re gunna want a peep hole plug

leebc — Today at 10:40 AM
is it missing in the photo?

spoony — Today at 10:40 AM

leebc — Today at 10:47 AM
Because someone going to ask, what was "School" for you for this?

spoony — Today at 10:50 AM
Ceramics 1&2 at spc
The click sequence confuses me every time XD
I believe that’s just the external version of the sitter in ths kiln 

leebc — Today at 10:55 AM
Does the kiln plug into that sitter and it ...does things based on the temperature it reads through the peep?

spoony — Today at 10:55 AM

It’s not quiiite as accurate as the sitter plugged directly in, but close enough for the temps I work with

leebc — Today at 1:59 PM
Please correct me if i'm wrong in how i'm writing this:
vitrify  --    To change or make into glass or a glassy substance, especially through heat fusion.
In ceramics, this is when the clay fuses completely together.  It doesn't leak or (mostly) absorb water.
The "Cone 6" label on the slip clay indicates that this clay vitrifies at cone 6, which is 2192° F, 1200° C.
If there is a 0 in front of the number, like 04, think of it as a negative sign.  As teh numbers get larger, the temperature gets hotter.   So, Cone 6 is hotter than 04.  You can look up the exact values on the internet (link to full table not included here).  The use of "Cone" is based on these ceramic...cones that you would stick in the kiln during the firing.  When the cone melted and "wilted", you knew the kiln was at the correct temperature and you could allow it to start to cool/remove heat.  Modern electric kilns use electric thermostats (in "kiln sitters)  to control the firing process, some with electronics that follow a program, making the whole process dramatically easier.  (see  below)    If you are using an old electric kiln, or a gas kiln, or a wood kiln, there is much more manual adjustment required.
The general ceramics workflow is:
Build piece; let piece dry until  "leather hard" (greenware); initial firing to Cone 04 (bisk firing); allow to cool; GLAZE here; fire to temperature (in your case Cone 6) so the piece will fully vitrify (called earthenware/stoneware). 
(copied from
Clay types and Temperature Ranges
* Low-fire / Earthenware
    Average is Cone 04 (1950° F / 1100° C)
    Ranges from Cone 015 - 1 (up to 2109° F / 1154° C)
    a relatively soft, porous clay body, and a clearly separate glaze layer.
    Glaze colors are generally more varied and bright than stoneware. 
* Mid-range
    Ranges from Cone 2 - 7 (2124-2264° F / 1162-1240° C)
    a relatively soft, porous clay body, and a clearly separate glaze layer.
    Glaze colors are generally more varied and brighter than stoneware.
    A common temperature range for industrial ceramics. 
* High-fire / Stoneware
    Average is Cone 10 (2381° F / 1305° C)
    Cone 8 -12 range (2305-22419° F / 1263-1326° C)
    a hard, vitrified, non-absorbent clay body
    a body-glaze layer that forms between the clay body and glaze. 
*Very High-fire / Porcelain
    Cone 10 -13 range (2381-2455° F / 1305-1346° C)
    a hard, vitrified, non-absorbent clay body
    a body-glaze layer that forms between the clay body and glaze 

The initial bisk process goes through several stages.  if you were performing these in a manual electric kiln, you would control the temperature by  starting with only  some of your heating coils on and the lid off, then closing the lid and adding more more heat.  Not necessarily in that order.   This initial slow process allows water to convert to steam and escape, initial expansion, air to escape (maybe), carbon based materials to burn off, and then clay particles to begin to vitrify.
If you heat up to fast, or cool to fast, your ceramics have a high chance of exploding.  (You also need to avoid trapped air bubbles for this reason)
See table linked here and read up from the bottom to get a better understanding of the firing process.
Ceramic, Pottery, Sculpture and Kintsugi Repair | China repair and ...
Kiln Temperature Ranges
Transformation of Ceramic Materials by Heat

spoony — Today at 2:03 PM
Okay so to make sure you add in: if you want to glaze it, the glaze goes on before the cone 6 fire
But after the bisque
You can’t really glaze it once vitrified

leebc — Today at 2:06 PM
Really?   I thought I'd been told you could.  Maybe with certain high-fire glazes?

spoony — Today at 2:07 PM
Depends on the clay, the glaze, etc
You can do an overglaze fire too, but that’s a third very very low fire

leebc — Today at 2:08 PM
maybe that's what i'm thinking

spoony — Today at 2:08 PM
General rule is once it’s fired to its top temp, don’t rely on being able to add glaze
If it’s like a cone 10 clay sometimes you can glaze fire to like six then add a higher fire glaze 
And sometimes you can add glazes to do a separate type of firing to get different effects

leebc — Today at 2:10 PM
could you high fire, then do a rakoo fire?

spoony — Today at 2:10 PM
I think it’s done sometimes but not sure
I’d be afraid of thermal shock exploding the piece

leebc — Today at 2:10 PM

spoony — Today at 2:10 PM
Cause there are clays specific for raku and they’re all low fire
Yeah stoneware is essentially glass

leebc — Today at 2:12 PM
AH.     Humm...  When I took "Experimental firing techniques" in ...90....+6, I think we used the same clay body for Raku and the high er soda/salt fires we did.
Then there's the wet fire body we made by adding a ton of silica sand.   That did NOT vitrify well.

spoony — Today at 2:14 PM
Yeah raku is designed for thermal shock which soda/salt fires definitely have XD
Salt fires are like high fire raku where you leave the piece in rather than pull out, iirc

leebc — Today at 2:16 PM
get it hot, throw shit on the clay?

spoony — Today at 2:16 PM
And keep it hot yeah

spoony — Today at 2:17 PM
Rather than throw it in a can to cool down naturally like with actual raku