From crazy temperature leaps to a cold back, the March firing at River Song Pottery was an odd one!
Simon Levin and I are hosting an Instagram chat this Thursday, at 10 am CST. Simon is generously offering his reading of the pots (like reading tea leaves, but on tea pots instead) and discussing firing guidance.
Here’s a little background and close-up pics from the firing, for your reference.
We’re firing a gorgeous train kiln! 67 cubic foot loading space.
The biggest issue I wanted to troubleshoot going into this firing: foaming shelves! Serious foaming. Not so much shelf drips— that has been a minor problem. The shelves foam, up through kiln wash, onto the wads, and onto the bottom of pots. We have nitride bonded silicon carbide shelves. As an aside, I want to keep using them, because I want shelves that are light enough for me to load by myself, and I stress out when I see clay shelves fail. If we cannot consistently and successfully troubleshoot foaming, I will have to figure out something else, but I am very motivated to fire my kiln such that the shelves stop being an issue.
Plan for a Foam-Free-Firing
Stop with the overstocking the firebox. I have a very experienced crew, myself included, who are good at fitting a lot of wood in there, which can be a problem since we’re using so much hard wood. Fire it very open leading up to body reduction, then consider the firebox “full” at 75% to avoid heavy reduction.
These changes were rather successful. The very front row of pots were an issue, but otherwise I was delighted with the decreased grinding. I will make some “cookies” for firing those few pots for the next run.
Downside— we had more oxidation than I’d like. I’m interested to discuss air and stoke cycles for the post-body-reduction phase.
The second goal, which is a common theme at the pottery, was to get the back to cone 10. This was the 6th firing of the kiln, and we’ve been puddling cone 12 and finishing at cone 9 soft in the cold spot in the back. The last firing, we had a few pieces fall over in the throat arch area, which effectively blocked heat on one side of the kiln for the entire firing. So we didn’t load any work in the throat arch to troubleshoot.
Don’t load any work in the throat arch. As cone 2 starts to go, close off air significantly, and use the back temp as an indication of when to stoke. So… big stoke from the front. Watch the back rise, and then as it tapers off and begins to fall, stoke the front again.
I’ll digress for a moment… We get our side stoke material from a stair company. Long strips of wood are loaded into my minivan (nirvana is when one of my crew has a BIG vehicle or a pick-up truck to use,) then we bring back the load, stack it, cut it with a chainsaw, then pile it up near the barn. It is thin and dry and burns FAST. We realized in the December firing that splitting logs seems to be a better boost to the back temp, and I have been tiring of running back and forth to the stair company… So we planned on firing with more of the split wood than the scrap. And I wanted to cut back on how much we were depending on side stoking for temperature rise. Too much ash burying the pots in the last firing. So I didn’t to go through a huge pile of side stoke.
Well, I am quite sure I underestimated how much we needed. I was waiting for the neighbors to wake up so I could run the splitter the morning we fired off!
Back to the plan… hold at cone 2, even the temps. Don’t go crazy side stoking so the heat can carry to the back. Unfortunately, the stoke aisles were kinda empty and asking for help!
A few notes from the log book…
Hours 1-10… nothing remarkable. Holding at 200, then climbing 50 degrees per hour to 400. Then 100 degrees per hour to 600.
The overnight crew typically runs into a stall when switching to the grates. Not this time! I had left the advice to let it go, and that’s exactly what they did. Which is great (grate?). I wanted to hold the kiln longer, earlier to build ash and even the temps. But! The kiln was ready for body reduction 4 hours earlier than planned, and I didn’t realize my directions weren’t thorough enough. And the barn’s metal siding makes for a really poor cellular connection. We were trying to communicate what the air/damper/stoking pattern was, while I was also hustling out the door to the barn. Which is a 30 min drive from home.
Hour 18: 1889 in the front and 1438 in the back. 450 degrees difference between front and back. Gulp. Miscommunication on when to start side stoking. We start side stoking and fill the firebox quite a bit more and also close more primaries and secondaries. Everything starts moving in the right direction…
Hour 22: 2139 in the front and 1901 in the back. Not paying attention, we had switched to pine…
Hour 23: 2215 in the front and 1967 in the back. Hot spot went from cone 6 to cone 11 soft in the last hour! At this point, zero primaries and secondaries are open. Front is HOT. We realize that we’re pulling pine from a different pallet (it’s heavy and wet, but sure enough, it’s pine, which explains the jump.) Switching to hard wood doesn’t make enough difference. We stoke all of the big logs I can lift, and call the neighbor and grab a few more from his yard. LOVE my neighbors! This very effectively drops the temp. We also close the damper an inch, which I believe extends firings…
Hour 24: Cone 12 drops in the hot spot.
Hour 25: 2039 in the front and 1999 in the back. That’s more like it.
Hour 26: 1849 and 1896. Damn it. Stoked too many of those big logs. Worried about what that will do to the glaze in the kiln. Fortunately, the big swing didn’t affect the back pyrometer as much, which is where we load glaze ware…. Time will tell.
Hours 28-37 2200 and 2100 in the back. Overnight crew does a great job holding the kiln without drama (at least, none that they shared!) They turn the kiln over to me at 4am! All we need to do is let her go and drop 9 in the back! Well, ideally 10. But at least 9.
Hours 37-43 Easier said than done. Holding, holding, dropping a little, dropping a little more…. Opened up primaries and split more side stoke at 8am (didn’t want to bother the neighbors at 4am, but I REALLY wish we had had more available then!)
With the primary goal being to avoid shelf foaming, I call final stoke at hour 42. Cone 9 is only soft in the back, and we don’t have a cone pack in the other (usually colder) spot in the back. Big stoke in the front, and I take an hour to burn it down.
Alas, dropping cones in the back remains the goal for the next firing!
The unload revealed that much of the back was affected by the low temps. The front, most of the middle, and the floor of the back held many delightful pots. The purpose of this conversation is to avoid the cold ones!