So, I'm loading our glaze kiln (Skutt 1231 PK) 31 inches tall, 28 wide, 11.5 cubic foot firing chamber. And the kiln slides on the floor. Now it's on casters, but the casters are suppose to be locked down, and even if they aren't the Envirovent coupler is spring loaded and should drag on the floor... Weird!?!
So, I get down on the floor to inspect this oddity, and find... what do I find? The Envirovent coupler is disconnected, not engaged, laying on it's side far away from the vent holes in the floor of the kiln. Meaning we have been firing our kilns unvented. How long? Unknown. Did I say Kilns? Well, me rolls over to inspect Meshach (Our glaze kiln is called Shadrach). Meshach, who does most of our bisque firings is... What's this? Disconnected as well?!? I have no idea how the couplers became disconnected as they are spring loaded and the spring is fairly hefty.
So, this means both glaze and bisque have been fired unvented! What are the implications of unvented firing? How might an unvented firing manifest its self in glaze faults? We've been having a series of faults, and now I'm wondering if I might have found the problem...
Your thoughts, other than... Go post this in the thread on "It's OK to make mistakes!" :-)
So to track this down defect wise .... you have to think what "no venting" means. Then take the ceramic physics and chemistry in firing and match the two up to see what "pops". Like being a doctor. Take the patient history, look at the symptoms, maybe runn a few tests, and see if you can track it down.
No active venting. So the kiln chamber had a far more static atmosphere than it would have if the vent wa operating. So what does that mean?
There was nothing helping get things like gases and fumes evolving out of the the clay body out of the kiln. There was no good supply of fresh air (and hence oxygen) into the kiln. There was reduced convective movment of gasses withing the chamber.
SO.............. I'll list one potential example here.......
If there was not a lot of fresh air circulating... there was not a lot of oxygen available for reactions that needed oxygen to go to completion. So for example, any carbonaceous contaminants in the clay body MAY not have been oxidized fully by the time the kiln had dropped below the reactive temperature. This fact might or might not be visible in any way on the fired bisque wares.
So it is possible that in the body, there remained stuff that could still be oxidized later in the up cycle in the following glaze firing. That action will cause more outgassing IF the surface of the glaze layer has not yet started to sinter or melt and become gas impermeable. Outgassing thru melting glaze can cause pinholes, blisters, and craters.
Another potential aspect of this is that the compounds that are looking to be oxidized, in a fast climbing glaze firing, may find that they can steal an oxygen atom/molecule from another compound in the clay body before any oxygen from the kiln atmosphere reaches it thru from the clay surface. So it can sometimes cause reactions like reducing the iron oxide in the clay body to the FeO state. This causes a color change in the iron... but more importantly FeO is a powerful low temperature acting flux on silica. So this can change the way that the body is melting. Which can change the color, maturity, and reaction with overlying glaze.
You can see many differnt potential scenarios depending on the exact clay bodys, the exact firing cycle, and the exact fiollowing glaze usage.
Another faster example...... sulphur concentrations in the clay body can readily cause color changes with certain materials. If there is not enough O2 present in the bisque, you don;t get all the Su changed to sulphur di and tri oxide. It then comes out on the glaze firing... to casue color changes.
Blistering, orangepeel surface, and color changes are all very possible from this lack of venting. I could even see the running scenario in certain situations... but less likely..... OTHER than the lack of convective heat transfer possibly causing som "hot spots" in the kiln that you have no idea exist... because you typically do not have cones all over the place.
PS: Want a real eye opener........ Take about 30 -40 cone packs and scatter them throughout your glaze firing kiln. Fire the load. Check out the cones. LOTS of assumption often go out the window there.
Want an even bigger eye opener? Make some cone packs up that have a lower end point cones in them. So maybe if you are firing to cone 9-10.... a set of packs with only 012, 010, and 08. Then another pack that has 02, 1, 3. Then another that has 4, 6, 7. Then anotehr that has 8, 9, 10. Make many multiples of these. Plan a firing load. Mark the exact locations that you will again scatter many, many cone packs in. The difference this time is that while you are intending to eventually go to cone 9....... you will use your main single sitter or controller to fire first to cone 010... and stop. The let it cool and open the kiln and check the cones. Replace with the next cone pack up the sale...... and fire to cone 1 and stop. repeat this all the way up. You might be amazed at how uneven the kiln is at intermediary times in the firing...... that evens out only at the end.... and your finish fire cone packs all look nice and even..... and you wonder why some results are different.