I've spent time off and on this last week cleaning the pots and getting them ready to sell. This has been a slow process, as the wads this time require quite a bit of work to remove them and get the surface clean. Unfortunately we have quite a few second quality pots this time. Most are sellable, but it's still annoying. I decided to list all the various causes of seconds for this firing so as to keep a record, and remember what to avoid them next time. While I was at it I took some pictures our various blunders and mishaps to share with all of you.
Bad Wadding Mix-
The wadding mix has caused seconds in numerous ways. It shrank as it fused, so we have some pots that stuck to the shelf. Not surprisingly I've chipped quite a few lids while separating them for the pot below, and damaged quite a few feet on bowls removing the wadding from inside the foot ring. The shrinking/fusing also caused some of Christy's serving pieces to warp in strange ways. Most curiously, though, the wadding has caused some cracks to appear on the bottom of pots. I first saw this on several bowls and scolded myself for poor compression while throwing (something you would think I would have learned by now). However I've since found it on several mugs and this large jar:It always occured right under the wad. Christy also had this problem on some of her tea boxes. Luckily this is an easy one to fix. We'll just avoid feldspathic sand next time.
Dunting cracks during cooling-
We fired the second chamber off of the waste heat from the first chamber until the first chamber fell to 1600 degrees. This was pushing the limit, but the main problem, I think, came from leaving the blow hole open. We probably should have closed it as soon as we finished firing the first chamber. This would have given us more time to fire off of the heat from the first chamber, since no cold air would have been pulled in through the blowhole. However, that wasn't the main problem. The main problem was that after we switched to firing the third chamber (it was empty; we just wanted push the moisture out of the kiln and surrounding ground) and sealed up the first chamber, we opened up the air inlet into the second chamber fire box, thinking the first chamber would cool slowly from there. Because we forgot to close the blow hole, cold air continued to be drawn in through the first chamber, causing dunting towards the back of the first chamber under the blow hole.
Floor bricks foaming
Not much to say here. It's the first time we've fired with them, and they foamed up. Certainly having wads that fused and shrank didn't help the situation. We'll cover the floor with a layer of kiln wash before next firing.
Mortar and Castable falling on pots
I thought we had cleaned the arches pretty well. However, some kiln debris still fell on our pots.
The dreaded white slip flaking
This is by far the most frustrating of all the problems we had this firing. First, we can't really sell these (all the rest of the problems are more aesthetic than functional, so we are simply selling them as seconds) because they are continually shedding glass. The worst thing, though, is that we are some what at a loss as to what caused the problem, much less how to fix the problem. We have used this slip extensively in the past, over numerous clay bodies, and we have never had a problem with it. The flaking happened under several different glazes, at different thicknesses, and it did not happen every time we used it. Our white slip consists of: Tile 6, feldspar, silica, and a little bentonite. If anybody has any insight into this please let us know.
This concludes the tour of first firing errors and mishaps!