My normally very reliable black glaze has had a little teenage moment of misbehaving.
A whole kiln full of black and white pieces came out blistered – a particularly nasty example is shown above.
I was not a happy bunny.
Luckily one of my studio mates happened to arrive as the crisis was unfolding and suggested re-firing them all to see if it could be got rid of.
I turned to my trusty potter’s dictionary, and then to the even more detailed and helpful “Ceramic faults and their remedies”. This helped me work through the (many) causes of blistering, which included under~ and over~firing, lack of oxygen caused by firing low pieces too close to the shelf or by packing pieces too tightly in the kiln and the kiln cooling too fast.
Once I’d finished flustering about this gave me a good list of potential problems to work through to check against my firing and narrow it down to a short list of possible contenders. (I think my research training may have kicked in at this point, I quite enjoyed this!).
Then I could work out a new firing cycle that addressed as many of the possible causes as I could.
In the end I re-packed them all, on tripods to raise them, less closely together. Fortunately we have two kilns so I could use the larger one. This has a fancier control box so I could add a longer soak and also control the cooling speed to make that slower.
Here’s the results on the same piece.
This one is still a little ‘scarred’ by its experiences, but the blistering is much reduced and the funny reddish tinge around the foot is also mostly gone. Luckily this was only a test piece for a new shape. On the stock items which were less blistered to start with the final finish was completely smooth.
The moral of the story is – it’s worth trying a re-firing!