Bloating: firing fault ruins good pots

After hours of work making, glazing and firing your pots, the opening of the kiln door can lead to some anxious moments. As the door opens your eyes quickly try to assess the setting; first looking for disasters like broken pots, are they all there? and they all standing? are any melted together etc. After the initial assessment you gauge the colour of your glazes; did you get the top temperature right, are the glazes under or over-fired? After the initial rush of excitement, providing everything has gone OK you happily and carefully unpack your work from the kiln, quietly satisfied that all the time and labour you invested was worth it.

Sadly, not every firing is a good firing and there might be times when a whole kiln load of work has failed for one reason or another. I have had many such firings, especially when I was starting off in ceramics, and again when I started experimenting with local materials. Failures are heartbreaking but they become part of the learning process. You slowing create a list of things that you won't do next time. This list will eventually lead to that process which will produce the results you seek.

My September firing was generally successful and on first glance I was very pleased with the majority of the work. Once I started unpacking I noticed that a few of my favourite pots had faults and were ruined by bloating. Bloating is a term used to describe a common firing fault. The fault can be described as bubbles or blisters in the clay body which distort the surface of the pot making it 'lumpy'. The photographs below show the top (where the bloating is barely visible) and side of a pot with some large lumps in the surface of the pot.

Top view

Side view- large lump in the side of the vessel

So what caused this bloating? 

Typically bloating is caused by:

• Over firing
• Impurities in the clay body
• Early reduction or excessive reduction
• Glaze melting early and trapping gases(as impurities burn away)within the clay body

Common solutions are:

• Firing to a lower temperature
• Slower bisque firing to a higher temperature to allow impurities in the clay body to burn away
• Starting reduction at a higher temperature
• Removing materials from the glaze recipe that melt early

Because I use local clay as my clay body I always bisque fire my work to 1030 C (Cone 05), although I have not bisque fired with cones for a while. I will be sure to bisque fire with cones next time to double check the accuracy of the pyrometer on the electric kiln that I use. Mind you, this is best practice, and I have been a little slack. You should never trust the pyrometer on your kiln and always use cones, especially if you have issues with bloating. However, as the bloating was not consistent with all my pots, some pots came out fine . What was consistent with the bloating pots was the glazes that where used, they all contained wood ash in the glaze recipe.

I have being doing a bit of experimentation with copper with some of my glazes of late. And as a consequence I lowered the point at which I begin reducing (reduction) oxygen to the kiln. I fire with wood and it can be tricky to be precise with the temperature - a bit of extra wood can easily create an atmosphere where there is more fuel than oxygen (reduction). So I am thinking that the likely cause of the bloating is the impurities in the wood ash combined with the lower reduction temperature.

As I am not planning to remove the ash from my glazes or stop using local clay, my plan for next time is:

• Slower bisque fire to 1030 C ( Cone 05) making sure to check temp with cones
• Slow wood firing down especially in the 600-800 c range to burn off impurities in glazes prior to reduction
• Be extra careful not a create reduction too early and reduce at a higher cone (Cone 06)

Oh..and hope for the best next time I crack the kiln door.


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